3
Multiple-choice Questions
3.1 Redox Reactions 5 The following equation shows What would be observed if
the reaction between metal X iodine solution is used instead
1 Which of the following of chlorine water?
substances are correctly ’11 and the salt solution of metal Y. A Bromide ion is oxidised to
’09 matched with their roles as X(s) + Y(NO3)2(aq) → bromine.
oxidising or reducing agent? X(NO3)2(aq) + Y(s) B Iodine is reduced to iodide ion.
Which metals could be X and Y? C No change is observed.
D A brown solution is formed.
Oxidising Reducing XY
agent agent 10 The diagram below shows the
A Copper Tin set-up of apparatus to investigate
A Sulphur dioxide Chlorine the transfer of electrons at a
B Copper Iron distance.
B Chlorine Sulphur dioxide
C Iron Tin Which of the following
C Potassium Bromine statements about this experiment
3 D Iron Zinc is correct?
manganate(VII) A The oxidation number of
manganese decreases from
D Potassium Potassium 6 Which of the following cannot +5 to +2.
bromide manganate(VII) B The iodide ions act as a
occur during reduction? reducing agent.
2 The manufacture of nitric acid ’09 A Loss of oxygen C The electrons are transferred
in the Ostwald process involves from electrode X to electrode
B Gain of hydrogen Y through dilute sulphuric acid.
’08 the following steps. C Donation of electrons D At electrode Y, H+ ions from
D Decrease in oxidation number sulphuric acid are reduced to
NH3 → NO → NO2 → HNO3 hydrogen gas.
7 Consider the following reaction:
Which of the following shows the 11 The following reaction occurs
correct sequence of changes in 2I–(aq) + 2Fe3+(aq) → when hydrogen peroxide is
the oxidation number of nitrogen? ’03 2Fe2+(aq) + I2(aq) added to an aqueous solution
A +3 → +2 → +4 → +3 of iron(II) salt.
B +3 → +2 → +4 → +5 Which of the following is true
C –3 → +2 → +4 → +3 H2O2(aq) + 2Fe2+(aq) + 2H+(aq) →
D –3 → +2 → +4 → +5 about the ionic equation? 2Fe3+(aq) + 2H2O(l)
3 Which of the following organic A Fe3+ is oxidised. Which of the following
compounds is reduced in the statements are true?
following reactions? B Fe3+ is a reducing agent.
A CH3CH2OH → CH3COOH
B CH3CH2OH → C2H4 + H2O C I– is an oxidising agent.
C CH3CHO → CH3CH2OH
D CH3CHO → CH3COOH D I– donates electrons to Fe3+.
4 Which of the underlined 8 In which of the following
substances in the following
equations acts as the oxidising reactions is copper oxidised?
agent? ’04 A The reaction of copper(II)
A Fe2O3 + 3CO → 2Fe + 3CO2
B H2S + Br2 → S + 2HBr oxide with magnesium.
C MgBr2 + Na2CO3 → B The reaction of copper with
MgCO3 + 2NaBr
D Mg + H2SO4 → MgSO4 + H2 silver nitrate solution.
C Electrolysis of copper(II)
sulphate solution using
carbon electrodes.
D Chemical cell with copper
and magnesium foils in
dilute sulphuric acid.
9 The following equation
represents the reaction between
’05 chlorine and potassium iodide:
2CKl2C(al(qa)q+) 2KBr(aq) →
+ Br2(aq)
Oxidation and Reduction 444
I The solution becomes IV Chlorine is oxidised. A magnesium
A I and II only B iron
colourless. B II and III only C lead
C III and IV only D copper
II Hydrogen peroxide molecule D I, II and III only
19 Which of the following does not
accepts electrons. 15 FeSO4 solution can be prepared prevent the corrosion of iron?
from Fe2(SO4)3 solution by A Coating iron with grease
III H+ ion acts as the oxidising B Coating iron with zinc
I adding zinc powder C Fixing a copper bar to iron
agent. II adding potassium D Fixing an aluminium bar to iron
IV Iron(II) salt is oxidised. manganate(VII) solution 20 Consider the experiments shown
A I and III only III passing chlorine gas in the diagram below.
B II and IV only IV passing sulphur dioxide gas
C I, II and III only
D II, III and IV only A I and III only
B II and IV only
12 The electronic configuration of C I and IV only
four elements are shown below. D I, III and IV only
Q: 2.8.2 Y: 2.8.7 3.2 Rusting as a Redox
X: 2.8.6 Z: 2.8.8 Reaction
Which of these elements will act 16 The diagram below shows the
rusting of iron.
as a reducing agent?
A Element Q C Element Y ’06
B Element X D Element Z
13 When an aqueous solution of 3
compound X is added to acidified
Which beaker contains the most
potassium dichromate(VI)
rust?
solution, the solution changes
A Beaker I C Beaker III
from orange to green. What
could be compound X? B Beaker II D Beaker IV
I Sulphur dioxide 21 The diagram shows the set-up
of apparatus to study the rusting
II Sodium sulphite of iron.
III Chlorine Which of the following equations
IV Iron(III) chloride occurs at the cathode?
A I and II only
B II and IV only A Fe2+ + 2e– → Fe
C III and IV only
D I, III and IV only
14 The diagram below shows the B Fe → Fe2+ + 2e–
arrangement of the C 4OH– → O2 + 2H2O + 4e–
apparatus to study the redox D O2 + 2H2O + 4e– → 4OH–
reaction between chlorine and
iron(II) sulphate solution. 17 Which of the following statements
about the rusting of iron are true?
I Rusting requires both oxygen
and water.
II Rusting is accelerated by The observations are recorded
below.
the presence of magnesium
chloride. Test tube Colour of solution
III An iron atom releases two 1 No change
2 Turns pale blue
electrons to form an iron(II) 3 Turns dark blue
ion.
Which of the following IV The chemical formula of rust
statements are true regarding the
A is Fe3O4. xH2O. Based on the observation, the
experiment shown above? I and III only
I Electrons flow from X to Y reactivity of the metal increases
B II and IV only
through the external circuit. in the order:
II Electrode X is the positive C I, II and III only A P < Q < R
B R < Q < P
electrode. D I, II, III and IV C Q < P < R
D Q < R < P
III The green colour of FeSO4 is 18 A metal X is placed in zinc
changed to brownish-yellow. sulphate solution and a reaction
occurs. The metal X is
445 Oxidation and Reduction
22 When metal X is twisted around BDAC SSSSnnnnOOOO2222 + C → Sn + CO2 30 Which of the following
+ 2C → Sn + 2CO reactions occurs at the
an iron nail, the iron nail rusts + 2CO → Sn + 2CO2 cathode and anode when
rapidly. However, if metal Y is + CaO → CaSnO3 concentrated copper(ll) chloride
is electrolysed using carbon
twisted around the iron nail, the 27 The diagram below shows the electrodes?
extraction of iron in a blast furnace.
iron nail does not rust. These
’06 Cathode Anode
observations show that
I metal X is more reactive than What is substance X? A H+ ions Cl– ions
A Coke reduced to oxidised to
metal Y. B Platinum hydrogen chlorine
II metal X is more reactive than C Aluminium
D Vanadium(V) oxide B Cl– ions Cu2+ ions
iron. reduced to oxidised to
III metal Y is more reactive than 28 Which of the following reactions chlorine copper
occur in the blast furnace for the
iron. extraction of iron from its ore? C Cu2+ ions OH– ions
IV metal Y can protect metal X reduced to oxidised to
I Coke (carbon) reduces iron copper oxygen
from corrosion. ore to iron.
A I and II only D Cu2+ ions Cl– ions
B III and IV only II Iron ore acts as an oxidising reduced to oxidised to
C I, II and III only agent. copper chlorine
D II, III and IV only
III Carbon dioxide is reduced by
23 An iron nail, with a piece of zinc coke to carbon monoxide.
foil wrapped around it, is placed IV Limestone (calcium carbonate)
acts as a reducing agent.
3 in a beaker of water.
A I and III only
Which of the following ionic B II and IV only 31 The diagram shows the set-up
C I, II and III only of apparatus for an experiment
equations represent the reactions D I, II, III and IV
’06 to study the electrolysis of
that occur after a few days? copper(II) sulphate solution.
I Zn(s) → Zn2+(aq) + 2e–
II Fe(s) → Fe2+(aq) + 2e–
III 2H+(aq) + 2e– → H2(g)
IV Fe2+(aq) + 2OH–(aq) →
Fe(OH)2(s)
A I and III only
B II and IV only
C I, II and III only
D II, III and IV only
3.3 Reactivity Series of Metals 3.4 Redox Reactions in The intensity of the blue colour
and Its Applications Electrolytic Cell and of copper(II) sulphate solution
Chemical Cell decreases as electrolysis proceeds.
24 What is the position of carbon in Which of the following statements
the reactivity series of metals? 29 What are the reactions that occur at best explains the observation?
the anode and the cathode during A Cu2+ ion is oxidised to form
’07 A Between zinc and iron the electrolysis of an aqueous
B Between aluminium and zinc solution of sodium sulphate? copper metal at the cathode.
C Between magnesium and B Cu2+ ion is reduced to form
aluminium Cathode Anode
D Between iron and lead copper metal at the cathode.
A Oxidation Reduction C OH– ion is oxidised to form
25 The following pairs of
substances are heated strongly. B Hpr2ogdausciesd Opr2ogdausceisd oxygen gas at the anode.
Which pair will produce a glow? D OH– ion is reduced to form
A Aluminium oxide and iron
B Iron(III) oxide and lead oxygen gas at the anode.
C Lead(II) oxide and copper
D Zinc oxide and aluminium 32 The diagram below shows a
simple chemical cell.
26 Which of the following reactions
does not occur during the C OH– ions H+ ions are
extraction of tin from tin ore
which contains tin(IV) oxide? are reduced oxidised
D oSOxid42is–eiodns are Na+ ions are
reduced
Oxidation and Reduction 446
Which of the following undergoes 36 When a dry cell supplies electric 39 The structure of a lead-acid
oxidation when the cell is current, battery is described below.
supplying electricity? A zinc container is oxidised.
A Zinc plate B the carbon rod acts as the Cell terminal P: Lead plate
B Copper plate anode. Cell terminal Q:
C Copper(II) ions C the ammonium ion acts as Lead plate coated with lead(IV)
D Hydrogen ions the reducing agent. oxide.
D ammonium ion is reduced to Electrolyte: Solution X
33 Which of the following statements nitrogen.
is true? Which of the following are true
A In chemical cells, anodes are 37 The electroplating of an iron about lead-acid battery?
positively-charged. spoon with copper is carried out I Cell terminal P is the positive
B In chemical cells, electrical using the apparatus shown in the
energy is converted to diagram below. terminal.
chemical energy. II Lead is oxidised to Pb2+ ions
C In electrolytic cells, anodes 3
are negatively-charged. at cell terminal P.
D In electrolytic cells, electrons III PbO2 is reduced to Pb2+ ions
move from the anode to the
cathode through the external at cell terminal Q.
circuit. IV Solution X is dilute
34 A piece of tin and a piece of hydrochloric acid.
copper are used as electrodes in A I and II only
a chemical cell as shown in the B II and III only
diagram below. C III and IV only
D I, II and III only
Which of the following statements is
true about this experiment? Which of the following redox 40 The diagram below shows a
A The size of the tin plate reactions occurs at the cathode magnesium-silver cell.
during electrolysis?
becomes smaller. A Cu → Cu2+ + 2e– ’11
B Bubbles of gas are formed
… oxidation V
around the tin plate. B Cu2+ + 2e– → Cu
C Electrons flow from the Mg plate Ag plate
… reduction
copper plate to the tin plate. C Fe → Fe2+ + 2e– MgSO4(aq) AgNO3(aq)
D The copper plate is covered
… oxidation
with a thin layer of a reddish- D 4OH– →2H2O + O2 + 4e–
brown substance.
… oxidation
35 In a dry cell,
A zinc ions are reduced to zinc 38 The apparatus set-up for a
metal. simple cell is shown below.
B carbon rod acts as the
negative terminal. What could be metals P and Q?
C manganese(III) oxide is oxidis
ed to manganese(IV) oxide Metal P Metal Q Which of the statements about
D ammonium chloride is this voltaic cell is true?
reduced to hydrogen and A Copper Lead
ammonia. B Lead Copper I Magnesium dissolves to form
C Iron Tin
D Aluminium Tin magnesium ions.
II Magnesium acts as the
reducing agent.
III The concentration of silver
nitrate decreases.
IV The ions in the solution flow
through the porous pot.
A I and III only
B II and IV only
C I, II and III only
D I, II, III and IV
447 Oxidation and Reduction
Structured Questions
1 Excess iron powder is added to an aqueous solution 3 (a) Diagram 2 shows an experiment for investigating
containing magnesium chloride and copper(ll) nitrate. the positions of metal X, metal Y and hydrogen
The mixture is shaken and filtered. in the reactivity series.
(a) Write the ionic equation for the reaction that
occurs. [1 mark]
(b) Explain your answer in (a). [2 marks]
(c) (i) Identify the substance in the residue after
filtration. [1 mark]
(ii) Describe what you see in this experiment.
[2 marks] Diagram 2
(d) Identify (i) the oxidising agent, (ii) the reducing The experimental results are shown in the following
table.
agent in this reaction. [2 marks]
(e) Explain your answer in (d). [2 marks] Metal Observation
oxide
2 (a) The following reaction occurs when ammonia is During heating After heating
passed over heated oxide of copper. Oxide of
3 metal X Glows brightly The colour of the oxide
’08 2NH3(g) + 3CuO(s) → N2(g) + 3H2O(l) + 3Cu(s) Does not glow changes from brown to grey
Oxide of
(i) State the changes in oxidation number of metal Y The oxide is yellow when hot
and white when cold
copper in this reaction. [1 mark]
(ii) Is ammonia oxidised or reduced in this (i) With the help of a diagram, explain how dry
reaction? Explain your answer. [1 mark] hydrogen is prepared in the laboratory.
(b) Diagram 1 shows the set-up of apparatus to [3 marks]
investigate electron transfer through a solution.
(ii) Explain one safety precaution that must be
taken in this experiment. [1 mark]
(iii) Predict the melting point and the boiling
point of liquid Z produced. [1 mark]
(iv) Suggest the identity of the oxide of X.
[1 mark]
(v) Suggest the identity of the oxide of Y.
[1 mark]
(vi) Arrange X, Y and hydrogen in order of the
reactivity series. [1 mark]
(vii) Explain your answer in (vi). [2 marks]
(b) Diagram 3 shows the arrangement of apparatus in
an experiment for investigating the redox reactions
of three elements: magnesium, zinc and carbon.
Diagram 1
(i) What is meant by oxidising agent. [1 mark]
(ii) Name the oxidising agent and the substance
that is reduced in the experiment.
[2 marks]
(iii) Which is the positive electrode, X or Y ?
[1 mark]
(iv) Write the half-equations that take place at Diagram 3
the negative electrode. [1 mark] (i) What is the function of potassium
(v) Name one substance that can be used to manganate(VII) in this experiment?
replace dilute sulphuric acid. [1 mark] [1 mark]
(c) State the changes that can be observed at (ii) Record the experimental results expected to
electrodes X and Y after 5 minutes. [2 marks]
be obtained in the following table.
Oxidation and Reduction 448
Observation 6 The apparatus set-up of two cells, P and Q, are shown
in diagram 4.
Element Nature of flame/ Nature of residue
Magnesium glow (if any)
Zinc
Carbon
[3 marks]
(iii) Describe the steps to be taken so that the
redox reactions can occur. [2 marks]
4 (a) State two conditions for the rusting of iron.
[1 mark]
’07 (b) Based on your answer in (a), draw a labelled
diagram to show what happens when iron rusts.
Your diagram should also include the ionisation
of iron and the flow of electrons. [3 marks]
(c) Describe the reactions that take place at the edge 3
of a water droplet. [3 marks] Diagram 4
5 When metal X is added to iron(III) chloride solution, (a) (i) Name the types of reactions that occur in
a redox reaction occurs and a green solution is
cells P and Q. [1 mark]
’09 formed.
(ii) What is the difference between cell P and
(a) Which metal given in the list below is most likely
to be metal X? cell Q in terms of energy change? [1 mark]
Explain your answer.
Answer the following questions by referring to cell P.
(b) Write the half-equations for the reaction at (i) the
anode, (ii) the cathode. [2 marks]
(c) (i) Write the chemical equation for the overall
copper, lead, magnesium, potassium reaction. [1 mark]
(ii) Explain the above reaction in terms of
oxidation and reduction. [2 marks]
[3 marks] Answer the following questions by referring to cell Q.
(b) (i) What is the change in oxidation number for (d) State the observations that occurred at (i) the
both the reactants? [2 marks] copper plate, (ii) the zinc plate.
(ii) Write the half-equations involved in the (e) (i) Write the ionic equation for the overall
redox reaction. [2 marks] reaction. [1 mark]
(c) Explain the role that iron(III) chloride plays in this (ii) Explain the above reaction in terms of
reaction? [2 marks] oxidation and reduction. [2 marks]
Essay Questions
1 (a) Describe an industrial method for the extraction of iron from its ore. [6 marks]
(b) In terms of electron arrangement, explain why metals (such as sodium, magnesium and aluminium)
act as reducing agents and non-metals (such as oxygen and chlorine) act as oxidising agents. [10 marks]
(c) Explain why galvanising can prevent iron from rusting. [4 marks]
2 (a) Magnesium is more reactive than copper. Design two experiments to prove this statement. [10 marks]
[10 marks]
(b) Some substances act as an oxidising agent in one reaction but a reducing agent in another reaction.
Using iron(II) sulphate as an example, describe how you would prove this statement.
449 Oxidation and Reduction
Experiments
1 (a) Table 1 shows the set-up of apparatus and the result of the experiment to study the reaction of
copper(II) oxide with carbon.
Table 1
Set-up of apparatus Observation on the mixture
Bright glow
3 What inference can you make based on the above observation. [3 marks]
(b) The experiment is repeated using magnesium oxide, lead(II) oxide and sodium oxide mixed with [3 marks]
carbon. The results of the experiment are shown in Table 2. [2 marks]
Table 2
Mixture of metal oxide with Observation on the
carbon mixture
Magnesium oxide + carbon No change
Lead(II) oxide + carbon Faint glow
Sodium oxide + carbon No change
(i) State one hypothesis for the experiment.
(ii) Based on Tables 1 and 2, construct a table to classify the metals into two groups.
(c) Diagram 1 shows the set-up of apparatus to determine the reactivity of metals with oxygen.
Diagram 1 [1 mark]
(i) What is the purpose of heating potassium manganate(VII)?
(ii) The experiment is carried out using powders of copper, iron, lead and magnesium.
The results of the experiments are shown in Table 3.
Oxidation and Reduction 450
Table 3
Set-up of apparatus Observation of the metal Set-up of apparatus Observation of the metal
Faint glow
Look at the flame or glow in each of the experiments. Then complete Table 3 by stating the [3 marks] 3
observations of the reaction of metals with oxygen. [3 marks]
(d) State one hypothesis for the experiment.
(e) Based on the observations in Table 3, arrange the metals, copper, iron, lead and magnesium in
descending order of reactivity with oxygen.
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [3 marks]
Reactivity of metal with oxygen decreases [3 marks]
[17 marks]
(f) Complete Table 4 by stating the action to be taken for each variable.
Table 4
Variables Action to be taken
(i) Manipulated variable (i) How to manipulate the variable
(ii) Responding variable (ii) What to observe in the responding variable
(iii) Constant variable (iii) How to maintain the constant variable
2 Steel but not iron is used to build bridges. This is because iron rusts easily when exposed to air and water
but steel is more resistant to rusting.
You are asked to plan an experiment to show that steel is more difficult to rust than iron.
Your experiment should include the following aspects:
(a) Statement of the problem
(b) Statement of the hypothesis
(c) All the variables
(d) Lists of materials and apparatus
(e) Procedure of the experiment
(f) Tabulation of data
451 Oxidation and Reduction
4CHAPTER FORM 5
THEME: Interaction between Chemicals
Thermochemistry
SPM Topical Analysis
Year 2008 2009 2010 2011
Paper 1 2 31 2 31 2 31 2 3
Section ABC ABC ABC ABC
Number of questions 3 1 – – – 2 1 – – – 3 – – 1 – 4 – – – –
ONCEPT MAP
Energy level diagram Calculations
(a) Number of moles of reactants
Example: Exothermic reaction
or products = x
N2 + 3H2 2NH3; ∆H = –92 kJ THERMOCHEMISTRY (b) Heat released or absorbed
• The branch of chemistry that
LULYN` = (–m––3––c––3––θ–) kJ = y kJ
studies the changes in heat 1000
5 / energy in chemical reactions (c) From (a) and (b),
heat of reaction = – —y kJ mol–1
¬/ $ ¶ R1 x
5/
Heat of precipitation Heat of displacement
Example: Heat of displacement of copper by zinc
Example: Heat of precipitation of silver chloride Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq); ∆H = –210 kJ mol–1
Ag+(aq) + Cl–(aq) → AgCl(s); ∆H = –63 kJ mol–1
Heat of neutralisation Heat of combustion
Example: Heat of neutralisation of hydrochloric acid Example: Heat of combustion of ethanol
with sodium hydroxide C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l);
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l);
∆H = –1371 kJ mol–1
∆H = –57 kJ mol–1
Heat of neutralisation of a strong Heat of neutralisation of a strong Choice of suitable fuels
acid with a strong base is always acid with a weak base Important factors:
–57 kJ mol–1 because strong (or a weak acid with a strong base) • High fuel value (kJ/g)
acids and strong bases dissociate is always less than –57 kJ mol–1 • Burns easily
completely. because weak acids and bases • Do not produce air pollutants
absorb heat energy on dissociation. • Cheap
• Can be obtained easily
4.1 Energy Changes in Exothermic Reactions SPM
Chemical Reactions
’09/P1,
’11/P1
1 Exothermic reactions are reactions that release
1 Energy exists in various forms. In chemistry, heat energy to the surroundings.
the important forms of energy are
(a) heat energy, 2 Figure 4.1 shows a simple experiment for
(b) light energy,
(c) chemical energy, measuring the temperature change when
(d) electrical energy,
(e) kinetic energy. anhydrous copper(II) sulphate solid is dissolved
2 According to the law of conservation of in water.
energy, energy cannot be created or destroyed.
However, energy can be converted from one
form to another as shown in Table 4.1.
Table 4.1 Examples of conversion of energy
Energy change Example Figure 4.1 An exothermic reaction
3 In this reaction, heat energy is released to the
(a) Electrical energy Electrolysis 4
surroundings (Figure 4.2).
→ chemical
energy The term ‘surroundings’ means the water in
which the chemical is dissolved, the container in
(b) Chemical energy Chemical cells (batteries) which the chemical reaction occurs, the air and the
thermometer.
→ electrical
energy SPM
(c) Light energy → • Photosynthesis ’07/P1
chemical energy • Photochemical reactions
CH4 + Cl2 → CH3Cl + HCl
or action of sunlight on
photochromic glass
3 Chemical energy is the energy stored in all CuSO4
chemical substances. +
4 During a chemical reaction, the chemical energy H2O
stored in the reactants are converted mainly
into heat energy and other forms of energy Figure 4.2 Heat energy is released
(such as light energy). to the surroundings in an
exothermic reaction
5 Thermochemistry is the branch of chemistry
that studies the changes in heat energy in 4 When an exothermic reaction occurs,
chemical reactions. (a) heat is released and is transferred from
the reactants to the surroundings,
6 Chemical reactions can be divided into two (b) the reaction mixture and the container
classes based on the energy changes that occur become hot,
during the reaction. (c) the temperatures of the reaction mixture
(a) Exothermic reactions and the container rise,
(b) Endothermic reactions (d) the chemical energy is converted into heat
energy.
Heat energy can be transferred from the reactants
or the products to the surroundings and vice versa. 5 Figure 4.3 shows the changes in temperature
However, the total energy remains unchanged at the when an exothermic reaction occurs. Initially,
end of the reaction. the temperature of the reaction mixture rises
until the highest temperature is reached. When
453 Thermochemistry
the reaction is completed, the temperature of (a) Combustion of fuels
the reaction mixture falls until it reaches room
temperature. CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Figure 4.3 Variation of temperature with time for (b) Oxidation of food in the respiration
an exothermic reaction process
6 Examples of exothermic reactions involving C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(g)
physical changes glucose
(a) Condensation process (gas to liquid)
H2O(g) → H2O(l) (c) Rusting of iron
steam water 4Fe(s) + 3O2(g) + 2xH2O(l) → 2Fe2O3.xH2O
HCl(g) → HCl(l) rust
Condensation process (gas to solid)
CO2(g) → CO2(s) (d) Dissolving soluble bases (metal oxides)
dry ice in water
I2(g) → I2(s)
(b) Freezing (solidification) process (liquid CaO(s) + H2O(l) → Ca(OH)2(aq)
to solid)
4 H2O(l) → H2O(s) (e) Neutralisation reactions between acids
water ice and bases
Fe(l) → Fe(s)
(c) Dissolving alkalis and acids (especially HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
concentrated acids) in water. H2SO4(aq) + CuO(s) → CuSO4(aq) + H2O(l)
NaOH(s) + water → Na+(aq) + OH–(aq)
sodium hydroxide (f) Reaction between acids and metals or
sHu2lSpOhu4r(ilc)a+ciwd ater → 2H+(aq) + SO42–(aq) metal carbonates
(d) Dissolving anhydrous salts, such as
anhydrous copper(II) sulphate (CuSO4) Zn(s) + H2SO4(aq) → ZnSO4(aq)+ H2(g)
and anhydrous sodium carbonate (Na2CO3) Na2CO3(s) + 2HCl(aq) →
in water. 2NaCl(aq) + H2O(l) + CO2(g)
CuSO4(s) + water → Cu2+(aq) + SO42–(aq)
Na2CO3(s) + water → 2Na+(aq) + CO32–(aq) (g) Displacement reaction of a metal from its
salt solution by a more reactive metal
7 Examples of exothermic reactions involving
chemical changes Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(h) Haber process for the manufacture of
ammonia
N2(g) + 3H2(g) 2NH3(g)
(i) Contact process for the production of
sulphur trioxide
2SO2(g) + O2(g) 2SO3(g)
Endothermic Reactions SPM
’09/P1
1 Endothermic reactions are reactions that absorb
heat energy from the surroundings.
2 Figure 4.4 shows a simple experiment to measure
the temperature change when copper(II) sulphate
crystals are dissolved in water.
Thermochemistry 454
Figure 4.4 An endothermic reaction 6 Examples of endothermic reactions involving 4
3 In this reaction, heat energy is absorbed from physical changes
(a) Dissolving ammonium salts such as
the surroundings and is transferred from water ammonium chloride (NH4Cl), ammonium
to the copper(II) sulphate crystals (Figure 4.5). nitrate (NH4NO3) and ammonium sulphate,
(NH4)2SO4 in water.
Figure 4.5 Heat energy is absorbed
from the surroundings in NH4Cl(s) + water → NH4+(aq) + Cl–(aq)
endothermic reactions NH4NO3(s) + water → NH4+(aq) + NO3–(aq)
(NH4)2SO4(s) + water → 2NH4+(aq) + SO42–(aq)
4 When an endothermic reaction occurs,
(a) heat energy is absorbed and is transferred (b) Dissolving crystalline salts such
to the reactants, as hydrated copper(II) sulphate
(b) the reaction mixture and the container (CuSO4.5H2O), hydrated magnesium
becomes cold, sulphate (MgSO4.7H2O) and hydrated
(c) the temperatures of the reaction mixture sodium carbonate (Na2CO3.10H2O).
and the container fall,
(d) the heat energy is converted into chemical CuSO4.5H2O + water →
energy. Cu2+(aq) + SO42–(aq) + 5H2O(l)
5 Figure 4.6 shows the variation of temperature Na2CO3.10H2O + water →
with time when an endothermic reaction 2Na+(aq) + CO32–(aq) + 10H2O(l)
occurs. Initially, the temperature of the reaction
mixture falls until it reaches the minimum (c) Melting process (solid to liquid)
temperature. When the reaction is completed,
the temperature of the reaction mixture rises H2O(s) → H2O(l)
until it reaches room temperature. ice water
Sn(s) → Sn(l)
Figure 4.6 Variation of temperature with time for
an endothermic reaction (d) Evaporation and boiling processes (liquid
to gas)
H2O(l) → H2O(g)
water water vapour
H2O(l) → H2O(g)
water steam
7 Examples of endothermic reactions involving
chemical changes
(a) The reaction between acids and sodium
or potassium hydrogen carbonate.
HCl(aq) + NaHCO3(s) →
NaCl(aq) + H2O(l) + CO2(g)
(b) Thermal decomposition of metal
carbonates, metal nitrates and ammonium
chloride.
ZnCO3(s) → ZnO(s) + CO2(g)
2Mg(NO3)2(s) →
2MgO(s) + 4NO2(g) + O2(g)
NH4Cl(s) NH3(g) + HCl(g)
(c) Photosynthesis
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
glucose
455 Thermochemistry
When an endothermic reaction occurs, the heat energy The reaction between an acid and sodium carbonate
is absorbed from the surroundings and converted into (Na2CO3) is an exothermic reaction, but the reaction
the chemical energy of the reactants. The loss of heat between an acid and sodium hydrogen carbonate
energy from the surroundings causes the temperature (NaHCO3) is an endothermic reaction.
of the solution to fall.
To study exothermic and endothermic reactions
Materials Plastic cup, thermometer and spatula. (b) Dissolving ammonium chloride solid in water
Apparatus
Sodium hydroxide solid, ammonium Reaction Initial Lowest
Procedure chloride solid, 1 mol dm–3 hydrochloric NH4Cl(s) + water temperature temperature
acid, 1 mol dm–3 sodium hydroxide
solution and 2 mol dm–3 sodium (°C) (°C)
hydrogen carbonate solution.
28 24
Activity 4.1 4 Discussion
1 When sodium hydroxide is dissolved in water,
the temperature of water rises. This means that
heat energy is given off to water.
2 When ammonium chloride is dissolved in water,
the temperature of water falls. This means that
heat energy is absorbed from water.
Figure 4.7 To investigate Conclusion
exothermic and
endothermic reactions 1 Dissolving sodium hydroxide solid in water is an
exothermic process.
(A) Dissolution of sodium hydroxide and ammonium
chloride in water 2 Dissolving ammonium chloride solid in water is
an endothermic reaction.
1 Using a measuring cylinder, 50 cm3 of water is
measured out and poured into a plastic cup. (B) The reactions between hydrochloric acid and
(a) sodium hydroxide, (b) sodium hydrogen
2 A thermometer is placed in the water as shown in carbonate
Figure 4.7 and the temperature of water is measured.
1 Using a measuring cylinder, 40.0 cm3 of 1.0
3 A spatula of sodium hydroxide solid is added to mol dm–3 hydrochloric acid is measured out and
the water in the plastic cup. poured into a plastic cup.
4 The water is slowly stirred with the thermometer 2 A thermometer is placed in the hydrochloric acid
to dissolve the sodium hydroxide. The highest and the temperature is recorded.
temperature obtained is recorded.
3 Using another measuring cylinder, 30.0 cm3
5 Steps 1 to 4 are repeated using ammonium chloride of 1.0 mol dm–3 sodium hydroxide solution is
solid instead of sodium hydroxide solid. The lowest measured out and poured into the hydrochloric
temperature obtained is recorded. acid.
Results 4 The reaction mixture is stirred slowly with
(a) Dissolving sodium hydroxide solid in water the thermometer and the highest temperature
achieved is recorded.
Reaction Initial Highest
NaOH(s) + water temperature temperature 5 Steps 1 to 4 are repeated using 50.0 cm3 of 1.0
mol dm–3 hydrochloric acid and 25.0 cm3 of 2.0
(°C) (°C) mol dm–3 sodium hydrogen carbonate solution.
The lowest temperature obtained is recorded.
28 36
Thermochemistry 456
Results Discussion
(a) Reaction between hydrochloric acid and sodium 1 When sodium hydroxide solution is added to
hydroxide solution hydrochloric acid, the temperature of the solution
rises. This means that heat energy is released to
Reaction Initial Highest the solution.
temperature temperature
2 When sodium hydrogen carbonate solution is
(°C) (°C) added to hydrochloric acid, the temperature of
the solution falls. This means that heat energy is
HCl(aq) + NaOH(aq) 30 35 absorbed from the solution.
(b) Reaction between hydrochloric acid and sodium Conclusion
hydrogen carbonate
1 The reaction between hydrochloric acid and
Initial Highest sodium hydroxide solution is an exothermic
reaction.
Reaction temperature temperature
2 The reaction between hydrochloric acid and sodium
(°C) (°C) hydrogen carbonate solution is an endothermic
reaction.
HCl(aq) + NaHCO3(aq) 30 27
7 For the general reaction, 4
A+B→C+D
• Heat is the transfer of energy caused by the Heat of reaction (ΔH)
temperature difference between the reacting particles = Total energy content of the products (H1)
and the surroundings. – total energy content of the reactants (H2)
= (HC + HD) – (HA + HB)
• Temperature is the measure of the average kinetic
energy of all the particles involved in the reaction. That is,
The unit of temperature is kelvin (K). ΔH = Hproducts –Hreactants
8 (a) For an exothermic reaction, the value of
• Heat will always flow from the region of high
temperature to the region of low temperature. ΔH is negative. That is,
Energy Level Diagram SPM H2(g) + —12 O2(g) → H2O(l); ΔH = –286 kJ mol–1
’08/P1 (b) This equation shows that when 1.0 mol
of water is produced from hydrogen and
1 Energy is defined as the ability to do work. oxygen, 286 kJ of heat energy are released.
The unit of energy is joule (symbol J).
(c) When 2.0 mol of hydrogen react with 1.0
1 kJ (kilojoule) = 1000 J mol of oxygen to form 2.0 mol of water,
(2 286 kJ) or 572 kJ of heat are released.
2 All substances contain energy. But different That is,
substances have different energy content.
2H2(g) + O2(g) → 2H2O(l); ΔH = –572 kJ
3 The absolute energy content of a substance is
9 (a) In endothermic reactions, the values of
given the symbol H (the unit of H is kJ or J). ΔH are positive. For example,
4 The absolute energy content of a given substance
—21 N2(g) + —12 O2(g) → NO(g); ΔH = +90 kJ mol–1
cannot be determined but the changes in energy
content that occur when the reactants are (b) The equation shows that when 1.0 mol
m ofonl iotrfongeitnroogexnideanids p—21r omduocledoffrooxmyge—n12,
converted to the products can be determined
and is given the symbol, ∆H. 90 kJ of heat are absorbed.
5 When substances react to produce new substances,
changes in energy content occur. This causes heat
to be released or absorbed during the reaction.
6 The changes in the energy content in a
chemical reaction that produces 1 mol of a
product, ∆H, is called the heat of that chemical
reaction. The unit for ∆H is kJ mol–1.
457 Thermochemistry
(c) When 1.0 mol of nitrogen reacts with 1.0 Figure 4.9 Energy level diagram for the reaction
mol of oxygen to form 2.0 mol of nitrogen between magnesium and oxygen
oxide, 180 kJ of heat are absorbed. That is,
4 In the reaction between magnesium and oxygen,
N2(g) + O2(g) → 2NO(g); ΔH = +180 kJ the total energy content of magnesium oxide is
less than the total energy content of magnesium
10 The changes in the energy content of and oxygen. The excess energy is released as heat.
exothermic and endothermic reactions can be Thus, the reaction is an exothermic reaction.
represented by energy level diagrams.
1
11 The energy level diagram is the diagram that
shows the total energy content of the reactants When 1.0 mol of zinc powder reacts with 2.0 mol
compared to the total energy content of the of hydrochloric acid, 120 kJ of heat are released.
products. Sketch the energy level diagram to represent this
reaction.
The Energy Level Diagrams for Exothermic
Reactions Solution
1 Figure 4.8 shows the energy level diagram
for an exothermic reaction. The difference
4 between the total energy content of the
reactants (H1) and the total energy content of
the products (H2) gives the heat evolved (ΔH)
during a reaction, that is, ΔH = H2 – H1.
2 For an exothermic reaction, the total energy
SPM content of the products is lower than the total
’10/P1 energy content of the reactants. Hence, the ΔH
value of an exothermic reaction is negative.
Figure 4.8 The energy level diagram for an The Energy Level Diagrams for Endothermic SPM
exothermic reaction ’09/P1
ΔH = Hproducts – Hreactants Reactions
= negative (if Hproducts < H )reactants 1 Figure 4.10 shows the energy level diagram for
3 When magnesium reacts with oxygen, heat an endothermic reaction.
energy is released.
Figure 4.10 The energy level diagram for an
2Mg(s) + O2(g) → 2MgO(s); ΔH = –1204 kJ endothermic reaction
The energy level diagram for this exothermic
reaction is shown in Figure 4.9.
Thermochemistry 458
2 For an endothermic reaction, the total energy Which of the following are true of this reaction? 4
content of the products is higher than the total I The reaction is exothermic.
energy content of the reactants. This means II The activation energy is x kJ.
that the value of ΔH for an endothermic III The heat of reaction is y kJ.
reaction is positive. IV The value of y increases in the presence of a
ΔH = Hproducts – Hreactants
= positive (if Hproducts > H )reactants catalyst.
A I and II only
2 B II and III only
C I and IV only
State (a) two facts that are given, and (b) two facts D I, II and IV only
that are not given by the energy level diagram in
Figure 4.11. Comments
The energy content of the products is lower than
Figure 4.11 that of the reactants. The reaction is exothermic.
Solution (I is correct)
(a) The energy level diagram in Figure 4.11 shows The activation energy is x kJ (II is correct). So the
value of x decreases in the presence of a catalyst.
that But indirectly, the value of y also decreases.
(i) the total energy content of the reactants is (IV is incorrect)
The heat of reaction (ΔH) is –(y – x) kJ, the
higher than the total energy content of the negative sign before (y – x) indicates an exothermic
products. This implies that the reaction is reaction. (III is incorrect)
exothermic, with negative ΔH value.
(ii) when 1.0 mol of nitrogen reacts with 3.0 Answer A
mol of hydrogen to form 2.0 mol of
ammonia, 92 kJ of heat energy are released. Relationship between Energy Change and
the Formation and Breaking of Bonds
N2(g) + 3H2(g) → 2NH3(g); ΔH = –92 kJ
1 The energy change in a reaction is caused by
(b) The energy level diagram in Figure 4.11 does (a) the formation of chemical bonds and
not give information concerning (b) the breaking of chemical bonds.
(i) the rate of reaction,
(ii) the conditions such as temperature, pressure 2 When a reaction occurs, energy is absorbed to
or the catalyst required to carry out the reaction.
break the bonds that exist between atoms in
1 ’05
the molecules of the reactants. Heat energy is
The energy profile of a reaction is shown below.
then released when new bonds are formed to
SPM
produce the products. This means that
’05/P1 (a) the breaking of chemical bonds is an
endothermic process, and
(b) the formation of chemical bonds is an
exothermic process.
3 Bond energies
(a) Table 4.2 shows the bond energies for
some chemical bonds.
Table 4.2 Bond energy
Covalent bond Bond energy
(kJ mol–1)
C–C
C=C 346
H–H 612
N≡N 436
Cl – Cl 946
242
459 Thermochemistry
4 (b) The bond energy is the energy required ΔH1 (bond breaking) = +678 kJ
to break one mole of covalent bonds. The ΔH2 (bond forming) = –862 kJ
same amount of heat energy is released ΔH (heat of reaction) = (+678) + (–862)
when 1.0 mol of the same covalent bonds
is formed. = –184 kJ
(c) The energy level diagram for the reaction
(c) Table 4.2 shows that the bond energy of
chlorine is +242 kJ mol–1. This implies between hydrogen and chlorine to form
that 242 kJ of heat energy are required to hydrogen chloride is shown in Figure
break the covalent bonds in one mole of 4.13.
chlorine molecules.
Figure 4.13
Cl2(g) → 2Cl(g); ΔH = +242 kJ mol–1 6 (a) If the energy absorbed to break the
Conversely, 242 kJ of heat energy are bonds is more than the energy released
released when 1.0 mol of chlorine gas is to form new bonds, then the reaction is
formed from chlorine atoms. endothermic. That is,
2Cl(g) → Cl2(g); ΔH = –242 kJ mol–1 ΔH (reaction) is positive if
ΔH1 (bond breaking) > ΔH2 (bond forming)
4 The heat of reaction is the sum of the energy
absorbed in breaking the bonds and the energy For example, the reaction between carbon
released in forming the bonds. and steam to form carbon monoxide and
hydrogen is an endothermic reaction.
ΔH (reaction) = ΔH1 (bond breaking) +
ΔH2 (bond forming) C(s) + H2O(g) → CO(g) + H2(g);
ΔH = positive
5 (a) If the energy absorbed to break the covalent
bonds is less than the energy released to (b) Figure 4.14 shows the breaking and the
form the new covalent bonds, the reaction formation of bonds when carbon reacts
is exothermic. That is, with steam.
ΔH1 (bond breaking) = +1645 kJ
ΔH (reaction) is negative if ΔH2 (bond forming) = –1513 kJ
ΔH1 (bond breaking) < ΔH2 (bond forming) ΔH (reaction) = ΔH1 + ΔH2
= (+1645) + (–1513)
For example, the reaction between hydrogen = +132 kJ
and chlorine to form hydrogen chloride is
an exothermic reaction.
H2(g) + Cl2(g) → 2HCl(g); ΔH = negative
(b) Figure 4.12 shows the breaking and
formation of bonds when hydrogen reacts
with chlorine.
Figure 4.12 The breaking and formation of bonds in the Figure 4.14 The breaking and formation of bonds in
reaction between hydrogen and chlorine the reaction between carbon and steam
Thermochemistry 460
2 ’07 (b) The reaction is very endothermic and 4
the water temperature can drop as much
The equation for the reaction between hydrogen as 18 oC, depending on the amount of
and iodine is shown below. ammonium nitrate used.
H2(g) + I2(s) → 2HI(g); ΔH = +11.3 kJ NH4NO3(s) + water → NH4NO3(aq);
Which of the following shows that the reaction is ΔH = +26 kJ mol–1
endothermic?
4 (a) There are several varieties of hot packs.
A The reaction releases 11.3 kJ of heat energy One variety of hot pack contains a small
when 2 mol of hydrogen iodide are formed. bag of water and a dry chemical such as
anhydrous calcium chloride or anhydrous
B The total heat absorbed to break the bonds is magnesium sulphate. When the pack is
more than the total energy released during the squeezed, the small bag breaks and the
formation of hydrogen iodide. anhydrous salt dissolves in water. The
dissolving process is very exothermic.
C The energy contained in hydrogen and iodine
is higher than the energy contained in hydrogen CaCl2(s) + water → Ca2+(aq) + 2Cl–(aq);
iodide. ΔH = –81 kJ mol–1
D The reaction is a slow reaction. MgSO4(s) + water → Mg2+(aq) + SO42–(aq);
Comments ΔH = –91 kJ mol–1
This is an endothermic reaction. The reaction absorbs
heat energy. In an endothermic reaction, the heat (b) Another variety of hot pack uses the
content of the products is higher than the heat oxidation of iron to produce heat. The
content of the reactants. hot pack contains wet iron powder and
Answer B sodium chloride put in a perforated bag.
When the perforated bag is taken out
Applications of Exothermic and and exposed to the air, a very exothermic
Endothermic Reactions in Everyday Life reaction occurs as iron reacts with oxygen.
1 Ice packs and hot packs are used to reduce 4Fe(s) + 3O2(g) → 2Fe2O3(s); ΔH = –1648 kJ
swelling and pain due to muscle or joint
sprain. In the hospitals, cold packs are put on The presence of water on the iron surface
the foreheads of patients to reduce fever. and sodium chloride increases the rate of
reaction.
2 In actual fact, an ice pack does not contain
SPM ice but contains chemicals that can react 3 ’06
’11/P1 endothermally with water. Thus, ice packs are
The following chemical equation shows the redox
often called cold packs. reaction between zinc and copper(II) oxide:
3 (a) Cold packs contain ammonium nitrate
Zn(s) + CuO(s) → ZnO(s) + Cu(s);
in a strong bag and water in a thin inner ΔH = –193 kJ mol–1
bag. When the cold pack is squeezed, the
inner bag containing water will break. The Which of the following is true about this reaction?
water then reacts with ammonium nitrate
(Figure 4.15). Type of reaction Heat change
A Endothermic Heat is released
Figure 4.15 The structure of a cold pack B Endothermic Heat is absorbed
C Exothermic Heat is absorbed
D Exothermic Heat is released
Comments
ΔH has a negative value. So the reaction is exothermic.
Answer D
461 Thermochemistry
Exothermic reactions
• Exothermic reactions are reactions that release heat energy to the surroundings.
• Energy level diagram • Energy profile diagram
• ΔH is negative because ΔH absorbed for bond breaking < ΔH released for bond forming.
• Hot packs contain chemicals that react with water to give out heat.
4 Energy changes in Application
chemical reactions • Cold pack and hot pack
Endothermic reactions
• Endothermic reactions are reactions that absorb heat energy from the surroundings.
• Energy level diagram • Energy profile diagram
• ΔH is positive because ΔH for bond breaking > ΔH released for bond forming.
• Hot packs contain chemicals that react with water to absorb heat.
4.1 Na2CO3(s) ⎯⎯wa⎯te⎯r → NΔaH2C=O–3(2a3q)k;J mol–1
1 Classify the following as exothermic or endothermic (a) Describe the changes in temperature of the
reactions: solution.
(a) Dissolving ammonium sulphate solid in water
(b) Dissolving sodium hydroxide in water (b) Construct an energy level diagram for the reaction.
(c) Adding sodium hydrogen carbonate to dilute
hydrochloric acid 3 When a hot pack is squeezed, a chemical reaction
(d) Adding sodium hydroxide solution to dilute occurs.
hydrochloric acid (a) Is the reaction exothermic or endothermic?
(b) Complete the table shown below describing the
2 When sodium carbonate solid dissolves in water, the characteristic of this reaction.
following reaction occurs:
Thermochemistry 462
Characteristic Reaction in the hot pack (a) Identify the step in the reaction that is
exothermic.
(i) Temperature change
(b) Identify the step in the reaction that is
(ii) Energy content endothermic.
of reactants and
products (c) State two facts that are given in the above energy
level diagram.
(iii) Energy involved in
bond breaking and 5 (a) Using the following data, calculate the mass of
bond forming anhydrous copper(II) sulphate needed to dissolve
in water so that 16.5 kJ of heat are released.
4 The energy changes for the reaction between nitrogen
and oxygen are represented in the energy level CuSO4(s) + water → CuSO4(ΔaHq)=; –66 kJ mol–1
diagram as shown below.
[Relative atomic mass: Cu, 64; S, 32; O, 16]
(b) The following equation shows the reaction between
nitrogen and oxygen.
N2(g) + 2O2(g) → 2NO2(g); ΔH = +66 kJ mol–1
Calculate the energy change when 12.0 dm3 of
nitrogen reacts with 24.0 dm3 of oxygen.
4
4.2 Heat of Precipitation N2(g) + 3H2(g) 2NH3(g);
The Concept of Heat of Reaction ΔH = –92 kJ …(2)
1 (a) The heat of reaction is the heat energy Equation (1) shows that 46 kJ of heat
SPM absorbed or released when the number of
’06/P1 moles of reactants, as shown in the chemical energy are released when 1.0 mol of
equation, react to form the products. For ammonia is formed from —21 mol of nitrogen
example, and —23 mol of hydrogen.
Hence, the heat of reaction for the
formation of ammonia is –46 kJ mol–1.
Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g); (b) Equation (2) shows that when 2.0 mol of
ΔH = –27 kJ
ammonia are produced from 1.0 mol of
nitrogen and 3.0 mol of hydrogen, 92 kJ
Hence, the heat of reaction (ΔH) is –27 kJ. of heat energy are released.
(b) The chemical equation that contains the
3 (a) The heat of reaction also depends on the
value of ΔH on the right of the equation
is called the thermochemical equation. direction of the reaction. For example,
(c) The thermochemical equation in (a) shows
that 27 kJ of heat are released, when 1.0 —12 N2(g) + —32 H2(g) NH3(g);
mol of iron(III) oxide reacts with 3.0 mol
of carbon monoxide to form 2.0 mol of ΔH = –46 kJ mol–1 … (3)
iron and 3.0 mol of carbon dioxide.
2 (a) Theheatofreactiondependsonthenumber NH3(g) —21 N2(g) + —23 H2(g);
of moles of reactants that are involved as ΔH = +46 kJ … (4)
shown by the equation. For example,
(b) If the formation of 1.0 mol of ammonia
—12 N2(g) + —32 H2(g) NH3(g); from nitrogen and hydrogen (equation
(3)) releases 46 kJ of heat energy, then the
ΔH = –46 kJ mol–1…(1) decomposition of 1.0 mol of ammonia
But, into nitrogen and hydrogen (equation
(4)) will absorb 46 kJ of heat energy.
463 Thermochemistry
4 ’06 For an exothermic reaction,
θ = rise in temperature
What is meant by heat of reaction? θ = T2 – T1
A The energy needed to start a reaction. For an endothermic reaction,
B The energy released on forming chemical bonds. θ = fall in temperature
C The change in the energy contained in the θ = T1 – T2
reactants and in the products. (T1 = initial temperature of solution, T2 = final
D The energy absorbed or released when matter temperature of solution)
changes its physical state. 4 In calculating heat of reaction, 1 cm3 of any
aqueous solution is assumed to have a density
Solution of 1.0 g cm–3. That is, 1 cm3 of any aqueous
Heat of reaction (ΔH) = Hproducts – Hreactants solution has a mass of 1.0 g.
where H is total heat content. 5 The other assumptions for calculating heat of
reaction are as follows:
Answer C (a) There is no loss of heat energy to the
4 The heat of reaction can be further classified surroundings or gain of heat energy from
as shown below, depending on the type of the surroundings.
reaction that occurs. (b) The container, the thermometer and all
other apparatus used in the experiment
4 Heat of reaction Type of reaction absorb a negligible amount of heat.
(a) Heat of Precipitation of an ionic 3
precipitation compound
When a spatula of ammonium chloride is dissolved
(b) Heat of Displacement of a metal in 300 cm3 of water at an initial temperature of
displacement from its salt solution by 28 °C, the minimum temperature obtained is 23 °C.
another metal Calculate the heat change for the reaction.
(c) Heat of (The specific heat capacity of solution = 4.2 J g–1 °C–1).
neutralisation Neutralisation reaction
between an acid and a base Solution
(d) Heat of Fall in temperature = 28 – 23 = 5 °C
combustion The reaction between an Heat energy absorbed = mcθ = 300 4.2 5
element or a compound with = 6300 J = 6.3 kJ
oxygen on burning Heat change = +6.3 kJ
Calculations Involving Heat Changes SPM 5 ’06
’04/P1 When 25.0 cm3 of dilute sulphuric acid is poured
’05/P1 into 25.0 cm3 of sodium hydroxide solution,
’06/P1 4200 J of heat are released.
What is the temperature change of the reaction
1 The unit for measuring heat energy is joule (J). mixture?
2 The specific heat capacity of a solution is the [Specific heat capacity of the solution = 4.2 J g–1 °C–1;
assume that the density of the solution = 1 g cm–3]
heat energy required to raise the temperature
of 1.0 g of the solution by 1.0 °C. A 2 °C B 5 °C C 10 °C D 20 °C
The specific heat capacity of water is Solution
4.18 J g–1 °C–1. ΔH = mcθ
4200 = (25 + 25) 3 4.2 3 θ
This means that 4.18 J of heat energy are θ = 20 °C
required to raise the temperature of 1.0 g of Answer D
water by 1.0 °C.
3 The amount of heat (in J) released or absorbed
in a reaction can be determined by using the
following formula:
ΔH = mcθ
where m = mass of solution,
c = specific heat capacity of solution,
θ = change in temperature.
Thermochemistry 464
When 10 g of ammonium chloride is dissolved in 250 Solution
cm3 of distilled water, the temperature of the solution Step 1: Calculate the heat energy released in the
falls by 3 °C.
Heat change = 250 4.2 3 J reaction
The heat absorbed on dissolving ammonium chloride (a) The average temperature of the solution
lowers the temperature of water. The mass of ammonium
chloride used is not included in the calculation. before reaction
The Concept of Heat of Precipitation SPM = —3—0—.5——+2—2—9—.—5
’04/P1 = 30.0 °C
’06/P1
’07/P1 Temperature change
1 The heat of precipitation is the heat change = 35.6 – 30.0
when 1.0 mol of a precipitate is formed from = 5.6 °C
its ions. (b) Volume of reaction mixture
2 The heat of precipitation of lead(II) sulphate = 25 + 25
is –51 kJ mol–1. Based on this information, we = 50 cm3
can write the thermochemical equation for the Assuming that the density of solution
= 1.0 g cm–3
reaction as follows.
Heat energy released = mcθ
Pb2+(aq) + SO42–(aq) → PbSO4(s); = 50 4.2 5.6 4
ΔH = –51 kJ mol–1 = 1176 J
= 1.176 kJ
3 The energy level diagram for the precipitation
reaction is shown in Figure 4.16. Step 2: Calculate the number of moles of AgCl
produced
Ag+(aq) + Cl–(aq) → AgCl(s)
0.023 mol of Ag+ ions react with 0.023 mol
of Cl– ions to produce 0.023 mol of AgCl.
Step 3: Calculate the heat of precipitation of AgCl
The precipitation of 0.023 mol of AgCl
Figure 4.16 The energy level diagram for the releases 1.176 kJ of heat energy.
precipitation of lead(II) sulphate
Therefore, the precipitation of 1.0 mol of
4
AgCl releases
25 cm3 of silver nitrate solution is added to 25 cm3
of potassium chloride solution. The results of the 1.176 —0—.01—2—3 0.023 mol → 1.176 kJ
experiment are shown below. 1.0 mol → ? kJ
Initial temperature of potassium chloride solution
= 30.5 °C = 51.1 kJ of heat energy.
Initial temperature of silver nitrate solution = 29.5 °C
Highest temperature of reaction mixture = 35.6 °C Hence, heat of precipitation = –51.1 kJ mol–1
Calculate the heat of precipitation of silver chloride.
(Specific heat capacity of solution = 4.2 J g–1 °C–1; Determining the Heat of Precipitation SPM
the solutions contain 0.023 mol of Ag+ ions and
0.023 mol of Cl– ions respectively) ’04/P2
’07/P1
1 The heat of precipitation for the ionic solid
Mn+Xn– is determined by measuring the
temperature change during precipitation when
a given volume of a solution of Mn+ ions is
added to a given volume of a solution of Xn–
ions. The concentrations of both the solutions
are known.
465 Thermochemistry
2 The following steps are involved when calculating the heat of precipitation.
Step 1 Step 2 Step 3
Calculate the number of Calculate the heat energy Heat of precipitation
moles of solid precipitated released = —–—xy kJ mol–1
(x mol) ΔH = —1m—0—c0—θ0 = y kJ
3 Activity 4.2 shows the experimental procedure for measuring the heat of precipitation of
silver chloride.
Precipitation, like crystallisation, is the reverse of dissolving. Crystallisation and precipitation are exothermic reactions.
If a solid comes out of a solution slowly, crystals are In one type of hot pack, the crystallisation of sodium
formed and the process is called crystallisation. But if ethanoate is used to produce heat.
the solid is formed very quickly, many tiny particles are
produced in the liquid. This process is called precipitation. CH3COONa + 3H2O → CH3COONΔaH.3=H2–O3;7.9 kJ mol–1
Activity 4.2 4 To determine the heat of precipitation of silver chloride
Apparatus Measuring cylinders, thermometer and 5 The mixture is stirred with a thermometer
Materials plastic cup. throughout the experiment and the highest
Procedure temperature obtained is recorded.
0.5 mol dm–3 silver nitrate solution and
0.5 mol dm–3 sodium chloride solution. Temperature NaCl(aq)
32.0
Highest temperature obtained (°C) 29.0
Initial temperature of sodium 28.0
chloride solution (°C)
Initial temperature of silver nitrate
solution (°C)
Figure 4.17 To determine the heat of precipitation Assumptions used for calculations
of silver chloride The specific heat capacity of solution = 4.2 J g–1 °C–1
Density of solution = 1.0 g cm–3
1 25 cm3 of 0.5 mol dm–3 sodium chloride solution
is measured and poured into a clean and dry Calculations
plastic cup using a measuring cylinder.
Step 1: Calculate the heat energy released in the
2 The initial temperature of sodium chloride
solution is measured and recorded. experiment:
3 Using another measuring cylinder, 25 cm3 of 0.5 (a) The average initial temperature of the
mol dm–3 silver nitrate solution is measured. The
initial temperature of the silver nitrate solution is solutions before mixing
measured and recorded.
= —2—9——+2——2—8 = 28.5 °C
4 The silver nitrate solution is poured quickly
and carefully into the sodium chloride solution The highest temperature of the reaction
(Figure 4.17).
mixture = 32.0 °C
Rise in temperature = (32.0 – 28.5) °C
= 3.5 °C
Thermochemistry 466
(b) Heat energy released Ag+(aq) + Cl–(aq) → AgCl(s); ΔH = –58.8 kJ mol–1 4
= mass of solution specific heat
4 In actual fact, the theoretical heat of precipitation
capacity rise in temperature of silver chloride is –66 kJ mol–1.
= (25 + 25) 4.2 3.5
= 735 J = 0.735 kJ Ag+(aq) + Cl–(aq) → AgCl(s); ΔH = –66 kJ mol–1
Step 2: Calculate the number of moles of AgCl
formed 5 The heat of precipitation obtained experimentally
(a) Number of moles of Ag+ ions used is usually less than the theoretical value. This is
= 0.5 —1—20—50—0 = 0.0125 because the calculation of the theoretical value is
(b) Number of moles of Cl– ions used based on two assumptions, namely,
= 0.5 —1—20—50—0 = 0.0125 • there is no loss of heat to the surroundings,
• the plastic cup and the thermometer used in the
Ag+(aq) + Cl–(aq) → AgCl(s) experiment do not absorb heat.
From the equation, 1.0 mol of Ag+ ions
react with 1.0 mol of Cl– ions to form 6 In order to prevent the loss of heat to the surroundings,
1.0 mol of AgCl. the following precautionary steps must be taken
Therefore, 0.0125 mol of Ag+ ions react when carrying out the experiment.
with 0.0125 mol of Cl– ions to form (a) Plastic cups (for example, polystyrene or
0.0125 mol of AgCl. polythene cup) are used because plastics are
Step 3: Calculate the heat of precipitation of silver poor conductors of heat.
chloride (b) The thermometer must be placed in the
From steps 1 and 2: solution for a few minutes before taking the
Precipitation of 0.0125 mol of AgCl reading. This is to ensure that the solution
releases 0.735 kJ of heat energy. has reached a uniform temperature.
Precipitation of 1.0 mol of AgCl will release (c) The solutions containing the reactants are
0.735 —0—.0—11—2—5 = 58.8 kJ of heat energy mixed quickly, so that the reaction can be
completed within a short time.
Discussion (d) The reaction mixture in the plastic cup must
be stirred slowly and continuously so that the
1 The reaction between silver nitrate and sodium temperature of the solution is uniform.
chloride solutions produces a white precipitate of (e) The thermometer should not be taken out of
silver chloride. the reaction mixture when taking the reading.
(f) The thermometer reading should be observed
2 The reaction between silver nitrate and sodium throughout the reaction so that the highest
chloride can be represented by a chemical temperature of the reaction mixture can be
equation or an ionic equation. recorded accurately.
(a) Chemical equation:
7 The energy level diagram for the precipitation of
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) silver chloride is shown in Figure 4.18.
(b) Ionic equation: Figure 4.18 Energy level diagram for the
precipitation of silver chloride
Ag+(aq) + Cl–(aq) → AgCl(s)
Conclusion
The ionic equation does not contain nitrate ions The heat of precipitation of silver chloride is
(NO3–) and sodium ions (Na+). This is because –58.8 kJ mol–1.
NO3– and Na+ ions are spectator ions and do not
participate in the precipitation reaction.
3 From the experimental results, the heat of precipitation
of AgCl is –58.8 kJ mol–1. The thermochemical
equation for the precipitation of silver chloride is
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq);
ΔH = –58.8 kJ mol–1
or
467 Thermochemistry
If potassium chloride solution is used to replace the sodium chloride in Activity 4.2, the heat of precipitation of silver
chloride will still be the same.
AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq); ΔH = –58.8 kJ mol–1
This is because K+ ions, like Na+ ions, do not take part in the reaction.
Calculations Involving the Heat of Precipitation
4 5 Step 2: Calculate the heat energy released when
0.06 mol of PbSO4 is precipitated
Consider the following equation:
The thermochemical equation shows that the
Pb2+(aq) + SO42–(aq) → PbSO4(s); heat of precipitation of PbSO4 is –50 kJ mol–1.
ΔH = –50 kJ mol–1 This means that the precipitation of 1.0
mol of PbSO4 releases 50 kJ of heat
Calculate the highest temperature reached when energy. Therefore, heat energy released
60 cm3 of 1.0 mol dm–3 lead(II) nitrate solution by the formation of 0.06 mol of PbSO4
reacts with 60 cm3 of 2.0 mol dm–3 sodium sulphate = 0.06 50 = 3.0 kJ
solution. The initial temperatures of both the
solutions are 29 °C. Step 3: Calculate the rise in temperature
[Specific heat capacity of solution = 4.2 J g–1 °C–1;
density of solution = 1 g cm–3] Let the rise in temperature = θ
Heat energy released (in J)
Solution = mcθ
Step 1: Calculate the number of moles of lead(II) = (60 + 60) 4.2 θ
sulphate precipitated in this experiment = 504 θ
Number of moles of Pb2+ ions used From step 2, heat energy released
= —1—.01—0—0—0—6—0 = 0.06 = 3.0 kJ = 3 1000 J
Number of moles of SO42– ions used 3 1000 = 504 θ
= —2—.0—1—0—0—06—0
θ = —3———5—01—40—00
= 0.12 = 6 °C
Pb2+(aq) + SO42–(aq) → PbSO4(s) θ = final temperature – initial temperature
From the equation, 1.0 mol of Pb2+ ions 6 = final temperature – 29
reacts with 1.0 mol of SO42– ions to form Final temperature = 29 + 6 = 35 °C
1.0 mol of PbSO4. Highest temperature reached is 35 °C.
0.06 mol of Pb2+ ions react with 0.06 mol of
SO42– ions to form 0.06 mol of PbSO4.
Thermochemistry 468
The meaning of heat of Determining the heat of precipitation
precipitation • Mix a known volume of solution of Mn+ with a known volume of
• Heat of precipitation is the
solution of Xn–. The concentrations of the two solutions are known.
heat change when 1.0 mol • Measure the highest temperature reached.
of the precipitate is formed
from its ions. Constructing the energy level diagram
Example:
Calculations involving heat of precipitation
Ag+(aq) + Cl–(aq) → AgCl(s); (a) Calculate the number of moles of salt precipitated = x mol
ΔH = –66 kJ mol–1 (b) Ca lculate the heat energy released (ΔH) = —1—m——0———c0——θ—0— kJ = y kJ
(c) From (a) and (b):
That is, heat of precipitation
of silver chloride is –66 kJ Precipitation of x mole of salt releases y kJ of heat energy.
mol–1. (d) Fro m (c): Heat of precipitation = – ——y—x kJ mol–1
4
4.2 (c) Calculate the heat of precipitation of the metal
carbonate.
1 The heat of precipitation of silver chloride is –66 kJ mol–1.
In an experiment, 200 cm3 of 0.1 mol dm–3 AgNO3(aq) (d) Sketch the energy level diagram for the
is added to 100 cm3 of 0.1 mol dm–3 CaCl2(aq). reaction.
(a) Write (i) the chemical equation, (ii) the ionic
equation, for the reaction between silver nitrate 3 When 100 cm3 of 0.5 mol dm–3 metal nitrate solution,
and calcium chloride. M(NO3)2, reacts with 100 cm3 of 1.0 mol dm–3
potassium chloride solution, the temperature of the
(b) Calculate the heat change in the experiment. reaction mixture increases by 10 °C.
2 When 100 cm3 of 0.5 mol dm–3 of a nitrate salt, M(NO3)2(aq) + 2KCl(aq) → MCl2(s) + 2KNO3(aq)
M(NO3)2 at 29.5 °C is added to 100 cm3 of 1.0
mol dm–3 sodium carbonate solution, the maximum What is the effect (if any) on the temperature change
temperature reached is 30.5 °C. [Specific heat if the experiment is repeated using
capacity of solution = 4.2 J g–1 °C–1; density of (a) 200 cm3 of 0.5 mol dm–3 M(NO3)2 solution and
solution = 1.0 g cm–3]
(a) Write the ionic equation for the reaction. 200 cm3 of 1.0 mol dm–3 KCl solution?
(b) Calculate the heat energy released in the (b) 100 cm3 of 1.0 mol dm–3 M(NO3)2 solution and
experiment.
100 cm3 of 2.0 mol dm–3 KCl solution?
4.3 Heat of Displacement SPM SPM Mg(s) + FeCl2(aq) → MgCl2(aq) + Fe(s);
ΔH = –202 kJ mol–1
’08/P1, ’11/P1
’10/P1
1 The heat of displacement is the heat released (b) When excess magnesium powder is added
when 1.0 mol of a metal is displaced from its to iron(II) chloride solution, 202 kJ of
salt solution by a more electropositive metal. heat are released when 1.0 mol of iron is
displaced from iron(II) chloride solution.
2 (a) The following thermochemical equation
shows that the heat of displacement of (c) In this reaction, the colour of the solution
iron by magnesium is –202 kJ mol–1. turns from green to colourless and iron
powder (grey in colour) is produced.
469 Thermochemistry
3 Figure 4.19 shows the energy level diagram for Determining the Heat of Displacement
the displacement reaction between magnesium
and iron(II) chloride. 1 The heat of displacement is determined by
adding excess metal powder to a given volume
Figure 4.19 The energy level diagram for the of the salt of another metal and measuring
displacement reaction of iron by the highest temperature reached. The metal
magnesium chosen must be more electropositive than the
metal to be displaced and the concentration
of the salt solution is known.
2 Activity 4.3 shows the method used to
determine the heat of displacement of copper
by zinc and iron from copper(II) sulphate
solution.
To determine the heat of displacement of copper by SPM
(a) zinc, (b) iron ’05/P2
/SA
Activity 4.3 4 Apparatus Thermometer, plastic cup, measuring Results
Materials cylinder, weighing bottle and electronic
Procedure balance. Type of metal Zinc powder Iron powder
0.2 mol dm–3 copper(II) sulphate Highest temperature 41 37.5
solution, zinc powder and iron obtained (°C)
powder.
Initial temperature of
copper(II) sulphate 31 30.5
solution (°C)
Increase in 10 7.0
temperature (°C)
Figure 4.20 To determine the heat of Calculations
displacement
Assumptions
1 Using a measuring cylinder, 25.0 cm3 of 0.2 mol Specific heat capacity of solution = 4.2 J g–1 oC–1
dm–3 copper(II) sulphate solution is measured Density of solution = 1.0 g cm–3
into a plastic cup. The temperature of the solution (A) Heat of displacement of copper by zinc
is measured with a thermometer. Step 1: To determine the heat energy released in
2 About 0.5 g of zinc powder (in excess) is this experiment
weighed out. Heat energy released
= mass of solution specific heat capacity
3 The zinc powder is poured into copper(II) of solution increase in temperature
sulphate solution in the plastic cup. The = 25 4.2 10 J
contents of the plastic cup is then stirred with = 1050 J
a thermometer (Figure 4.20) and the highest = 1.05 kJ
temperature reached is recorded. Step 2: To calculate the number of moles of copper
displaced by zinc
4 Steps 1 to 3 are repeated using iron powder to Number of moles of Cu2+ ions used
replace zinc powder. = —0—.2—1—0—0—02—5
= 0.005
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Thermochemistry 470
From the equation, 1.0 mol of Cu2+ ions 2 The copper metal displaced from the salt will be 4
produces 1.0 mol of copper metal. precipitated as a brown solid.
Therefore, 0.005 mol of Cu2+ ions produce Chemical equation:
0.005 mol of copper metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Step 3: To determine the heat energy released when
1 mol of copper is displaced Ionic equation:
From steps 1 and 2:
Displacement of 0.005 mol of copper metal Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
releases 1.05 kJ of heat.
Therefore, heat energy released when 1.0 3 Chemical equation for the displacement of
mol of copper is displaced copper by iron:
= 1.05 —0—.—01—0—5 Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
= 210 kJ
Ionic equation:
(B) Heat of displacement of copper by iron
Step 1: To determine the heat energy released in Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s)
this experiment 4 Excess zinc and iron are used to make sure that all
Heat energy released the copper from the copper(II) salt are displaced.
= mass of solution specific heat capacity
of solution increase in temperature 5 When calculating the heat of displacement of
= 25 4.2 7 J copper, the heat absorbed by the solids (copper
= 735 J = 0.735 kJ produced, excess zinc and iron remaining in the
solution) is very small and can be ignored.
Step 2: To calculate the number of moles of copper
displaced by iron 6 The following precautionary steps must be taken
Number of moles of Cu2+ ions used when carrying out the experiment:
(a) The initial temperature of copper(II) sulphate
= —0—.2—1—0—0—02—5 solution is taken only after the thermometer
= 0.005 has been placed in the solution for a few
minutes. This is to make sure that the solution
Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s) has reached a uniform temperature.
(b) The reactants must be mixed quickly so that
From the equation, 1.0 mol of Cu2+ ions the reaction can be completed in the shortest
produces 1.0 mol of copper metal. time. In this way, the loss of heat energy to
Therefore, 0.005 mol of Cu2+ ions produce the surroundings can be minimised.
0.005 mol of copper metal. (c) The reaction mixture in the plastic cup must
be stirred slowly and continuously so that the
Step 3: To determine the heat energy released when temperature of the solution is uniform.
1 mol of copper is displaced (d) The thermometer reading must be observed
From steps 1 and 2: continuously so that the highest temperature
Displacement of 0.005 mol of copper metal reached can be recorded. This is to ensure
releases 0.735 kJ of heat. that the reaction has completed and the heat
Therefore, heat energy released when 1.0 of reaction has all been released.
mol of copper is displaced (e) The zinc and iron metals used are in
powdered form to ensure that the reaction
= 0.735 —0—.—01—0—5 proceeds rapidly and completely. In this way,
= 147 kJ the loss of heat to the surroundings can be
reduced. Metal powder has a larger surface
Discussion area compared with pieces of metals with the
same mass.
1 Zinc and iron are more electropositive than copper.
This means that zinc and iron will displace copper 7 Figure 4.21 shows the energy level diagram
from an aqueous solution of copper salt. The for the displacement reaction between zinc and
displacement reaction is an exothermic reaction. copper(II) ions.
471 Thermochemistry
8 Figure 4.22 shows the energy level diagram for
the displacement of copper by iron.
Figure 4.21
Figure 4.22
Conclusion
(a) The heat of displacement of copper by zinc from aqueous solution of Cu2+ ions is –210 kJ mol–1.
(b) The heat of displacement of copper by iron from aqueous solution of Cu2+ ions is –147 kJ mol–1.
Calculations Involving Heat of Displacement
4 Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) … (1) 6 SPM
’07/P1
Zn(s) + Fe2+(aq) → Zn2+(aq) + Fe(s) … (2)
When excess aluminium powder is added to 100
In reaction 1, 1 mol of iron is displaced by 1 mol of cm3 of iron(II) sulphate solution, 0.48 g of iron
magnesium, but in reaction 2, 1 mol of iron is displaced is produced and the temperature of the solution
by 1 mol of zinc. Because different reactants are used, changes from 28 °C to 33.5 °C. Calculate the heat
the heat of displacement will not be the same for of displacement of iron by aluminium.
these two reactions.
3Fe2+(aq) + 2Al(s) → 3Fe(s) + 2Al3+(aq)
6 ’05 [Specific heat capacity of solution = 4.2 J g–1 °C–1;
density of solution = 1.0 g cm–3; relative atomic mass
The following equations show the values of heat of of iron = 56].
displacement of copper by zinc, iron and metal M.
Zn(s) + CuSO4(aq) → ZnSO4(a∆qH) +=C–u2(1s0);kJ mol–1 Solution
Fe(s) + CuSO4(aq) → FeSO4(aq∆)H+=C–u1(5s)0; kJ mol–1 Step 1: Calculate the heat energy released in this
experiment
M(s) + CuSO4(aq) → MSO4(aq∆) H+ Cu(s); (a) Rise in temperature = 33.5 – 28
= –100 = 5.5 °C
kJ mol–1 (b) Heat energy released
= mass of solution specific heat
Predict metal M by choosing from the list: capacity rise in temperature
aluminium, magnesium and tin. = 100 4.2 5.5 J
= 2310 J
Comments = 2.31 kJ
Iron is less electropositive than zinc. The heat of
displacement of copper by iron is less than the Step 2: Calculate the number of moles of iron
heat of displacement of copper by zinc. The heat displaced
of displacement of copper by metal M is less than Relative atomic mass of iron = 56
the heat of displacement of copper by iron. Hence,
metal M is less electropositive than iron. Number of moles of Fe produced
Solution = —r—e—la—t—iv—e—m—a—tao—sm—s—ic——m—a—s—s
Both Al and Mg are more electropositive than iron.
Hence, metal M is tin, which is less electropositive = —0—5.—46—8
than iron.
= 0.00857
Thermochemistry 472
Step 3: Calculate the heat of displacement of iron III The heat change if 2.56 g of copper metal is 4
by aluminium displaced is +8.4 kJ.
Displacement of 0.00857 mol of Fe releases
2.31 kJ of heat energy IV The temperature of the reaction mixture
Heat released from the displacement of 1.0 increases during the reaction.
mol of Fe
A I and III only C I, II and IV only
= 2.31 —0—.—0—01—8—5—7 = 269.5 kJ B II and IV only D I, III and IV only
Heat of displacement of iron by aluminium is
–269.5 kJ mol–1. Comments
Zn is oxidised. Oxidation number of zinc increases
7 ’03 from 0 in Zn to +2 in ZnSO4.
The negative value of ΔH shows that the reaction
The equation below shows the displacement reaction is exothermic. This means that the temperature
of copper metal and its heat of reaction. increases during the reaction.
The thermochemical equation shows that 210 kJ of
Zn + CuSO4 → ZnSO4 + Cu; ΔH = –210 kJ mol–1 heat are released when 1 mol (64 g) of copper is
displaced.
[Relative atomic mass of copper is 64]
N umber of moles of Cu displaced = —2—6.5—46
Which of the following statements are true about = 0.04
the reaction represented by the above equation?
I Zinc is oxidised. Heat released = 210 0.04
II The reaction is exothermic. = 8.4 kJ
That is, heat change (ΔH) is –8.4 kJ (not + 8.4 kJ).
Answer C
Heat of displacement
• Heat of displacement is the heat released when 1.0 mol of a metal is displaced from its salt solution by
another more electropositive metal. For example,
Mg(s) + Fe2+(aq) → Fe(s) + Mg2+(aq); ∆H = –202 kJ mol–1
ΔH is the heat of displacement of iron by magnesium.
Method for finding heat of Energy level diagram Calculations
displacement (a) Calculate the number of moles of
• Add excess of a more electro
metal displaced
positive metal (in powd ered = x mol
form) to a known volume of a (b) Calculate the heat energy evolved
salt solution of another metal of
known concentration. ΔH = —1m—0c—0θ0— = y kJ
• Measure the highest tempe
rature reached. (c) From (a) and (b):
Displacement of x mol of metal
produces y kJ of heat.
(d) H= e–at—oyx—f displacement
kJ mol–1
473 Thermochemistry
4.3
1 You are asked to carry out an experiment to determine If the initial temperature of the solution is 30 °C,
the heat of displacement of lead by magnesium. what is the maximum temperature reached in this
( a) (i) Name the chemicals needed for the experiment?
experiment. [Specific heat capacity of solution = 4.2 J g–1 °C–1;
(ii) Which of the chemicals must be used in relative atomic mass of Fe = 56]
excess? Why?
(b) (i) Draw a labelled diagram to show the 4 Five experiments were carried out to determine the
apparatus set-up for the experiment. heat of displacement of iron by magnesium. In each
(ii) What other apparatus are needed besides experiment, an increasing amount of magnesium
the ones shown in your diagram? was added to 50 cm3 of 0.25 mol dm–3 iron(II)
(c) Based on your answer in (b), select the data that chloride. The maximum increase in temperature of
is (i) required, (ii) not required for calculating the solution were recorded.
the heat of displacement.
Experiment Mass of Maximum increase in
2 The equation below shows a displacement reaction magnesium (g) temperature (°C)
and its heat of reaction. A 0.10 4
’08
Zn + CuSO4 → ZnSO4 + Cu; B 0.15 6
ΔH = –210 kJ mol–1
C 0.2 8
4 What is the change in heat energy if 2.56 g of copper D 0.3 12
is displaced?
[Relative atomic mass of copper: 64] E 0.35 12
3 An experiment is carried out by adding 0.7 g of iron [Specific heat capacity of solution = 4.2 J g –1 °C–1;
powder to 50 cm3 of 0.50 mol dm–3 copper(II) chloride density of solution = 1.0 g cm–3]
(a) Why was the rise in temperature increasing in
’03 solution. The energy level diagram for the reaction
between iron and copper(II) chloride solution is the first four experiments?
shown in Figure 4.23. (b) Why was the rise in temperature for experiments
Figure 4.23 D and E constant?
(c) Calculate the heat of displacement of iron by
magnesium.
(d) Another experiment is carried out by adding
magnesium powder to a solution of iron(II)
chloride in excess. What is the mass of magnesium
required to release 252 kJ of heat?
[Relative atomic mass of magnesium = 24.3]
4.4 Heat of Neutralisation SPM HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l);
ΔH = –57.3 kJ mol–1
’05/P1,
’10/P1 4 Figure 4.24 shows the energy level diagram for
SPM the neutralisation reaction between nitric acid
The Meaning of Heat of Neutralisation ’06/P1 and sodium hydroxide solution.
1 All neutralisation reactions are exothermic Figure 4.24 Energy level diagram for the reaction
2 rTehaecthioenats,otfhnateuist,raΔlHisnaeuttiohnasisatnheeghaetiavterevlaelausee.d between HNO3(aq) and NaOH(aq)
when 1.0 mol of H+ ions react with 1.0 mol of
OH– ions to produce 1.0 mol of water molecules.
H+(aq) + OH—(aq) → H2O(l);
ΔH = negative value
3 For example, the heat of neutralisation of
nitric acid with sodium hydroxide solution is
–57.3 kJ mol–1.
Thermochemistry 474
5 For the neutralisation reaction between 8 ’07 ’08 4
ethanoic acid and sodium hydroxide solution,
the heat of neutralisation is –55.2 kJ mol–1 2.4 g of magnesium is added to 100 cm3 of 2.0
(Figure 4.25). mol dm–3 copper(II) chloride solution at 30 °C.
The heat of reaction is –4.20 kJ mol–1. What is the
CH3COOH(aq) + NaOH(aq) → highest temperature for this experiment? [Specific
CH3COONa(aq) + H2O(l); heat capacity of solution = 4.21 g–1 °C–1; relative
atomic mass of magnesium = 24].
ΔH = –55.2 kJ mol–1
Solution
Figure 4.25 Energy level diagram for the reaction Number of moles of Mg
between CH3COOH(aq) and = —22—.44
NaOH(aq) = 0.1
6 Hydrochloric acid, nitric acid and ethanoic acid Number of moles of Cu2+
are monobasic acids and sulphuric acid is a = —1—M0—0V—0
dibasic acid. The heat of neutralisation between
sulphuric acid and sodium hydroxide solution = —2—.—0—1—30—0—01—0—0
is –57.3 kJ mol–1 and the thermochemical
equation can be written as follows: = 0.2
—12 H2SO4(aq) + NaOH(aq) → —12 Na2SO4(aq) Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
+ H2O(l); Number of moles of Cu displaced = 0.1
ΔΔH = –57.3 kJ mol–1 Heat of reaction = –4.2 kJ mol–1
Δ H for the reaction = —01—..—01 3 4.2 kJ
(ΔH represents the heat released on the
formation of 1 mol of water). = 0.42 kJ
= 420 J
Let temperature rise = θ °C
ΔH = mcθ
420 = 100 3 4.2 3 θ
θ = 1 °C
Highest temperature = (30 + 1) °C
= 31 °C
The reaction between hydrogen chloride gas and Determination of the Heat of
sodium hydroxide solution is a neutralisation reaction. Neutralisation
However, the heat energy released when 1 mol of
HCl(g) reacts with 1 mol of NaOH(aq) is not –57.3 1 The heat of neutralisation can be determined
kJ mol–1. This is because the solubility of hydrogen by adding a known volume of acid (with a
chloride in water to form hydrochloric acid releases known concentration) to a known volume of
heat energy. an alkali (with a known concentration) and
measuring the highest temperature obtained.
HCl(g) + water → HCl(aq); ΔH = negative … (1)
2 Experiment 4.1 shows the method used to
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l); determine the heat of neutralisation between
ΔH = –57.3 kJ mol–1 … (2) sodium hydroxide with (a) hydrochloric acid
(strong acid) and (b) ethanoic acid (weak
acid).
475 Thermochemistry
4.1
To determine the heat of neutralisation between an acid and an alkali
Problem statement Results
How are heats of neutralisation determined and (A) Reaction between hydrochloric acid and
compared? sodium hydroxide solution
Hypothesis Initial temperature of sodium hydroxide 30 °C
solution
The heat of neutralisation between hydrochloric 31 °C
acid and sodium hydroxide is higher than the heat Initial temperature of hydrochloric acid 43.5 °C
of neutralisation between ethanoic acid and sodium
hydroxide. Highest temperature obtained
Variables Average initial temperature of solutions before
Manipulated variable : Different types of acids neutralisation
Responding variable : Heat of neutralisation = —3—0——+2—3—1 = 30.5 °C
Constant variable : Concentrations and volumes
Rise in temperature during neutralisation
of acid and alkali used = 43.5 – 30.5 = 13 °C
Experiment 4.1 4 Apparatus (B) Reaction between ethanoic acid and sodium
Thermometer, plastic cup and measuring cylinder. hydroxide solution
Materials Initial temperature of sodium hydroxide 30 °C
2.0 mol dm–3 hydrochloric acid, 2.0 mol dm–3 solution
ethanoic acid and 2.0 mol dm–3 sodium hydroxide 30 °C
solution. Initial temperature of ethanoic acid 42 °C
Procedure Highest temperature reached
Average initial temperature of solutions before
neutralisation = 30 °C
Rise in temperature = 42 – 30 = 12 °C
Assumptions
Specific heat capacity of solution = 4.2 J g–1 °C–1
Density of solution = 1.0 g cm–3
Figure 4.26 To determine the heat of Calculations
neutralisation
Section (A)
1 100 cm3 of 2 mol dm–3 sodium hydroxide solution (a) Heat energy released (mcθ)
is poured into a plastic cup by using a measuring
cylinder. The initial temperature of the alkali is = mass of solution specific heat capacity
recorded (Figure 4.26). rise in temperature
= (100 + 100) 4.2 13
2 Using another measuring cylinder, 100 cm3 of = 10 920 J = 10.92 kJ
2 mol dm–3 hydrochloric acid is measured. The
initial temperature of the acid is recorded. (b) Number of moles of HCl used
= —2——1—0—01—00—0 = 0.2
3 The hydrochloric acid is then poured quickly and
carefully into the sodium hydroxide solution. Number of moles of NaOH used
The mixture is stirred with a thermometer and = —2—1—0—0—10—0—0 = 0.2
the highest temperature obtained is recorded.
H+(aq) + OH–(aq) → H2O(l)
4 Steps 1 to 3 are repeated using 100 cm3 of 2 mol
dm–3 ethanoic acid instead of hydrochloric acid. Number of moles of water molecules produced
= 0.2
Thermochemistry 476
(c) From (a) and (b) acid (–54.6 kJ mol–1) is higher than the heat of 4
Formation of 0.2 mol of H2O molecules releases neutralisation between sodium hydroxide and
10.92 kJ of heat energy. ethanoic acid (–50.4 kJ mol–1).
2 The theoretical value for the heat of neutralisation
(d) Amount of heat energy released when 1 mol of of a strong acid and a strong alkali is –57.3 kJ
water is formed mol–1 and the theoretical value for the heat of
neutralisation of a weak acid (CH3COOH) and a
= 10.92 —0—1.—2 = 54.6 kJ strong alkali is –55.2 kJ mol–1.
Heat of neutralisation for strong acids and strong HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l);
alkalis is –54.6 kJ mol–1. ΔH = –57.3 kJ mol–1
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l); CH3COOH(aq) + NaOH(aq) →
ΔH = –54.6 kJ mol–1 CH3COONa(aq) + H2O(l); ΔH = –55.2 kJ mol–1
Section (B) 3 The heats of neutralisation determined by
(a) Heat energy released (mcθ) experiments are usually less than the theoretical
values because some of the heat released are lost
= (100 + 100) 4.2 12 to the surroundings or absorbed by the apparatus
= 10 080 J = 10.08 kJ (thermometer and plastic cup). Both these losses
of heat are ignored in the calculation.
(b) Number of moles of CH3COOH used
= —2——1—0—01—00—0 = 0.2 4 The following precautionary steps must be taken
when carrying out the experiment.
Number of moles of NaOH used (a) The initial temperatures of the solutions
= —2——1—0—01—00—0 = 0.2 of sodium hydroxide, hydrochloric acid
and ethanoic acid are only taken after the
H+(aq) + OH–(aq) → H2O(l) thermometer is left in the solution for a few
minutes. This is to ensure that the solutions
Number of moles of water molecules produced have reached a constant temperature.
= 0.2 (b) The reactants (hydrochloric acid and ethanoic
acid) must be added quickly and stirred so
(c) From (a) and (b): that the reaction can be completed in a very
Formation of 0.2 mol of H2O molecules releases short time.
10.08 kJ of heat energy. (c) The thermometer reading must be observed
throughout the experiment so that the highest
(d) Amount of heat energy released when 1 mol of temperature reached can be recorded.
water is formed
Conclusion
= 10.08 —0—1.—2 = 50.4 kJ The heat of neutralisation for strong acids and strong
alkalis is higher than the heat of neutralisation for
Heat of neutralisation for weak acids and strong weak acids and strong alkalis. The hypothesis is
alkalis is –50.4 kJ mol–1. accepted.
CH3COOH(aq) + NaOH(aq) →
CH3COONa(aq) + H2O(l); ΔH = –50.4 kJ mol–1
Discussion
1 The experiment shows that the heat of neutralisation
between sodium hydroxide and hydrochloric
The heat of neutralisation is NOT the heat released when 1.0 mol of an acid neutralises 1.0 mol of an alkali.
The reaction between 1.0 mol of an acid with 1.0 mol of a base can produce 2.0 mol of water. For example,
H2SO4 + Mg(OH)2 → MgSO4 + 2H2O; ΔH = 2 heat of neutralisation
In actual fact, the heat of neutralisation is defined as the heat evolved when 1.0 mol of H+ ions react with 1.0 mol of
OH– ions to form 1.0 mol of water.
477 Thermochemistry
Comparing the Heats of Neutralisation of a SPM CH3COOH(aq) CH3COO–(aq) + H+(aq)
’04/P1, 100 mol 4 mol
Reaction Involving a Strong Acid and Strong ’05/P1,
’10/P2
Alkali with a reaction involving a weak acid
and a weak alkali This means that an aqueous solution of ethanoic
acid contains mainly CH3COOH molecules and
1 The energy level diagram for the heat of very few H+ ions.
snteruotnragliaslaktaiolin(oNfaaOsHtr)oinsgshaociwdn(HinNFOig3u)rwe i4t.h24a 7 (a) When neutralisation occurs, some of the
and the heat of neutralisation of a weak acid
(CH3COOH) with a strong alkali (NaOH) is heat released are absorbed by ethanoic acid
shown in Figure 4.25. molecules to break the O–H bonds in the
molecules so that the acid will eventually
2 For the neutralisation reaction between a dissociate completely.
strong acid and a strong alkali, the heat of
O
neutralisation is –57.3 kJ mol–1. For example, i
CH3 – C – O – H + Na+ + OH– →
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l); breaking O – H bond in CH3COOH
ΔH = –57.3 kJ mol–1 molecule absorbs heat energy
4 HNO3(aq) + KOH(aq) → KNO3(aq) + H2O(l); O
ΔH = –57.3 kJ mol–1 i
3 This is because strong acids and strong alkalis CH3 – C – O–Na+ + H – O – H
dissociate completely in aqueous solutions
to form hydrogen ions and hydroxide ions forming O – H bond in H2O
respectively. molecule releases heat energy
HCl(aq) → H+(aq) + Cl–(aq) (b) As a result, the heat of neutralisation of a
weak acid with a strong alkali is less that
1 mol 1 mol –57.3 kJ mol–1.
NaOH(aq) → Na+(aq) + OH–(aq) 8 (a) Ammonia solution is a weak alkali. This is
because ammonia dissociates only partially
1 mol 1 mol in aqueous solution.
4 When a strong acid is neutralised by a strong NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)
alkali, the reaction that occurs is the bonding
of H+ ions with OH– ions to form water This means that an aqueous solution of
molecules. Hence, the heat of neutralisation ammonia contains mainly ammonia
of strong acids and strong alkalis have the molecules and very few OH– ions. We can
represent the reaction between aqueous
same value, that is, –57.3 kJ mol–1. ammonia and hydrochloric acid by the
chemical equation
H+(aq) + OH–(aq) → H2O(l); ΔH = –57.3 kJ mol–1
strong strong
acid alkali
5 The heat of neutralisation of a weak acid with NH4 +(aq) + OH–(aq) + HCl(aq) →
a strong alkali is less than –57 kJ mol–1. For (ammonia solution)
NH4Cl(aq) + H2O(l)
example,
(b) The heat of neutralisation of hydrochloric
CH3COOH(aq) + NaOH(aq) → acid and ammonia solution is –51.5 kJ
CH3COONa(aq) + H2O(l); mol–1. The energy level diagram for the
heat of neutralisation of a strong acid
ΔH = –55.2 kJ mol–1 (hydrochloric acid) with a weak alkali
(ammonia solution) is shown in Figure
6 Weak acids dissociate only partially in water 4.27.
to produce very few hydrogen ions in aqueous
solutions.
Thermochemistry 478
9 ’05
The energy level diagram of a reaction is shown
below.
Figure 4.27 The energy level diagram for the heat Which of the following pairs of acid and alkali can
of neutralisation of hydrochloric acid
and ammonia solution be used to replace hydrochloric acid and sodium
(c) The heat of neutralisation for a strong hydroxide to obtain the same ΔH value?
acid with a weak alkali is less than A
–57.3 kJ mol–1. This is because the weak B Ethanoic acid (CH3COOH) and NaOH 4
alkali does not dissociate completely in C Nitric acid (HNO3) and KOH
aqueous solution. Therefore, when the Carbonic acid (H2CO3) and NaOH
neutralisation reaction occurs, some of D Hydrochloric acid (HCl) and ammonia solution
the heat energy released is absorbed in
the dissociation of the weak alkali. Comments
Sulphuric acid is a strong acid and sodium hydroxide
9 (a) The heat of neutralisation for ethanoic acid is a strong alkali. The heat of neutralisation of a
and ammonia solution is –50.4 kJ mol–1. strong acid and a strong alkali is –57 kJ mol–1.
The energy level diagram for the heat of Ethanoic acid and carbonic acid are weak acids and
neutralisation of a weak acid (ethanoic ammonia solution is a weak alkali.
acid) with a weak alkali (ammonia solution)
is shown in Figure 4.28. Answer B
Figure 4.28 The energy level diagram for the Calculations Involving the Heat of
heat of neutralisation of ethanoic Neutralisation
acid and ammonia solution
7
(b) The heat of neutralisation (ΔHneut) of
a strong/weak acid with a strong/weak When 100 cm3 of 1.2 mol dm–3 hydrochloric acid is
alkali decreases in the order: added to 100 cm3 of 1.2 mol dm–3 ammonia solution,
the temperature of the solution changes from 30 °C
ΔHneut strong acid/strong alkali > ΔHneut to 37.5 °C. Calculate the heat of neutralisation of
strong acid/weak alkali or ΔHneut weak hydrochloric acid with ammonia.
acid/strong alkali > ΔHneut weak acid/weak [Specific heat capacity of solution = 4.2 J g–1 °C–1;
alkali density of solution = 1.0 g cm–3]
(c) This is because when neutralisation between Solution
a weak acid and a weak alkali occurs, (a) Calculate the heat energy released in this
some of heat energy released is absorbed
not only in the dissociation of the weak experiment
acid but also in the dissociation of the Heat energy released (mcθ)
weak alkali. = (100 + 100) 4.2 (37.5 – 30)
= 6300 J
= 6.30 kJ
479 Thermochemistry
4 (b) Calculate the number of moles of water Experiment 2
molecules produced Since the same heat energy is released in
Number of moles of HCl used Experiments 1 and 2, the number of moles of water
molecules produced are the same. That is, 0.2 mol
= —1—.2—1—0—0—01—0—0 of water molecules are produced in Experiment 2.
= 0.12
Number of moles of NH3 used 2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + 2H2O(l)
= —1—.2—1—0—0—01—0—0 The equation shows that 2 mol of HCl reacts with
= 0.12 1 mol of Ca(OH)2 to form 2 mol of H2O.
Therefore, 0.2 mol of HCl will react with 0.1 mol
HCl(aq) + NH4+(aq) + OH–(aq) → of Ca(OH)2 to form 0.2 mol of H2O.
NH4Cl(aq) + H2O(l) That is, number of moles of Ca(OH)2 used = 0.1 … (1)
Let concentration of calcium hydroxide,
0.12 mol of H+ ions react with 0.12 mol of OH– Ca(OH)2 = M mol dm–3
ions to produce 0.12 mol of water molecules. Number of moles of Ca(OH)2 used
(c) From (a) and (b)
Formation of 0.12 mol of water molecules = —M——1—0—0—10—0—0
release 6.30 kJ of heat. = 0.1 M … (2)
(d) Heat released when 1.0 mol of water molecules
are formed From (1) and (2), we have,
0.1 M = 0.1
= 6.30 —0—.1—1—2 M = 1.0 mol dm–3
= 52.5 kJ
That is, heat of neutralisation of hydrochloric
acid with ammonia is –52.5 kJ mol–1.
8 10 ’04
When 200 cm3 of 1.0 mol dm–3 NaOH is added to Which of the following neutralisation reactions
100 cm3 of hydrochloric acid (in excess), x kJ of releases the least heat?
heat energy is released.
The experiment is repeated using 100 cm3 of A 1 mol of nitric acid and 1 mol of potassium
Ca(OH)2 solution and 100 cm3 of hydrochloric acid hydroxide
(in excess). The heat energy released is also x kJ.
What is the concentration of calcium hydroxide? B 1 mol of ethanoic acid and 1 mol of potassium
hydroxide
Solution
Experiment 1 C —21 mol of sulphuric acid and 1 mol of sodium
Number of moles of NaOH used hydroxide
= —1—.0—1—0—0—02—0—0
= 0.2 D 1 mol of hydrochloric acid and 1 mol of
sodium hydroxide
H+(aq) + OH–(aq) → H2O(l)
Comments
HCl (in excess) reacts with 0.2 mol NaOH to In each reaction, 1 mol of H2O is formed. Ethanoic
produce 0.2 mol of water molecules and x kJ of acid is a weak acid, which dissociates partially in
heat energy is released. aqueous solution. Part of the heat of neutralisation
released is used to dissociate ethanoic acid. Hence,
the heat of neutralisation for the reaction between
a weak acid and a strong alkali is less than that
between a strong acid and a strong alkali.
Answer B
Thermochemistry 480
• Heat of neutralisation is the Heat of Method for determining heat of
heat energy released when 1.0 neutralisation neutralisation
mol of H+ ions react with 1.0 • Add a known volume of an acid (of
mol of OH– ions to form 1.0
mol of water molecules. known concentration) to a known volume
of an alkali (of known concentration)
and measure the maximum temperature
reached.
Energy level diagram Method of calculation
(a) Strong acid/strong alkali neutralisation: (a) Number of moles of H2O produced
ΔH = –57.3 kJ mol–1 = x mol
(b) Strong acid/weak alkali, weak acid/strong alkali (b) Heat energy released
and weak acid/weak alkali neutralisation:
ΔH less than –57.3 kJ mol–1 = —1m—00—cθ0—
(c) This is because weak acids/weak alkalis undergo = y kJ
partial dissociation in water. Part of the heat
released during neutralisation is absorbed for (c) From (a) and (b):
the dissociation of the weak acid/weak alkali
molecule. Formation of x mol of H2O releases y kJ of heat 4
From (c),
(d)
Heat of neutralisation = —–x—y kJ mol–1
4.4 (a) What is meant by ΔH = –114 kJ?
(b) What is the heat of neutralisation between
Use the following data for all the calculations. Specific
heat capacity of solution = 4.2 J g–1 °C–1; density of sulphuric acid and sodium hydroxide solution?
solution = 1.0 g cm–3 (c) When 50 cm3 of 1.0 mol dm–3 nitric acid is
1 Figure 4.29 shows the energy level diagram for
added to 50 cm3 of a sodium hydroxide solution,
the reaction between sulphuric acid and sodium the temperature rise is 6.5 °C. What is the
hydroxide solution. concentration of sodium hydroxide solution?
Figure 4.29 Energy level diagram for the reaction 2 Consider five neutralisation reactions.
between H2SO4 and NaOH
I 50 cm3 of 1.0 mol dm–3 HNO3 + 25 cm3 of 1.0
mol dm–3 NaOH
II 50 cm3 of 1.0 mol dm–3 HCl + 50 cm3 of 1.0
mol dm–3 NaOH
III 50 cm3 of 1.0 mol dm–3 CH3COOH + 25 cm3 of
IV 1.0 mol dm–3 NmHo3l dm–3 CH3COOH + 25 cm3 of
25 cm3 of 1.0 NaOH
2.0 mol dm–3
V 50 cm3 of 1.0 mol dm–3 H2SO4 + 25 cm3 of 1.0
mol dm–3 KOH
481 Thermochemistry
(a) Which two reactions will release the same if 100 cm3 of 0.8 mol dm–3 ethanoic acid is mixed
amount of heat? with 100 cm3 of 0.8 mol dm–3 sodium hydroxide
solution.
(b) Which reaction will release the highest amount
of heat? 4 When 50.0 cm3 of 1 mol dm–3 sodium hydroxide
solution is added to an excess of acid, HA, 2.65 kJ
(c) Which reaction will release the lowest amount of of heat is released.
heat? (a) Calculate the heat of neutralisation.
(b) Is the acid, HA, a strong acid or a weak acid?
Briefly explain your answers. (c) State the assumptions that you made in answer
(b).
3 When 100 cm3 of 1 mol dm–3 ethanoic acid is mixed
with 100 cm3 of 1 mol dm–3 sodium hydroxide
solution, the temperature of the reaction mixture
increases by 5.5 °C. Calculate the rise in temperature
4.5 Heat of Combustion 6 The heat of combustion of the compounds,
methanol and methane, are –715 kJ mol–1 and
The Meaning of Heat of Combustion SPM –890 kJ mol–1 respectively.
’04/P1
4 CH3OH(l) + —32 O2(g) → CO2(g) + 2H2O(l);
1 Combustion is a redox reaction in which a ΔH = –715 kJ mol–1
substance reacts rapidly with oxygen with the
production of heat energy. Combustion is CH4 (g) + 2O2(g) → CO2(g) + 2H2O(l);
always accompanied by a flame and light. ΔH = –890 kJ mol–1
2 During combustion, the elements are converted Determining the Heat of Combustion
to their oxides. For example, carbon (C) burns
to form carbon dioxide gas (CO2) while 1 The heat of combustion can be determined
hydrogen burns to form water (H2O). SPM by burning a known mass of a fuel (such as
’11/P1 ethanol) and the heat evolved is used to heat
3 All combustion reactions are exothermic
reactions. water. From the increase in the temperature
of water, the heat of combustion of the fuel
4 The heat of combustion is the heat released can be determined.
when 1 mol of a substance is burnt completely 2 The following relationship is used for
in excess oxygen. calculation:
5 (a) The heat of combustion of the elements,
carbon and hydrogen, are –392 kJ mol–1
and –286 kJ mol–1 respectively.
C(s) + O2(g) → CO2(g);
ΔH = –392 kJ mol–1
H2(g) + —12 O2(g) → H2O(l); kJ mol–1 Heat evolved during = Heat absorbed
ΔH = –286 combustion of fuel by water
(b) Figure 4.30 shows the energy level diagram 3 Experiment 4.2 shows the simple apparatus
for the combustion of the element used for determining the heat of combustion
carbon. of alcohol. The copper container is called the
calorimeter. Other metal calorimeters can also
SPM be used in the experiment. The calorimeter
is made from substances that are good
’07/P1 conductors of heat to minimise the loss of
heat. Thus, the rise in temperature during
combustion can be determined accurately.
Figure 4.30 Energy level diagram for the
combustion of carbon
Thermochemistry 482
4.2
To determine the heat of combustion of various alcohols
Problem statement How does the number of carbon atoms per molecule of an alcohol affect the heat of
combustion?
Hypothesis As the number of carbon atoms per molecule in an alcohol increases, so does the heat of
Variables combustion.
(a) Manipulated variable : Type of alcohol
(b) Responding variable : Heat of combustion
(c) Constant variable : Volumes of water and alcohol, metal container (calorimeter) and spirit lamp
Apparatus Copper container, spirit lamp, measuring cylinder, thermometer, electronic balance, tripod
stand, asbestos screen, wooden block and Bunsen burner.
Materials Methanol, ethanol, propan-1-ol and butan-1-ol.
Procedure
Figure 4.31 Determining the heat of combustion of ethanol Experiment 4.2 4
1 Using a measuring cylinder, 250 cm3 of water is measured into a copper container.
2 The copper calorimeter is placed on the tripod stand. The initial temperature of water is measured and recorded.
3 The spirit lamp is half-filled with ethanol. The spirit lamp and ethanol are weighed and the mass is recorded.
4 The spirit lamp is placed below the copper calorimeter and the wick is lighted (Figure 4.31). The flame of
the spirit lamp is shielded from the draught (blow of wind) by using an asbestos screen.
5 The water in the calorimeter is stirred throughout the experiment.
6 When the temperature of water increases by about 30 °C, the spirit lamp is extinguished.
7 The spirit lamp and ethanol are weighed again and the mass is recorded.
8 The experiment is repeated using other alcohols as shown below to replace ethanol:
(a) Methanol (b) Propan-1-ol (c) Butan-1-ol
Results
Alcohol Volume Initial Highest Rise in Initial mass Final mass
of water temperature temperature temperature of lamp + of lamp +
of water (°C) of water (°C) of water (°C) alcohol (g) alcohol (g)
(cm3)
Ethanol 250 30.5 (t1) 59.5(t2) t2 – t1 218 (m1) 216.6 (m2)
Methanol 250 t3 t4 t4 – t3 m3 m4
Propan-1-ol 250 t5 t6 t6 – t5 m5 m6
Butan-1-ol 250 t7 t8 t8 – t7 m7 m8
Manipulated Constant Responding variable
variable variable 483
Thermochemistry
Calculations (a) of the loss of heat to the surroundings,
(b) the combustion of ethanol is incomplete,
Assumption (c) some of the ethanol escapes due to
Specific heat capacity of water = 4.2 J g–1 °C–1
Density of water = 1.0 g cm–3 evaporation.
(a) Calculate the heat energy released during the 3 (a) When ethanol burns, some of the heat released
combustion of ethanol in the experiment is absorbed by the copper calorimeter and
Rise in the temperature of water some is lost to the surroundings.
= t2 – t1 = 59.5 – 30.5 = 29 °C (b) In complete combustion, all carbon atoms in
ethanol are converted into carbon dioxide.
Heat energy evolved
= mass of water in the copper container specific C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l);
heat capacity rise in temperature ΔH = –1371 kJ mol–1
= 250 4.2 29 = 30 450 J = 30.45 kJ
(b) Calculate the number of moles of ethanol burnt In partial combustion, carbon atoms in
in the experiment ethanol are converted to carbon dioxide,
Mass of ethanol burnt carbon monoxide and also soot. As a result
= (m1 – m2) = 218 – 216.6 = 1.4 g of incomplete combustion, less heat energy is
released.
Relative molecular mass of ethanol (C2H5OH) (c) Ethanol is a volatile liquid. When combustion
= (2 12) + (1 6) + 16 = 46 occurs, the temperature around the spirit
lamp becomes higher. As a result, ethanol
4 Therefore, the number of moles of ethanol burnt tends to evaporate off. Hence, the spirit lamp
and the remaining alcohol after combustion
= ——re—l—a—ti—v—em—ma—so—sl—eo—cf—ue—lta—hr—amn—o—al—sb—su—or—fn—et— th—a—n—o—l = —1—4.—64 must be weighed as soon as possible.
Precautionary steps
= 0.03
1 The following actions must be taken to avoid
(c) Calculate the heat energy released when 1 mol of loss of heat to the surroundings.
ethanol is burnt (a) An asbestos screen is placed around the
From (a) and (b), combustion of 0.03 mol of copper calorimeter and the tripod stand. The
ethanol produces 30.45 kJ of heat energy. asbestos screen protects the flame of the spirit
Therefore, heat evolved when 1 mol of ethanol lamp from the disturbance of air currents.
burns (b) The spirit lamp is placed on a wooden block
so that the spirit lamp is at a minimum
= 30.45 —0—.—10—3 = 1015 kJ distance from the copper calorimeter. In this
That is, heat of combustion of ethanol = –1015 kJ way, a bigger area of the flame can be in
mol–1. contact with the calorimeter.
(d) The energy level diagram for the combustion of 2 The copper calorimeter and the water in it are
ethanol is shown in Figure 4.32. heated directly by spirit lamp. A wire gauze is
not used because it is a good conductor of heat
Figure 4.32 and will absorb heat energy given off by the fuel.
(e) Heat of combustion of the other alcohols can 3 The water in the copper calorimeter must always
be obtained by the same method used for be stirred so that its temperature is uniform.
determining the heat of combustion of ethanol.
4 Precautionary steps must also be taken when
Sources of error carrying out the experiment because the liquid
fuel (ethanol) is very volatile and catches fire easily.
1 In fact, the actual value of the heat of combustion
of ethanol is –1371 kJ mol–1. Conclusion
2 The heat of combustion of ethanol obtained in this 1 The heat of combustion of ethanol = –1371 kJ
experiment is very much lower than the actual mol–1
value (–1371 kJ mol–1) because
2 The heat of combustion increases as the number
of carbon atoms per molecule in the alcohol
increases.
Thermochemistry 484
Heats of Combustion of Various Alcohols 3 The heat of combustion of ethanol is higher
than the heat of combustion of methanol.
1 The heat of combustion of methanol, ethanol, This is because one molecule of ethanol
propan-1-ol and butan-1-ol are shown in Table contains one carbon atom and two hydrogen
4.3. When the relative molecular mass of the atoms more than one molecule of methanol.
alcohol increases, the heat of combustion also When carbon and hydrogen burns, heat energy
increases. is released. Hence, the greater the number
of carbon atoms and hydrogen atoms per
Table 4.3 Heat of combustion of some alcohols molecule, the higher the heat of combustion
of the alcohol.
SPM Alcohol Heat of
Equation for reaction combustion
’04/P1 The Fuel Values of Various Fuels
’05/P2 (kJ mol–1)
/SA
’07/P1
Methanol CH3OH(l) + —32 O2(g) → –715 1 Fuels are substances that can burn easily in air
CO2(g) + 2H2O(l) to produce heat energy.
Ethanol C2H5OH(l) + 3O2(g) → –1371 2 Fuels can be classified into three groups
2CO2(g) + 3H2O(l) depending on their physical states.
(a) Solid fuels: coal, charcoal, coke and peat.
(b) Liquid fuels: petrol, diesel and kerosene. 4
(c) Gaseous fuel: hydrogen, natural gas and
Propanol C3H7OH(l) + —92 O2(g) → –2010 coal gas.
3CO2(g) + 4H2O(l)
3 Different fuels have different fuel values. Fuel
Butanol C4H9OH(l) + 6O2(g) → –2673 value is the heat energy released when 1.0 g
4CO2(g) + 5H2O(l) of fuel is burnt in excess oxygen. Fuel value is
also known as heat value. The units for fuel
2 If the heat of combustion of alcohols are plotted value is kJ g–1.
against the number of carbon atoms in the
alcohol molecules, a straight line is obtained 4 Table 4.4 shows the fuel values of some fuels.
(Figure 4.33). For example, 1.0 g of hydrogen releases 143
kJ of heat energy and 1.0 g of coal releases
only 34 kJ of heat energy.
Table 4.4 Fuel values
Fuel Fuel value (kJ g–1)
Hydrogen 143
Natural gas (methane) 52
Propane 51
Butane 50
Petrol 42
Coal 34
Wood 21
Figure 4.33 The heat of combustion of alcohols 5 In terms of heat energy released per g of fuel,
against the number of carbon atoms hydrogen is the best fuel and other fuels such
per molecule as natural gas and petrol are better fuels than
coal.
6 The fuel values are used in industry to compare
the energy cost of various fuels.
485 Thermochemistry
9 They are good fuels because they do not
produce soot or poisonous gases that
The table below shows the fuel value (in kJ g–1) and pollute the air. Liquid fuels and gaseous
the cost (in ringgit per kg) for two fuels, fuel X and fuels do not leave ashes after combustion.
fuel Y. They are therefore better fuels than solid
fuels.
Fuel Fuel value (kJ g–1) Cost of fuel (c) Cost per gram of fuel
X 52 RM 0.35 per kg Coal is a cheap fuel and fuels such as
Y 42 RM 1.00 per kg butane and petrol are quite expensive.
2 In terms of the economy and environment,
Which is the more expensive fuel, fuel X or fuel Y, good fuels have the following characteristics:
in terms of (a) fuel cost and (b) energy cost? (a) Produce a large amount of heat energy
when burnt
Solution (b) Do not cause pollution
(c) Can be obtained cheaply
(a) Fuel cost (d) Can be obtained easily
(e) Can be burnt easily
Fuel Y is more expensive than fuel X in terms of (f) Can be kept and transported easily and
safely
cost per gram of fuel. 3 We use a particular fuel for a specific purpose.
For example, charcoal is used to roast meat,
(b) Energy cost of fuel X coal is used to generate electricity in power
stations and petrol is for car engines.
1 kg of fuel X costs RM 0.35. 4 Besides traditional fuels such as coal and
petrol, scientists continue to look for new fuels
1 kg of fuel X produces 52 1000 J of heat for various purposes. For example, hydrazine
(N2H4) is produced by scientists as fuel for
4 energy. From fuel value rockets.
Therefore, 52 1000 J heat energy costs N2H4(l) + O2(g) → N2(g) + 2H2O(g);
ΔH = –622 kJ mol–1
RM 0.35.
The combustion of acetylene in air is highly
That is, 1000 J of heat energy costs RM —0—5.3—25 exothermic and gives a very hot flame. The
oxyacetylene flame is used to weld and cut
= RM 0.0067 … (1) metals. Ethyne (C2H2) is commonly known as
acetylene.
Energy cost of fuel Y
1.0 kg of fuel Y costs RM1.00.
1.0 kg of fuel Y produces 42 1000 J of heat
energy. From fuel value
Therefore, 42 1000 J of heat energy costs
RM1.00.
That is, 1000 J of heat energy costs RM —14—.02—0
= RM 0.024 … (2)
Based on answers (1) and (2), fuel Y is more
expensive than fuel X in terms of energy cost.
The Selection of Suitable Fuels for Specific
Purposes
1 When selecting a suitable fuel for a specific
purpose, three main factors must be
considered.
(a) Fuel value
The higher the fuel value, the more energy
is released per gram of the fuel.
(b) Effect on the surroundings
Most fuels produce a lot of soot when
burnt and this causes air pollution. Fuels
such as hydrogen are known as clean fuels.
Thermochemistry 486
Calculations Involving Heat of Combustion Solution 4
(a) Calculate the heat energy released in this
10
experiment
When 2.7 g of glucose (C6H12O6) is burnt completely Heat energy released (mcθ)
in excess oxygen, the heat released increases the = 250 4.2 10 J
temperature of 600 g of water by 12.5 °C. Calculate = 10 500 J
the heat of combustion of glucose. = 10.5 kJ
[Specific heat capacity of water = 4.2 J g–1 °C–1;
density of water = 1.0 g cm–3; relative atomic mass: (b) Calculate the heat released by the combustion
H, 1; C, 12; O, 16] of 1.0 mol of methane
0.0125 mol of methane gives off 10.5 kJ of heat
Solution energy. Heat energy given off by 1.0 mol of
(a) Calculate heat energy released in this experiment methane
Heat energy released = 10.5 —0—.0—11—2—5
= mass of water (m) specific heat capacity (c) = 840 kJ
rise in temperature (θ) (c) Calculate the heat released by the combustion
= 600 4.2 12.5 J of 1.0 g of methane
= 31 500 J
= 31.5 kJ Relative molecular mass of methane (CH4)
(b) Calculate the number of moles of glucose burnt = 12 + (4 1)
in this experiment = 16
Relative molecular mass of glucose (C6H12O6) Fuel value = ——81—46—0
= (12 6) + 12 + (6 16) = 52.5 kJ g–1
= 180
11 ’07
Number of moles of glucose burnt
= —12—8.—70 In which of the following is the heats of combustion
for methanol, ethanol and propanol correctly
= 0.015 matched?
(c) Calculate the heat energy released when 1 mol Methanol Ethanol Propanol
of glucose is burnt (kJ mol–1) (kJ mol–1) (kJ mol–1)
From (a) and (b), 0.015 mol of glucose produces
31.5 kJ of heat energy. A –715 –2010 –1321
Therefore, heat energy released by 1 mol of
glucose B –715 –1321 –2010
= 31.5 —0—.01—1—5 C –2010 –1321 –715
= 2100 kJ
D –2673 –2010 –715
That is, heat of combustion of glucose is
–2100 kJ mol–1. Comments
The heat of combustion increases (more exothermic)
11 from methanol to propanol.
When 0.0125 mol of methane (CH4) is completely Answer B
burnt, the heat energy released raises the temperature
of 250 cm3 of water by 10 °C. What is the fuel
value of methane?
[Specific heat capacity of water = 4.2 J g–1 °C–1;
density of water = 1.0 g cm–3; relative atomic mass:
H, 1; C, 12]
487 Thermochemistry
Heat of combustion
• Heat of combustion is the heat
released when 1.0 mol of the
substance is burnt completely in
excess oxygen.
Method of determining heat Calculation Energy level diagram
of combustion
(a) Calculate number of moles
• Find the mass of the fuel of fuel used (x mol)
used.
(b) Calculate heat evolved
• Find the volume of water to ΔH = —m——1—0—c0—0——θ = y kJ
be heated. (c) Heat of combustion
= —–x—y kJ mol–1
• Find the initial and final
temperatures of the water.
4
Differences in heats of Selection of suitable fuel Differences in fuel values
combustion • For specific purposes (kJ g–1)
• Examples:
For various alcohols • Coal: 34 kJ g–1
• CH3OH; ΔH = –715 kJ mol–1 Coal, natural gas, petrol, • Petrol : 42 kJ g–1
• C2H5OH; ΔH = –1371 kJ mol–1 hydrogen • Natural gas: 52 kJ g–1
• Caused by an additional –CH2
Factors to consider
group
Does it burn easily? Is it easy to store? Does it produce Can it be Is it cheap or
soot or leave obtained easily? expensive?
residue after
combustion?
Problems of safety Problems of
pollution
Advantages Disadvantages/limitations
Thermochemistry Type of fuel recommended
for the specific purpose
488
12 ’04 (b) If 1.14 g of propan-2-ol is used for complete
combustion, what is the rise in temperature of
The heat of combustion of three alcohols are shown 500 cm3 water?
below.
(c) What is the rise in temperature, if 500 cm3 of
Alcohol Heat of combustion (kJ mol–1) water is heated by the heat energy produced
by the complete combustion of 2.28 g of
CH3OH –715 propan-1-ol?
C2H5OH –1371
C3H7OH –2010 (d) What is the rise in temperature if 250 cm3 of
water is heated by the heat energy produced
Which of the following factors cause the increase by the complete combustion of 0.57 g of
propan-1-ol?
in the heat of combustion of alcohols?
(e) 2C3H7OH(l) + 9O2(g) → 6CO2(g) Δ+H8=H2–Ox(lk)J;
I The increase in the molecular size of alcohols. Determine the value of x in the thermochemical
equation given above.
II The increase in the number of carbon atoms
3 The thermochemical equation for the combustion
per molecule. of heptane is shown below.
III The increase in the number of hydrogen atoms ’04 C7H16(l) + 11O2(g) → 7CO2(g) + 8H2O(l);
ΔH = –5520 kJ mol–1
per molecule.
The combustion of heptane in excess oxygen
IV The increase in the forces of attraction releases 1104 kJ of heat energy. What is the mass
of heptane used?
between the molecules of alcohol. [Relative atomic mass: C, 12; H, 1]
4 What are the advantages and disadvantages of
A I and IV only C I, II and III only using (a) coal, (b) natural gas as fuel? 4
B II and III only D I, II, III and IV 4.6 Appreciating the
Existence of Various
Comments Energy Sources
The increase in molecular size and forces of
attraction increase the boiling points of alcohols but Various Sources of Energy
not the heat of combustion. (I and IV are incorrect).
The increase in the heat of combustion of alcohols 1 Fuels can be classified as renewable source of
is due to the combustion of carbon and hydrogen: energy or non-renewable source of energy.
C(s) + O2(g) → CO2(g); ΔH = –394 kJ mol–1 2 The non-renewable sources of energy are fossil
2H2(g) + O2(g) → 2H2O(l); ΔH = –572 kJ fuels such as coal, petroleum and natural gases.
The renewable sources of energy are:
Answer B (a) Nuclear power (e) Solar energy
(b) Hydroelectric power (f) Biomass and biogas
4.5 (c) Geothermal energy (g) Energy from tides
(d) Wind power
1 (a) A student wants to carry out an experiment
Choosing an Energy Source
to determine the heat of combustion of
In choosing an energy source, we need to know
pentane m(Cu5sHt 1t2a)k. eStianteortdherer ethmatetahseurehmeaetnotsf (a) the advantages and disadvantages of using that
that he
particular energy source,
combustion can be determined. (b) the costs involved,
(c) the environmental impact and
(b) The heats of combustion of propane, carbon (d) whether there are suitable sites for harnessing
and hydrogen are –2220 kJ mol–1, –394 kJ
mol–1 and –286 kJ mol–1 respectively. this source of energy. For example, although
Predict the heat of combustion of butane.
2 In an experiment, the complete combustion of
1.14 g of propan-1-ol (oCf3Hw7aOteHr )byin1c8re.2as°eCs. the
temperature of 500 cm3
[Specific heat capacity of water = 4.2 J g–1 °C–1;
density of water = 1.0 g cm–3; relative atomic mass:
H, 1; C, 12; O, 16]
(a) Give the structural formula of propan-1-ol and
propan-2-ol.
489 Thermochemistry
4 tide-powered plants produce great amounts of 5 Solar energy
energy, there are few suitable coastal sites that There are three methods to harness solar energy:
can be used to build such plants. (a) The solar energy can be absorbed on a solar
collector placed on a rooftop. This method is
Technology for Harnessing Various called solar heating and is used for providing
Energy Sources hot water or for heating the building.
(b) The solar energy can be converted into
1 Nuclear energy electricity in photovoltaic cell or more
Nuclear power stations use nuclear reactors simply, solar cells. Solar cells are now
to generate electricity. In the nuclear reactors, widely used to power small electronic
uranium-235 nuclei split to form lighter nuclei devices such as electronic calculators.
(fission reaction) and energy is released. (c) The solar energy is concentrated to heat
water and produce steam, which is used
2 Hydroelectric power to produce electricity. This is called solar
Hydroelectric power is electricity generated by thermal electric power.
the energy of water power. The most common
form of hydroelectric power uses dams on 6 Biomass, biogas and biodiesel
rivers to create large reservoirs of water. Water (a) Biomass is plant material and animal
released from the reservoirs flows through manure used as sources of energy. The
turbines, causing them to spin and to generate energy produced by biomass is called
electricity when connected to generators. biomass energy.
(b) Most biomass is
3 Geothermal energy • burnt directly for heating, cooking or
The layers of rock deep below the earth’s surface other industrial uses, or
is very hot. Heat contained in underground • converted to gaseous and liquid biofuels.
rocks is an important source of energy. When (c) The synthetic natural gas (gaseous biofuel)
water is pumped into the hot rocks, it is turned produced from biomass is called biogas.
into steam which can be used to operate Methane in biogas is produced from
turbines and generate electricity. machines known as digesters. The digesters
can convert animal manure, such as cow
4 Wind power dung and chicken dung into methane
Wind power can be used to generate electricity. gas. Methane gas produced in this way is
called biogas.
Wind turbines used to produce electricity (d) Biodiesel is a biofuel made from vegetable
individually or in groups called wind farms. oils such as palm oil. Biodiesel can be
used to power motor vehicles.
The kinetic energy of the moving air is
converted into mechanical energy for the 7 Energy from the tides
turbines. The turbine then drives a generator The power of tides is another potential source
that converts mechanical energy into electrical of energy. Twice a day during high and low
energy. Wind turbines usually have two or tides, water that flows into and out of coastal
three blades. The longer the blades and the bays can spin turbines to produce electricity.
faster the wind speed, the more electricity
the turbine generates. To produce the most The Advantages and Disadvantages in using
electricity, wind turbines must be located in
areas where the wind blows at a constant speed. Various Energy Sources
1 The main advantages of these energy sources
is that, they are renewable and they do not
produce carbon dioxide gas (except biomass)
that causes greenhouse effect or acidic gases
that cause acidic rain or poisonous gases that
cause air pollution.
2 Table 4.5 shows the disadvantages of these
renewable energy resources.
Thermochemistry 490
Energy resource Table 4.5 4
Nuclear energy
Hydroelectric power Disadvantages
Geothermal energy
(a) Nuclear reactors produce radioactive wastes, which are difficult to dispose off.
Wind power (b) Accidents in nuclear reactors can cause thousands of deaths.
Solar energy
(a) It provides low-cost electricity but the construction cost is high.
Biomass, biogas and (b) Danger of collapse which can cause catastrophic floods.
biodiesel
Energy from the tides (a) The geothermal power is low in cost (second only to hydropower) but can only
be obtained in areas where steam or hot water is at or near the surface.
(b) The wastewater is quite salty and its disposal is a problem.
(a) Steady wind needed.
(b) Contributes to noise pollution when located near populated areas.
(a) The installation for solar heating is expensive. Solar collectors need to be
exposed to the sun 60% of the time.
(b) Solar cells are not very efficient. Generation of enough energy to meet demand
would require large areas of land with solar cells.
Carbon dioxide emissions.
Electricity can be generated only when the tide is coming in or going out.
1 An exothermic reaction is a reaction that releases heat θ (in °C) = change in temperature.
11 The heat of precipitation is the heat released when
energy to the surroundings. The temperature of the
1.0 mol of a precipitate is formed from its ions.
reaction mixture increases. ΔH has a negative sign. 12 The heat of displacement is the heat released
2 An endothermic reaction is a reaction that absorbs heat
when 1.0 mol of a metal is displaced from its salt
energy from the surroundings. The temperature of the
solution by another metal.
reaction mixture decreases. ΔH has a positive sign. 13 The heat of neutralisation is the heat released
3 The energy level diagram shows the total energy
when 1.0 mol of H+ ions react with 1.0 mol of OH–
content of the reactants and that of the products.
ions to form 1.0 mol of water molecules.
4 Exothermic reaction: Total energy content of products 14 For the neutralisation reaction between a strong acid
is lower than that of the reactants. and a strong alkali, the heat of neutralisation is –57
5 Endothermic reaction: Total energy content of products kJ mol–1.
15 For the neutralisation of weak acid and strong alkali
is higher than that of the reactants.
or the neutralisation of strong acid and weak alkali,
6 Bond breaking is an endothermic process while
the heat of neutralisation is less than –57 kJ mol–1.
bond making is an exothermic process.
This is because weak acid and weak alkali do not
7 ΔH of reaction = ΔH1 (bond breaking) + ΔH2 (bond
making) dissociate completely in aqueous solution. Some of
8 Cold packs contain chemicals such as NH4NO3(s) the heat released is absorbed to dissociate the weak
and water. The reaction between NH4NO3(s) and
water is endothermic. acid or weak alkali.
16 The heat of combustion is the heat released when
9 Hot packs contain chemicals such as anhydrous
1.0 mol of a substance is burnt completely in excess
CwaaCtel2r and water. The reaction between CaCl2 and
is exothermic. oxygen.
17 The heat of combustion of alcohols increases as the
10 The heat change in a reaction can be calculated using
relative molecular mass increases.
the formula: 18 Fuel value is the heat energy released when 1.0 g
ΔH = mcθ of fuel is burnt in excess oxygen.
where ΔH (in J) = heat released or absorbed in 19 Hydrogen is the best fuel because it has the highest
aqueous solution,
fuel value and is a clean fuel (that is, does not produce
m (in g) = mass of solution,
air pollutants). However, it is an expensive fuel.
c (in J g–1 °C–1) = specific heat capacity of solution
491 Thermochemistry
4
Multiple-choice Questions
4.1 Energy Changes in fever. products is 570 kJ.
Chemical Reactions ’11 Which pair of substance is used C the reactant has more energy
1 Which of the following in cold packs? than the products.
operations absorbs heat energy A Ammonium nitrate and water D more bonds are formed than
B Anhydrous sodium carbonate
’08 from the surroundings? are broken.
A Dissolving sodium hydroxide and water
in water C Fused calcium chloride and 7 The reaction between
B Evaporating sodium chloride methane and chlorine to form
solution water
C Adding sodium chloride D Zinc powder and copper(II) ’11 chloromethane and hydrogen
solution to silver nitrate solution chloride is exothermic.
D Adding ethanoic acid to sulphate solution
ammonia solution CH4(g) + Cl2(g) → CH3Cl(g) +
5 A simple experiment as shown HCl(g) ΔH = –102 kJ
2 The energy profile diagram is carried out.
for the reaction, P → X + Y, is Which statement is correct about
4 this reaction?
’05 shown below. A Heat is absorbed during the
What is the heat of reaction? Which of the following will occur reaction.
during the experiment? B The heat absorbed on bond
A The pH of the solution
breaking is the same as the
increases. heat released on bond making.
B Heat energy is absorbed C The heat absorbed on bond
breaking is more than the heat
from the surroundings. released on bond forming.
C The temperature of the D The heat absorbed on bond
breaking is less than the heat
solution decreases. released on bond forming.
D The total energy content of
A –40 kJ C +40 kJ 8 When 25.0 cm3 of lime juice is
B +30 kJ D +70 kJ the products is lower than
the total energy content of added to 185.0 cm3 of milk to
the reactants. ’04 prepare yogurt, the temperature
3 Water can be converted to
ice or steam as shown in the 6 The energy level diagram of a of yogurt increases by 2.4 °C.
reaction is shown below.
’09 diagram below. What is the total amount of heat
’09
released?
energy
steam [The specific heat capacity of
CaO + CO2 yogurt is X J g–1 °C–1; the density
of the solution is 1 g per cm3].
A 444X J C 504X J
B 444X kJ D 504X kJ
water ice 9 The diagram shows an energy
profile diagram for a chemical
reaction.
In which conversion is heat energy = + 579 kJ mol–1
absorbed from the surroundings?
A Ice to steam CaCO3
B Steam to ice TC 58
C Water to ice Based on the diagram, it can be
D Steam to water deduced that
A it is an endToCthe5r9mic reaction.
4 Cold pack is used to put on the B the total energy of the
forehead of a patient to reduce
Thermochemistry 492
Which energy changes are the What is the rise in temperature? IV Use a screen to block the wind.
activation energy and heat of A I and III only
reaction for the uncatalysed [Specific heat capacity of B II and IV only
reaction? C I, II and III only
solution = 4.2 J g–1 °C–1] D I, II, III and IV
A 7 °C C 12 °C 16 The equation shows the reaction
between calcium chloride and
For the uncatalysed reaction B 10 °C D 14 °C sodium sulphate.
Activation Heat of 13 The following table shows the CaCl2(aq) + Na2SO4(aq) →
energy reaction volumes of 1.0 mol dm–3 silver CaSO4(s) + 2NaCl(aq);
nitrate (AgNO3) solution and
A ΔH1 ΔH3 1 mol dm–3 potassium chloride ΔH = –x kJ mol–1
solution used for determining
B ΔH1 ΔH3 – ΔH1 the heat of precipitation of silver Which of the following pairs of
chloride in four experiments. solutions will produce the same
C ΔH2 ΔH4 Which of the experiments gives value for ∆H?
the highest rise in temperature? (The concentration and volume
D ΔH2 ΔH4 – ΔH2 of the solutions are kept constant).
BACD CCCCaaaa((CCNNll22OO((aa33qq))22))((aa++qq))HHCN++lO(NHa3qa2(SaC)Oql()4a(qa)q)
4.2 Heat of Precipitation Volume of Volume 4.3 Heat of Displacement
Experiment AgNO3 of KCl
10 Ag+ (aq) + Cl–(aq) → AgCl(s) 17 When 6 g of magnesium powder
(cm3) (cm3) (in excess) is added to 50 cm3
of copper(II) sulphate solution
What is the heat change when A1 55 5 containing 0.2 mol of Cu2+ ions,
B2 50 10 the rise in temperature is t °C.
5.74 g of AgCl is precipitated C3 40 20 The heat of displacement of
D4 35 25 copper from copper(II) sulphate
when mixing solutions of silver solution by magnesium is 4
nitrate and potassium chloride? A – ( ——5———0—————1———0——4—0——.—20——————————t ) kJ mol–1
B – ( —5——0—0——.—2———————4———1.—2——0———0——0———t ) kJ mol–1
[Relative atomic mass: Ag, 108;
C – ( —5———6——————1———0—4—0——.—20——————————t ) kJ mol–1
Cl, 35.5; ∆H = –58.8 kJ mol–1] D – ( —5——0—6——.—2———————4———1.—2——0———0——0———t ) kJ mol–1
A 2.35 kJ C 23.6 kJ
14 The energy level diagram for the 18 The following shows the energy
B 3.38 kJ D 33.8 kJ reaction between lead nitrate level diagram for the displacement
solution and sodium bromide
11 The following equation solution is shown below. ’03 of iron by the metal, M.
represents the reaction between
Pb2+ ions and CrO42– ions. energy
Pb2+(aq) + 2Br–(aq)
Pb2+(aq) + CrO42–(aq) →
PbCrO4(s)
H = –x kJ mol–1
When 50 cm3 of 0.5 mol dm–3 PbBr2(s)
lead(II) nitrate solution is added
to 50 cm3 of 0.5 mol dm–3 What is the energy change
potassium chromate(VI) solution,
the temperature rises by t °C. when 1.0 mol of Pb2+ ions is
If the experiment is repeated added to 0.5 mol of Br– ions?
using 50 cm3 of 1.0 mol dm–3 A –—41 x kJ C – x kJ
lead(II) nitrate solution and B –—21 x kJ
50 cm3 of 1.0 mol dm–3 D – 2x kJ
potassium chromate(VI) solution,
what is the rise in temperature? 15 An experiment is carried out
to determine the heat of
A —21 t oC C 2t °C precipitation of silver chloride
for the reaction between silver
B t °C D 4t °C nitrate solution and sodium
chloride solution. Which of the
12 The heat of precipitation of following steps must be taken What is the temperature
copper(II) hydroxide in the to obtain accurate results?
reaction between copper(II) reached if excess M is added to
sulphate solution and sodium I Stir the solution continuously.
hydroxide solution is –59 kJ II Use a container that is a good 50 cm3 of 0.2 mol dm–3 iron(II)
mol–1. In one experiment, 50
cm3 of 2.0 mol dm–3 sodium conductor of heat. sulphate solution at 30 °C?
hydroxide solution is poured III Add the silver nitrate solution
into 50 cm3 of 1.0 mol dm–3 [Specific heat capacity of
copper(II) sulphate solution. quickly to the sodium
chloride solution. solution = 4.2 J g–1 °C–1]
A 14.4 °C C 37.8 °C
B 22.2 °C D 44.9 °C
493 Thermochemistry
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