3.2 HEAT OF REACTION

Comparing the Heat of Neutralisation

1) Type of reaction Heat of neutralisation between strong acid and a strong
alkali

Heat of neutralisation Equal with : - 57 kJ mol-1

Example HCl + NaOH → NaCl + H2O ΔH = -57 kJ/mol
Explanation: HNO3 + KOH → KNO3 + H2O ΔH = -57 kJ/mol
ü Strong acid and strong alkali ionise completely in water

ü To produces a high concentration of hydrogen ions and
hydroxide ions respectively

2) Type of reaction Heat of neutralisation between weak acid and a strong
alkali

Heat of neutralisation Less than: - 57 kJ mol-1

H+ + OH-→ H2O ΔH = - 55 kJ mol-1

Example CH3COOH + NaOH → CH3COONa + H2O ΔH = -55 kJmol-1
Explanation:
ü Ethanoic acid, CH3COOH is weak acid. Acid ionise partially
in water

ü To produces a lower concentration of hydrogen ions.

ü Some of the heat energy release during the neutralisation
is used to ionises completely the remaining ethanoic
molecules to form hydrogen ions.

ü The heat release is less than 57 kJ.

3) Type of reaction Heat of neutralisation between strong acid and a weak
alkali

Heat of neutralisation Less than: - 57 kJ mol-1

H+ + OH-→ H2O ΔH = - 52 kJ mol-1

Example HCl + NH3(ak) → NH4Cl + H2O ΔH = -52 kJmol-1
Explanation:
ü Ammonia aquoues, NH3 is weak alkali, that ionises
partially in water

ü To produces a lower concentration of hyroxide ions.

ü Some of the heat energy release during the neutralisation
is used to ionises completely the remaining ammonia
molecules to form hydroxide ions.

ü The heat release is less than 57 kJ.

The heat of neutralisation between weak acids and weak alkalis is the lowest:
ü More heat energy is needed to completely ionise both weak acids and weak

alkalis.
ü Therefore, hydrogen ions, H+ and hydroxide ions, OH− produced can completely

react to form 1 mole of water.

Ø Hydrochloric acid, HCl is a monoprotic acid while sulphuric acid, H2SO4 is a
diprotic acid.

Ø Complete neutralisation between a strong diprotic acid and a strong alkali
produces twice the quantity of heat compared to a strong monoprotic acid.

One mole of a strong diprotic acid such as sulphuric acid, H2SO4 ionises in water to
produce two moles of hydrogen ions, H+:

Two moles of hydrogen ions, H+ will produce two moles of water, H2O when they
react with two moles of hydroxide ions, OH−. 114 kJ heat is released because two
moles of water are formed.

The heat of neutralisation of sulphuric acid, H2SO4 with sodium hydroxide, NaOH
solution is still the same, which is -57 kJ, because the meaning of the heat of
neutralisation is the heat released for the formation of one mole of water.







1 : Solve the question

100 cm3 of 2.0 mol dm-3 hydrochloric acid, HCl at 29.5 0C is added to 100 cm3 of 2.0 mol dm-3 sodium
hydroxide, NaOH solution which is also at 29.5 0C in a plastic cup. The mixture is then stirred and the
highest temperature reached is 41.5 0C.
Calculate the heat of neutralisation for the experiment.

2 : Solve the question

20 cm3 of 2.0 mol dm-3 nitric acid is poured into a polystyrene cup and the initial temperature is
recorded. 40 cm3 of 0.5 mol dm-3 potassium hydroxide, NaOH solution is poured into the same cup.
The solution mixture is stirred slowly and the highest temperature is recorded. The results are shown
below.

Calculate:
a) heat change
b) heat of reaction











REACTION B

Hypothesis:

The reaction between a strong acid and weak alkali will produce a higher heat of
neutralisation as compared to the reaction between a weak acid and weak alkali.

Variables:

Manipulated : Hydrochloric acid, HCl and Ethanoic acid, CH3COOH
Responding : Heat of neutralisation
Constant : Volume and concentration of ammonia

Procedure:

1. Measure and pour 50 cm3 of 1.0 mol dm−3 of hydrochloric acid, HCl into a
polystyrene cup.
2. Measure and pour 50 cm3 of 1.0 mol dm−3 of ammonia, NH3 solution into
another polystyrene cup.
3. Put a thermometer into each solution and record the initial temperature of each
solution.
4. Pour the hydrochloric acid, HCl quickly and carefully into the ammonia, NH3
solution.
5. Stir the mixture using the thermometer.
6. Record the highest temperature.
7. Repeat steps 1 to 6 by replacing hydrochloric acid, HCl with 1.0 mol dm-3 of
ethanoic acid, CH3COOH.

29.0 29.0 28.0 28.0
29.0 29.0 28.0 28.0

35.5 34.0 34.0 30.0



DISCUSSION:

5. heat of neutralisation between a strong acid and a strong alkali is higher than
the heat of neutralisation using a strong acid with a weak alkali and strong alkali
with a weak acid. Heat of neutralisation is the lowest for the reaction between a
weak acid and a weak alkali





5 : Solve the question

- ethanoic acid is a weak acid while hydrochloric acid is a strong acid
- ethanoic acid will ionises partially in water to produce low concentration of H+ ion, so some acid still
remains in form molecules.
- while hydrochloric acid will ionises 100 % in water to form high concentration of H+ ion
- so, some of heat release during neutralisation between ethanoic acid and sodium hydroxide is used
// absorbed again to ionises completely the remaining ethanoic molecules to form hydrogen ions.







Heat of combustion
burning of different fuels will produce different energy value

o The thermochemical equation shows that when one mole of methane, CH4 is
completely burnt in oxygen, O2, heat released is 394 kJ.

o The heat released is known as heat of combustion





1 : Solve the question



b) Write the chemical equation for the complete combustion of propanol

c) Based on the results of the experiment, calculate:
(i) heat released when propanol is burnt
[Given that the specific heat capacity for water is 4.2 J g-1 °C-1]

(ii) number of moles of propanol burnt.
[Given that relative molecular mass of propanol is 60]

iii)Heat of combustion of propanol.in this experiment

(d) The heat of combustion obtained in this experiment is less than the actual
theoretical value.
Suggest two precaution that should be taken to obtain a more accurate value of the
heat of combustion.
ü Use wind shield to reduce heat release to surrounding
✓ switch off the fan
✓ spirit lamps must as near as to cooper tin
✓ do not touch thermometer on copper tin
ü stir the water continuously

2 : Solve the question



2 : Solve the question





o Alcohol molecules consist of carbon atom, C, hydrogen atom, H and oxygen
atom, O. Complete combustion of alcohols produces carbon dioxide, CO2 and
water, H2O. Combustion of alcohol also releases energy, which is an exothermic
reaction.

o Figure 3.14 shows the graph of heat of combustion of alcohols against the
number of carbon atoms per molecule of alcohols.

Figure 3.14 Graph of heat of combustion of alcohol against the number of
carbon atoms per molecule of alcohol

ü As the number of carbon atoms per alcohol molecule increases, the
combustion of alcohol produces more carbon dioxide and water molecules.
Therefore, more heat is released.

ü The increment in the heat of combustion between successive members of
alcohols is almost the same. This is because each member differs from the
following member with one CH2 group.