Heat of displacement

Method and duration

Learning area At the end of the module, pupils are able to:

11.0 Thermochemistry State the meaning of heat of
displacement.
Content Standard
determining the heat of displacement
11.2 Heat of reaction of metal from its salt solution

Learning Standard Write thermochemical
equation for
11.2.2 displacement reaction
Determine heat of
displacement through construction of energy level
activity diagram

Solve numeric problems involving the
heat of precipitation

Theory recall:
A more electropositive metal can displaces a less
electropositive metal from its salt solution.

What is the Reactivity
Series?! Between
sodium and zinc,
which is more reactive/
electropositive?

If you are not sure, please scan the QR
code to identify it.

EXAMPLE

Zinc can displace copper from a copper (II) sulphate solution ,CuSO4.
Chemical equation:

Zn (s) + CuSO4 (aq) Cu (s) + ZnSO4 (aq)

Thermochemical equation:

Zn (s) + Cu2+ (aq) Cu (p) + Zn2+(aq) DH = -210 kJ mol-1

Explanation:

A total of 210 kJ of heat is released when 1 mole of copper is displaced from
the copper (II) sulphate solution, CuSO4 by 1 mole of zinc.

Heat of displacement,DH is -210 kJ mol-1.

How can the heat of displacement
of copper from a solution of
copper (II) sulphate be
determined?

Scan the QR code to determine
the heat of displacement of
copper from its salt solution.



Assumptions:

1. The solution is dilute. It has the same density as
water, which is 1.0 g cm-3.

2. The solution has the same specific heat capacity
as water, which is 4.2 J g-1 °C-1

3. No heat is absorbed by the apparatus of the
experiment.

Three steps to deterrmine the heat of reaction, ∆H.

1) Number of mole
n = MV
1000 (unit = mole)

2) Heat change // heat released

H = mcθ

1000 (unit = kJ )

3) Heat of reaction, ∆H = H
n

= +/- heat change (unit = kJ mol-1)
number of mole

A. If Ө, temperature change is given in the question

Question requirement: Determine the heat of reaction, ∆H

Number of mole, M = Molarity/ concentration (unit = mol dm-3)
n = MV/1000 V = volume of solution (unit = cm3)

Heat change, m = mass of the solution, g
H = mcϴ c = specific heat capacity of the solution, 4.2 J g-1 °C-1
ϴ = temperature change in the solution, °C
During the calculation, the unit for the answer obtained
is the Joule J.

g x J g-1 °C-1 x °C

∆H = mcϴ  mcϴ / 1000 to change the unit into kilojoule,
n x 1000
kJ.

 This can be done in step 2.
 Put a ‘+’ or ‘-’ sign in front of the value.
 Temperature rises, ‘-‘ sign
 Temperature drops, ‘+‘ sign

B. For the question which ∆H is given

 Find from energy level diagrams or thermochemical equations.
 Question requirements: Calculate the change in temperature, °C

Number of mole, n Heat change, H Temperature change, Ө

n = H = n x ∆H n x ΔH = mcӨ
1000 Ө = n x ΔH

• During the calculation, the unit for the answer mc

obtained is kilojoule, kJ.  Ensure that
• mol x kJ mol-1 n x ∆H in the
• Change into the unit J (x 1000) to continue with unit J

the following steps.

Scan the following QR code for examples to solve numeric problems
involving the heat of precipitation

Example 1

1. In an experiment, copper powder (excess) was added into 100 cm3 of 0.5
mol dm-3 silver nitrate solution. The recorded temperatures are shown below:

Initial temperature of silver nitrate solution 28.0 o C
The highest temperature of the mixture 5.0 o C

a) Wrirte the chemical equation for the reaction that took place in this experiment.
b) Wrirte the ionic equation for the reaction that took place in this experiment.
c) i) State an observation in this experiment.

ii) State the reason for the observation in c) i)
d) State another precaution in this experiment.

e) Calculate:
i) the number of moles of silver nitrate solution
ii) the heat change in this experiment
[Specific heat capacity of solution= 4.2 J g-1 °C-1;density of solution = 1.0 g cm-3]

iii) heat of displacement

f) Draw an energy level diagram for the reaction took place

g) Explain why that copper in excess is added.
h) Predict the change in heat of displacement if copper is replaced by aluminium.

Explain your answer.

Answers for example 1

a) Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
b) Cu + 2Ag+ → Cu2+ + 2Ag
c) i) The temperature rises// the colorless solution turns blue//

a grey solid is formed
ii) Exothermic reaction// copper (II) ions are formed// silver
metal is

formed
d) Zinc powder is poured quickly and carefully// the solution

mixture is constantly stirred// zinc powder is used, not zinc
granules

Answers for example 1

e) i) Number of moles = MV e) iii) ∆H = H
1000 mol n

= (0.5) (100) = 5.25 kJ / 0.05
1000
= - 105 kJ mol-1
= 0.05 mol
From the equation,

2 mol Ag+ : 2 mol Ag
0.05 mol Ag+ : 0.05 mol Ag

e) ii) H = mcӨ
= (100)(4.2)(12.5)
= 5250 J // 5.25 kJ

* (Ө = 40.5 – 28.0 = 12.5 ˚C)

Answers for example 1

f) Heat

Cu + 2AgNO3

DH = - 105 kJ mol-1
Cu(NO3)2 + 2Ag

g) To ensure that copper can displace all silver ions from the solution.
h) Higher heat of displacement will be obtain. Aluminum is more

electropositive than copper.

Example 2

1. The thermochemical ioinic equation below represents the reaction
between magnesium powder and a solution of iron (II) sulphate.
Mg (p) + Fe2+ (aq)  Mg2+ (aq) + Fe(p) DH = - 189 kJ mol-1
Calculate the rise in temperature when excess magnesium powder is
added to 80 cm3 iron (II) sulphate solution 0.4 mol dm-3.
[Specific heat capacity of solution= 4.2 J g-1 °C-1;
density of solution = 1.0 g cm-3]

Answer for example 2

Step1 : Calculate the numbe of Step 2 : Calculate the heat
moles of iron displaced
No. of mole = MV released, H

1000 in the experiment
= (0.4) (80) ∆H = H

1000 n
= 0.032 mol H = ∆H x n
From the equation,
1 mol Fe2+ : 1 mol Fe = 189 kJ mol-1 x 0.032 mol
0.032 mol Fe2+ : 0.032 mol Fe
= 6.048 kJ

= 6048 J

Step 3: Calculate the
temperature change, Ө

H = mcӨ
6048 J = 80 x 4.2 x Ө

Ө = 18 ˚C



Have you reached the
learning standard?

Let’s do the following
exercises to test on

your level of
mastery.

30 minit

Objective Questions

1. Figure 1 shows the energy level diagram for a displacement reaction.

Heat

Figure 1

What is the increase in temperature if 50 cm3 iron (II) chloride solution 0.25
mol dm-3 reacted with excess magnesium powder?

[Specific heat capacity of solution= 4.2 J g-1 °C-1]

A 12 oC B 16 oC C 22 oC D 24 oC

2. The diagram shows the arrangement of the apparatus for determining the
heat of displacement between metal M and solution N. Which material
produced the highest heat of displacement?

Metal M

Solution N

Metal M Solution N TP 4
A Copper Silver nitrate
B Magnesium Copper (II) sulphate
C Zinc Copper (II) nitrate
D Magnesium Iron (II) chloride

3.

Which of the following statements is true about the above equation?
[Relative atomic mass: Mg = 24 ; Fe = 56 ; Cl = 35.5]
A 200 kJ heat is released when 56 g iron is deposited in the reaction.
B 12.5 kJ heat is released when 7 g iron is deposited in the reaction.
C 20 kJ heat is released when 0.1 g iron is displaced by magnesium.
D 2 kJ heat is released when 0.01 g magnesium is soluble in solution.

4. In an experiment, 6.5 g zinc powder is added into 100 cm3 iron (II) sulphate
solution 2.0 mol dm-3. The temperature of the mixture increased by 1.0 oC. What
is the heat of displacement in the experiment?
[Specific heat capacity of solution= 4.2 J g-1 °C-1; relative atomic mass, Zn= 65]

A - 0.42 kJ mol-1
B - 0.84 kJ mol-1
C - 4.20 kJ mol-1
D - 8.40 kJ mol-1

5. The reaction between zinc and copper (II) sulphate solution is exothermic.
Which of the following energy level diagrams shows the reaction
mentioned?

Heat Heat

Heat Heat

TP 4

Structure Questions

1. Figure 1 shows the arrangement of the apparatus for an experiment for
determine the heat of displacement of copper.

5 g zinc powder

Plastic 50m cm3, 0.2 mol
cup dm-3 copper (II)
sulphate solution

Diagram 1

The following data is Initial temperature of copper (II) sulphate solution = 28 ˚C
: The highest temperature of the mixture = 36 ˚C
obtained

a) Define the meaning of ‘heat of displacement’. [1 mark] TP 2

b) State two other observations in this experiment. [2 marks] TP 2

c) Why plastic cups were used in the experiment? [1 mark] TP 2

d) Write the ionic equation for the reaction. [1 mark] TP 2

e) Based on the information given in the experiment, calculate;

(i) the heat released in the experiment [1 mark]
[Specific heat capacity of solution= 4.2 J g-1 °C-1;
density of solution = 1.0 g cm-3]

(ii) heat of displacement of copper [2 marks]
f) Draw an energy level diagram for the reaction. [2 marks]

Self- reflection

Tick (/) if you are able to do this.

State the meaning of heat of displacement.
Determining the heat of displacement of metal from its salt solution
Write thermochemical equations for displacement reactions.
Construct an energy level diagram.
Solve calculation problems involving the heat of displacement.

15608x00.2
01.0010

Answer scheme

Objective questions

1. A 2. B 3. A 4. C 5.C

Structural questions
1 a) The change heat released when one mole of metal is displaced from

its salt solution by a more electropositive metal.
b) Brown solids are formed. The intensity of the blue solutions decreases

until they become colorless.
c) To reduce the heat lost to the surrounding.
d) Zn + Cu2+ → Zn2+ + Cu

e) i) H = (50)(4.2)(8)
= 1680 J

ii) No. of mole CuSO4 = 50 x 0.2
1000

= 0.01 mol

Heat of displacement of copper = - 1680
0.01

= -168 kJmol-1
f)

Heat

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