## Thursday, 26 February 2015

### 4.3 Heat of Displacement

Heat of displacement
■ Heat of displacement

The heat change that occurs when one mole of metal salt solution displaced at standard conditions by a more electropositive metal.

Example of displacement reaction:
 ○ Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s), ∆H = -210kJ mol-1
■ This video contains information regarding heat of displacement.

Worked-example 4.3(a)
50cm3 of 1.0mol dm-3 copper (II) chloride, is poured into a polystyrene cup and the initial temperature of the solution is recorded. 4g of zinc powder (in excess) is added to the copper (II) chloride. The mixture is stirred slowly, and the highest temperature recorded. The experimental results are as follows:
The initial temperature = 28°C
The highest temperature of mixture = 69°C
[Specific heat capacity = 4.2Jg-1 °C-1, density of a solution of 1g cm-3]
(a) Calculate the heat of displacement of copper by zinc metal salt solution.
(b) Draw an energy level diagram for the reaction that occurs.

Solution:
(a) Step 1 Write chemical equation
 ○ CuCl2(aq) + Zn(s) → ZnCl2(aq) + Cu(s) ○ Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Step 2 Calculate heat change = mc θ
= 50 X 4.2 X (69 – 28)
= 8610J
Step 3 Calculate the number of moles of Cu2+
= $\frac{\text{MV}}{1000}$
= $\frac{50\mathrm{×}1}{1000}$
= 0.05mol
Step 4 Calculate the heat of reaction by linking the number moles of reactants with the heat change which occurs
0.05 mol of copper chloride react with excess zinc to produce 8610 J
Thus, 1 mol of copper chloride react with 1 mol of zinc to produce
= $\frac{8610}{0.05}$
= 172200J
= 172.2kJ
Heat of displacement of copper from copper(II) chloride solution, ∆H = -172.2kJ mol-1
(b)

Worked-example 4.3(b)
About half a spatula of excess magnesium powder is added to 50cm3 of iron (II) chloride 0.25mol dm-3 and the mixture is stirred with a thermometer. Changes of temperature during the reaction is recorded. The following results were obtained.
The initial temperature of the iron (II) chloride = 28°C
The highest temperature reached by the mixture = 40°C
(a) State two precautions that must be taken during the experiment.
(b) Name a suitable container that can be used to perform the reaction between magnesium and iron (II) chloride. Give reasons for your selection
(c) What is the heat change in the experiment?
(d) Calculate the heat change when one mole of iron displaced in the reaction between magnesium and iron (II) chloride.
[Specific heat capacity = 4.2Jg-1 °C-1, density of a solution of 1g cm-3]

Solution:
 (a) The magnesium powder must be poured quickly into the iron(III) chloride solution. The solution mixture in the polystyrene cup is stirred throughout the activity to ensure an even temperature. (b) A polystyrene cup is used to reduce the loss of heat to the environment. (c) Heat change = mc θ = 50 X 4.2 X (40 – 28) = 2520J (d) Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) Number of moles of Fe2+ = $\frac{\text{MV}}{1000}$ = $\frac{50\mathrm{×}0.25}{1000}$ = 0.0125mol 0.0125 mol of ferum(II) chloride react with excess magnesium to produce 8610 J Thus, 1 mol of ferum(II) chloride react with 1 mol of magnesium to produce = $\frac{2520}{0.0125}$ = 201600J = 201.6kJ Heat of displacement of iron from iron(II) chloride solution, ∆H = -201.6kJ mol-1