Thursday, 26 February 2015

4.3.1 - Laboratory Activity : Heat of Displacement of Copper

Laboratory Activity 4.3.1:
Heat of Displacement of Copper
Aim: To determine the heat of displacement of copper.
Problem statement: What is heat of displacement?
Hypothesis: Copper will be displaced from copper (II) sulphate by different elements with different values of heat of displacement.
» Fixed variable : Volume and concentration of copper (II) sulphate, mass of metallic elements and polystyrene cup.
» Manipulated variable : Types of metal
» Responding variable : Heat of displacement

» Zinc powder
» Magnesium powder
» Copper (II) sulphate 0.5mol dm-1

» Polystyrene cup
» Thermometer
» Measuring cylinder

The following animation shows the arrangement of apparatus and the observation of the experiment.

1. 25cm3 of copper (II) sulphate is measured by using a measuring cylinder and poured into a clean polystyrene cup.
2. The initial temperature is measured with thermometer and recorded.
3. About 1g of zinc powder, Zn, is poured into the copper (II) sulphate solution.
4. The mixture is stirred with thermometer, and the highest temperature achieved is recorded.
5. The experiment is repeated with magnesium powder to replace zinc.

Metals used for displacement Zinc Magnesium
Initial temperature of copper (II) sulphate (°C) T1 T3
Maximum temperature of the mixture (°C) T2 T4
Increase in temperature (°C) T2 – T1 = ∆θ1 T4 – T3 = ∆θ2

Displacement of copper(II) sulphate with zinc
Heat change = mcΔθ = 25 X 4.2 X ∆θ1 = y joules (Specific heat of solution = 4.2Jg-1 °C-1)
Number of moles of Cu2+ that has been displaced
= MV1000
= 0.5×251000
= 0.0125mol

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
0.0125mol of Cu2+ ion will liberate y J of heat energy.
Thus, 1mol of Cu2+ ion will liberate y0.0125 = xJ = x1000kJ
The heat of displacement of copper from copper(II) ion, ∆H1 = x1000kJ

Displacement of copper(II) sulphate with magnesium
The displacement of by magnesium is calculated in the same way.
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)

Magnesium and zinc are more electropositive than copper in the electrochemical series. Therefore, they will displace copper ions from copper (II) sulphate.

The blue copper(II) sulphate solution fades because more blue coloured copper(II) ions are changing into brown solid, copper metal.
Cu2+(aq) + 2e → Cu(s)

The zinc powder (magnesium powder) dissolves to form zinc ion(magnesium ion) in the solution.
Zn(s) → Zn2+(aq) + 2e
Mg(s) → Mg2+(aq) + 2e

The displacement of copper(II) sulphate with zinc
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) , ∆H1
The displacement of copper(II) sulphate with magnesium
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s) , ∆H2
∆H1 is different from ∆H2 because of the different metals being displace with different quantities of heat energy in different solution.

The energy level diagram for the displacement of copper from it salt is shown in the following diagram.

Precautionary steps:
The initial temperature of the copper (II) sulphate was recorded after the thermometer is left in the solution for several minutes to obtain a constant temperature.
The final temperature recorded must be the highest temperature reached by the mixture after the reaction is complete, and all the heat displacement reaction has been released.
Metal powder is used instead of metal pieces to ensure that the reaction occur quickly, properly and reduce heat loss to the environment.

The experiment value will always be less than the theoretical value, due to the heat lost to the surroundings and hat absorbed by the polystyrene cup.

The heat of displacement of copper ion from copper (II) sulphate by magnesium and zinc are different.

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