Enthalpy change of a displacement reaction

8. Apparatus and equipment :

Safety spectacle          x 1      Spatula               x 1

25 cm3 pipette           x 1      Balance               x 1

Pipette filler             x 1      Thermometer(0-100 oC)  x 1

Polystyrene cup with lid   x 1       Digital timer          x 1

Weighting bottle         x 1

9. Procedure :

1. An empty polystyrene cup was weighted.

2. 5.9g of zinc powder (in excess)was weighted and then added into the polystyrene cup.

3. The temperature of the copper (II) sulphate solution in beaker was recorded every half minute for 2.5 minutes and then 25.0 cm3 of the copper (II) sulphate solution was pipetted into a polystyrene cup.

   4. Thermometer was put through the hole of the lid. The solution was stirred continuously and the temperature was recorded every half minute for 6 minutes.

10. Observation :

The blue colour of the solution disappears. Brown solid is formed. Some zinc powder remains there.

11. Data, Calculation and Results :

a. Results Table

   b. Calculations

     1&2.              Graph of temperature against time

3. Energy change of solution = 25 x 10-3  x 4.18 x (76.5 - 25)

                              = 5.38175 kJ mol-1

        Energy change of the cup = 2.27 x 10-3 x 1.3 x (76.5- 25)

                             = 0.1519765 kJ mol-1

        Enthalpy change for the quantities used = - (5.38175 + 0.1519765)

                                                            = - 5.5337265 kJ mol-1

        Assumptions :
       (1)  The specific heat capacity of the solution is the same as that of water.

        (2)  Heat is not lost to the surrounding.

4.  No. of mole of CuSO4 = 1 x 25 x 10-3 = 0.025 mol

         Required enthalpy change = - [(5.5337265)/0.025]

                             = - 221 kJ mol-1

Zn (s) + CuSO4(aq) ⎯→ ZnSO4(aq) + Cu(s)   ΔH = - 221 kJ mol-1

12. Conclusion :

It was found that the enthalpy change of a displacement reaction (Between zinc and copper (II) ions) was - 221 kJ mol-1.

13. Answer to questions

1.  % error = {[- 221 – (- 217)]/- 217 }x 100% = 1.84%
  The experimental value is more negative than the accepted value.

2.  Reasons :
  (1) Heat is lost to the surrounding.
  (2) The specific heat capacity of the solution is not the same as that of water.
3.  CuSO4 is not in excess which is used up totally. No matter that more zinc is  
  used or powder is very finely divided, same amount of the solution is used so
  there is same increase in temperature.