Determining an enthalpy change of reaction

Procedure :

Apparatus needed : safety spectacles, a pipette with 25 cm3, a pipette filler, a polystyrene cup with lid, a weighing bottle, a spatula, a thermometer which is started form 0 oC to 100 oC with 0.1 oC graduations, a watch, a balance, a beaker

1. The pipette was washed with pipe water, and then distilled water was used to clean the pipette. Finally, it was cleaned with the solution which would be used later.
2. 25.0 cm3 of the copper(II) sulphate solution was pipetted into a polystyrene cup.
3. About 6 g of zinc powder was weighed in the weighing bottle. As this was provided in excess, there was no need to be accurate.
4. The polystyrene cup was covered by a lid and a thermometer was put through the hole in the lid. The polystyrene cup was put in the beaker.
5. The solution was kept stirring and the temperature was recorded to the nearest 0.1 oC every half minute for 2 1/2 minutes.
6. At accurately 3 minutes, the zinc powder was added to the cup.
7. The mixture was kept on stirring and the temperature was recorded for an additional 6 minutes to complete a copy of Result Table.

Calculation :

Result Table

1. Plot the temperature ( y-axis ) against time ( x-axis ).
2. Extrapolate the curve to 3.0 minutes to establish the maximum temperature rise.

Result :

Initial temperature : 26.0 oC

Maximum temperature : 80.0 oC

Assumption :

1. The heat absorbed by cup ,beaker ,lid , thermometer, air, zinc is neligible.
2. The density of solution = 1 gcm-3 
3. The specific heat capacity of the solution = 4.18 Jg-1K-1

Calculation :

Zu(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

E  =  mcT

   =  (25)(4.18)(80.0-26.0)

   =  5643 J

No. of mole of Cu2+                        No. of mole of Zn

= 25 x 10-3 x 1                    = 6 / 65.4

= 0.025 mol                      = 0.092 mol

Since Cu2+ is a limiting, 0.025 mol of Zn is reacted.

H = -E / N

   = -5643 / 0.025

   = -22.6 kJ mol-1

Discussion :

    In this experiment, we should use the zinc powder instead of zinc granule. Since the zinc powder has a larger surface area, and it can increase the rate of reaction.

    We can use some good insulator to reduce heat loss to the surroundings, such as polystyrene cup. We can also cover the cup by a lid to reduce the heat loss to the surroundings by convection. Our hand should not touch the cup directly, it is used to prevent heat loss to our hands by conduction.

Conclusion :

The enthalpy change of the reaction is – 22.6kJ mol-1.