Name:
Date:
Determining an enthalpy change of reaction
Aim
The purpose of this experiment is to determine the enthalpy change for the
displacement reaction:
Zn (s) + Cu2+ (aq)  Cu (s) + Zn2+ (aq)
Introduction
By adding an excess of zinc powder to a measured amount of aqueous
copper (II) sulphate, and measuring the temperature change over a period of
time, you can then calculate the enthalpy change for the reaction.
Assessment
You will not be assessed on this practical.
Requirements
 Safety glasses
 Pipette, 25cm3
 Pipette filler
 Polystyrene cup with lid
 Copper (II) sulphate solution, 1.00M CuSO4 – harmful if swallowed!
 Weighing bottle
 Spatula
 Zinc powder
 Balance
 Thermometer, 0-100oC
 Watch or stopclock
Procedure
1. Read through all of the instructions below and construct a suitable results table in
preparation for your results.
2. Pipette 25.0cm3 of the copper (II) sulphate solution into a polystyrene cup.
3. Weigh about 6g of zinc powder in the weighing bottle. Since this is an excess, there is
no need to be accurate.
4. Put the thermometer through the hole in the lid, stir and record the temperature to the
nearest 0.1oC every half minute for 2½ minutes. Create an appropriate data table in
which to record quantitative and qualitative results.
5. At precisely 3 minutes, add the zinc powder to the cup.
6. Continue stirring and record the temperature for an additional 6 minutes.
Calculations
1. Plot the temperature (y-axis) against the time (x-axis) for your results. YOUR GRAPH
MUST BE DONE BY HAND ON GRID PAPER.
2. Extrapolate the curve to 3.0 minutes to establish the maximum temperature rise.
3. Calculate the enthalpy change for the quantities used, making appropriate
assumptions.
4. Calculate the enthalpy change for one mole of Zn and CuSO4 (aq) and write the
thermochemical equation for the reaction.
Questions
1. Compare your result with the accepted value of –217 kJ mol-1 by calculating the
percentage error in your answer:
error = experimental value – accepted value
accepted value
x 100%
2. List possible reasons for any difference between your value and the accepted value.
3. Why do you think the temperature increases for a few readings after adding the zinc?
(Hint: it does not increase if a large excess of zinc is used or if the powder is very
finely divided).