Enthalpy – Displacement

Aim

The purpose of this experiment is to determine the enthalpy change for the displacement reaction:

Introduction

By adding an excess of zinc powder to a measured amount of aqueous copper(II) sulfate, and measuring the temperature change over a period of time, you can then calculate the enthalpy change for the reaction.

Requirements

  • safety glasses
  • measuring cylinder, 25 cm3
  • pipette filler
  • polystyrene cup with lid
  • copper(II) sulphate solution, 1.00 mol dm-3
  • weighing boat
  • spatula
  • zinc powder
  • balance
  • thermometer, 0 – 100°C (0.2 °C graduations)
  • stopwatch

Procedure

  1. Add 25.0 cm3 of the copper(II) sulphate solution into a polystyrene cup.
  2. Weigh about 6g of zinc powder in the weighing bottle. Since this is an excess, there is no need to be accurate.
  3. Put the thermometer through the hole in the lid, stir and record the temperature to the nearest 0.1°C every 30 seconds for 2 minutes.
  4. At precisely 3 minutes, add the zinc powder to the cup.
  5. Continue stirring and record the temperature every 30 seconds for an additional 6 minutes. Recording each value in the results table.

 

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Calculations

  1. Plot the temperature (y-axis) against time (x-axis).
  2. Extrapolate the curve to when the metal was added to the solution (3 minutes). This is to establish the theoretical maximum temperature rise.

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  1. Calculate the enthalpy change for the quantities used, making the same assumptions as in the preceding exercise.
  2. Calculate the enthalpy change for one mole of Zn (s) and CuS04 (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:

Percentage error calculations

  1. List some possible reasons for any difference between your value and the accepted value.