Experiment: Determining the heat of displacement

Determining the heat of displacement is a Form 5 practical in the Thermochemistry chapter. It brings together displacement reactions in the reactivity series and the measurement of an energy change per mole, and it shows again how a temperature change is turned into a heat of reaction. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward. The observations are qualitative; use your own measured readings for any calculation.

Aim

To determine the heat of displacement when a more reactive metal displaces a less reactive metal from a solution of its salt, by measuring the temperature change in an insulated container.

Apparatus and materials

• Polystyrene cup (with a lid) as an insulated calorimeter
• Thermometer
• Measuring cylinder
• Spatula and balance
• Copper(II) sulfate solution of known concentration
• Zinc powder (the more reactive metal, used in excess)
• Stirrer

Procedure

1. Measure a known volume of copper(II) sulfate solution with a measuring cylinder and pour it into the polystyrene cup.
2. Measure and record the initial temperature of the copper(II) sulfate solution with the thermometer.
3. Weigh out an excess of zinc powder, so that all the copper ions in the solution can be displaced.
4. Add the zinc powder to the copper(II) sulfate solution in the cup all at once, and stir the mixture gently.
5. Watch the thermometer and record the highest (maximum) temperature reached by the mixture.
6. Observe the colour of the solution and any solid that forms during the reaction.
7. Calculate the temperature change, then use it to find the heat released and, dividing by the moles of the salt that reacted, the heat of displacement per mole.

Expected observations

When zinc powder is added to the blue copper(II) sulfate solution, the temperature of the mixture rises to a maximum shortly after mixing and then falls slowly as heat is lost. The blue colour of the solution fades as the copper ions are removed from solution, and a reddish-brown solid (copper) is deposited. Because excess zinc is used, some grey zinc powder remains unreacted. The rise in temperature from the starting value to the highest value is the key measurement for the calculation.

Inference and conclusion

The temperature rises because displacement is an exothermic reaction: the more reactive zinc displaces the less reactive copper from the copper(II) sulfate solution, releasing heat. The fading of the blue colour and the reddish-brown deposit show that copper has been displaced from the solution, and the leftover zinc shows the reactive metal was in excess so the reaction was complete. Using the temperature change, the heat released is calculated with heat released = mass of solution x specific heat capacity x temperature change (mc-theta), and dividing by the moles of salt that reacted gives the heat of displacement per mole. The conclusion is that the displacement of a less reactive metal by a more reactive one is exothermic.

Science process skills (Paper 3 style)

Making an inference. From the observations that the blue colour fades and a brown solid forms, infer that copper has been displaced from the solution by the zinc.

Identifying variables. If the investigation compared different metals displacing copper, the manipulated variable is the metal added, the responding variable is the temperature change, and the controlled variables are the volume and concentration of the copper(II) sulfate solution and the insulated cup.

Operational definition. Define the reaction as complete operationally: the displacement is complete when excess metal remains and the blue colour no longer fades on further stirring.

Tabulating and calculating. Record the initial and highest temperatures and the temperature change, then show the calculation of the heat released and of the heat of displacement per mole, with units.

Safety precautions

• Wear safety goggles, because copper(II) sulfate solution is harmful and an irritant.
• Add the zinc powder carefully so that the mixture does not splash out of the cup.
• Wash any solution splashes off the skin at once with plenty of water, to limit irritation.
• Handle the thermometer gently and do not stir vigorously with it, so that it does not break.
• Dispose of the mixture and the metal residue as instructed, rather than pouring them down the sink.

Common errors

• Not using excess metal. If too little zinc is added, not all the copper ions are displaced and the reaction is incomplete; use excess so the salt solution is the limiting reactant.
• Reading the temperature too late. Heat is lost quickly after mixing, so watch closely and record the maximum temperature.
• Using a glass beaker. Glass conducts heat away and lowers the measured temperature change; use an insulated polystyrene cup with a lid.
• Adding the metal slowly. Adding zinc a little at a time spreads out the temperature rise and lowers the maximum; add it all at once and stir.
• Inventing readings. Use the temperatures you actually measure in the calculation; do not copy values from another source.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can link the observations to the reactivity series, explain why excess metal is used, and set out the calculation of heat per mole clearly. Heat of displacement ties together the reactivity of metals and energy changes across SPM Chemistry, and the same measure-the-temperature-change method reappears in the heat of neutralisation and combustion, and in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).