 # Heat of Displacement

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Heat of Displacement - Energy changes in metal displacement reactions
When a more reactive (electropositive) metal displaces a less reactive metal from a solution of its salt, heat is given out. The heat of displacement is the heat change when one mole of metal is displaced from its salt solution by a more reactive metal. It can be determined by measuring the heat change occurred when a more reactive metal is added in excess to a known quantity of a salt of a less reactive metal.

The heat of displacement of a metal is different when it is displaced by different metals in the reactivity series. For example, the heat of displacement of copper by magnesium is higher than the heat of displacement of copper by zinc. This is because magnesium is more reactive than zinc.

Method
First draw up a table to record the temperature change before and after adding the zinc powder.
1, Measure 25 cm3 of 1M of copper sulfate solution and pour into a polystyrene cup.
2. After a few minutes, measure the initial temperature of the solution and record the temperature.
3. Weigh 6 g of zinc powder in a weighing bottle.
4. Add the zinc powder quickly and carefully into the copper(II) sulfate solution.
5. Stir the mixture in the polystyrene cup using the thermometer.
6. Record the highest temperature reached.

Questions
1. After the maximum temperature is reached, why does the temperature eventually begin to fall?

2. Would you expect the maximum temperature reached to be higher or lower if we repeat the above experiment with magnesium instead of zinc? Why?

3. What, if any, are the parallels between the heat energy produced in these reactions and the electrical energy generated in the appropriate electrochemical cells?

4. How can the reaction be monitored continuously?

• Show Solutions
1. The temperature increases because the reaction is exothermic. The temperature rises until the reaction is complete. Once the reaction stops, the temperature will begin to fall because heat will be lost to the surroundings.
2. I would expect the maximum temperature reached to be higher because magnesium is more reactive than zinc.
3. Combinations of metals that are further apart in reactivity series will give out more heat. This is similar to electrochemical cells; where combinations of metals that are further apart in reactivity series used for the electrodes will give a higher voltage.
4. We can use a temperature sensor linked to a computer.

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