Entropy changes in simple chemical reactions

This sequence of experiments illustrates the principle of entropy, which you might like to check before you look at the practical work. In each of the following cases work out the sign for the entropy change of the system and the sign for the entropy change of the surroundings. Remember each of these reactions takes place, that is, they are spontaneous. This means that the total entropy change in each of the reactions must be positive. Consequently, either the entropy change of the system, the entropy change of the surroundings, or both of these changes must be positive. Make sure that you reach this conclusion in each of the following cases. You can check your answers by clicking on the entropy change conclusions link at the bottom of the page. Several of the following reactions are endothermic. Don't let this give you a false impression - most reactions are exothermic.

1 Dissolving

This simply involves dissolving some solid ammonium nitrate in water. Write an equation for this process and assess the entropy change of the system. Make a note of the temperature change involved from the video, and work out the sign of the entropy change of the surroundings.

Video - dissolving ammonium nitrate solid in water

2 A neutralization reaction

Having watched the video, try to write an equation for the reaction with state symbols. Consider the entropy changes in the system and the surroundings. Remember that the total entropy change must be positive.

Video - the reaction of ethanoic acid and ammonium carbonate

3 A combustion reaction

Again, watch the video, then try to write the equation (this one is easy!), and consider the various entropy changes.

Video - the burning of magnesium ribbon

4 A redox reaction

Sodium nitrite is the old name for sodium nitrate(III), NaNO₂. Given that this reaction is endothermic, work out the sign of the entropy change of the surroundings. Write down the equation for the reaction, and work out the sign of the entropy change of the system. It may help you to know that the gas released is very unreactive.

Video - the reaction of solid ammonium chloride and solid sodium nitrite

5An acid-base reaction

In this reaction, the two solids dissolve in their own water of crystallization. This is followed by a further change. You should be able to detect one of the products from its smell. Watch the video, write an equation for the reaction and assess the signs of the two entropy changes.

Video - the reaction of solid hydrated barium hydroxide and solid ammonium chloride

6 A thermal decomposition

This reaction is clearly not spontaneous at room temperature, indicating that the total entropy change is negative at this temperature. However, it does occur on heating to a much higher temperature. We shall use limewater to detect any released carbon dioxide gas. Again assess the signs of the two entropy changes and write an equation for this reaction.

Video - the thermal decomposition of zinc carbonate

Entropy change conclusions