Energy changes in reversible reactions

A reversible reaction can proceed in either direction: reactants form products in the forward reaction, and the products can change back into reactants in the reverse reaction. The two directions transfer energy in opposite ways.

If the forward reaction transfers energy to the surroundings, it is exothermic. The reverse reaction must take in the same amount of energy from the surroundings, so it is endothermic. Equally, if the forward reaction is endothermic, the reverse reaction is exothermic.

The reverse reaction exactly undoes the chemical change of the forward reaction. For the same amounts of substances under the same conditions, the two energy changes are therefore equal in size but opposite in sign: energy given out in one direction is taken in when the change is reversed.

Energy changes in the copper sulfate reaction

Hydrated copper sulfate contains water. Heating it removes this water and forms anhydrous copper sulfate. This direction is endothermic: it takes in energy from the surroundings.

In the reverse direction, adding water to anhydrous copper sulfate forms hydrated copper sulfate. This direction is exothermic: it transfers energy to the surroundings. For the same amount of copper sulfate, the energy transferred out in this reverse change is equal in size to the energy taken in during the forward change.

Hydration and dehydration of copper sulfate

Blue hydrated copper sulfate contains water within its crystal structure. This water is called water of crystallisation; it is part of the crystals rather than simply water on their surface.

When hydrated copper sulfate is heated, it loses its water of crystallisation. The blue solid changes into white anhydrous copper sulfate. Anhydrous means that it contains no water of crystallisation. The loss of water is called dehydration.

The change is reversible. Adding water to white anhydrous copper sulfate forms blue hydrated copper sulfate again. This gain of water is called hydration.