Question

When is the heat evolved or absorbed in certain chemical reactions?

Answer 1

Heat is evolved in the reaction when:

1. When the formation of bonds occurs during a chemical reaction.
2. These are known as exothermic reactions.

For example:

$\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+393.5\mathrm{KJ}$

Heat is absorbed in the reaction when:

1. When the breaking of bonds occurs during a chemical reaction.
2. These are known as endothermic reactions.

For example:

The melting of ice to form water.

Hence, In endothermic reactions, heat is absorbed and in exothermic reactions, heat is evolved.