Question

Separate out exothermic and endothermic process from the following:

Heat is absorbed when water evaporates

Answer 1

Step 1: Energy changes during a reaction

Energy in the form of heat is absorbed or released during a chemical reaction. This energy will be equal to the energy difference between the products and the reactants. The excess energy is released in the form of heat if the energy of the products is lower than the reactants, and the excess energy required is absorbed by the reactants if the energy of products is higher than the reactants.

Reactions that involve the liberation of heat are called exothermic reactions, and the reactions that involve absorption of heat are called endothermic reactions.

Step 2: Evaporation of water

When water evaporates, water $\left({\mathrm{H}}_2\mathrm{O}\right)$ in its liquid state is converted to steam, that is, in its gaseous state. This is a phase change, and energy is absorbed by water in the form of heat during the change. The equation for the phase change is:

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{Heat}\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right) \\ \left(\mathrm{water}\right)(\mathrm{water}\mathrm{vapour}) \end{gathered}$$

Since heat is absorbed by water during the phase change, the energy of water vapour is greater than that of water.

Therefore, it is an endothermic process.