Question

Separate out exothermic and endothermic process from the following:

Temperature of water increases on adding sulphuric acid to it.

Answer 1

Energy changes during a reaction

Chemical changes in a substance include the breaking or formation of chemical bonds. This causes a change, that is, an increase or decrease in energy. Therefore, the energy of the products of a reaction can have higher or lower energy than the reactants.

• If the products have higher energy, reactants require extra energy for the conversion, which will be absorbed in the form of heat.
• If the products have lower energy, the excess energy of the reactants will be liberated in the form of heat.

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

Dissolution of sulphuric acid in water

When sulphuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ is added to water $\left({\mathrm{H}}_2\mathrm{O}\right)$, it dissociates into hydrogen ions $\left({\mathrm{H}}^{+}\right)$ and bisulphate ions $\left({\mathrm{HSO}}_4^{-}\right)$. The dissolution reaction can be given as:

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{HSO}}_4^{-}\left(\mathrm{aq}\right)+\mathrm{Heat} \\ \left(\mathrm{sulphuric} \\ \mathrm{acid}\right)\left(\mathrm{proton}\right)\left(\mathrm{bisulphate}\mathrm{ion}\right) \end{gathered}$$

The temperature increases due to the evolution of heat during the reaction. Therefore, the energy of the products is less than that of the reactants.

Thus, the given process is an exothermic process.