Question

Separate out exothermic and endothermic process from the following:

Temperature drops when ammonium nitrate dissolves in water.

Answer 1

Energy changes during a reaction

The reactions that involve the absorption of heat are called endothermic reactions. The reactions that involve the evolution of heat are called exothermic reactions.

The heat evolved or absorbed depends on the energy difference between the reactants and products. If the energy of the products is greater than the reactant, energy will be produced during the conversion, which is liberated in the form of heat. The reactant will absorb energy if the energy of the products is higher than that of the reactants.

Dissolution of ammonium nitrate

When ammonium nitrate $\left({\mathrm{NH}}_4{\mathrm{NO}}_3\right)$ is dissolved in water, it dissociates into ammonium ions $\left({\mathrm{NH}}_4^{+}\right)$ and nitrate ions $\left({\mathrm{NO}}_3^{-}\right)$. The ionic bond between the ions is broken, and the dissolution can be represented as:

$$\begin{gathered} {\mathrm{NH}}_4{\mathrm{NO}}_3\left(\mathrm{s}\right)+\mathrm{Heat}\to {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{NO}}_3^{-}\left(\mathrm{aq}\right) \\ \left(\mathrm{ammonium} \\ \mathrm{nitrate}\right)\left(\mathrm{ammonium}\mathrm{ion}\right)\left(\mathrm{nitrate}\mathrm{ion}\right) \end{gathered}$$

The temperature drops because heat is absorbed by the reactants. This causes a decrease in heat content in the surroundings, causing a decrease in temperature. This means that the energy of the products is higher than the reactants.

Therefore, it is an endothermic process.