Use adaptive quiz-based learning to study this topic faster and more effectively.

# Endothermic and exothermic reactions

A reaction is classified as either endothermic or exothermic based on the enthalpy change ($\Delta H$) during the reaction.

An endothermic reaction absorbs heat at constant pressure and has a positive $\Delta H$. In such a reaction, heat is converted to chemical energy stored in bonds.

$$\ce{N2 {(g)} -> 2N {(g)}}\hspace{5pt}\Delta H = 472.7\text{ kJ/mol}$$

The splitting of nitrogen gas ($\ce{N2}$) into nitrogen atoms ($\ce{N}$) is endothermic and requires $472.7\text{ kJ/mol}$ of heat. The $\Delta H$ is $472.7\text{ kJ/mol}$.

An exothermic reaction releases heat and has a negative $\Delta H$. In such a reaction, chemical energy from bonds is converted to heat.

$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}\hspace{5pt}\Delta H = -2,220\text{ kJ/mol}$$

The combustion of propane ($\ce{C3H8}$) is exothermic and releases $2,220\text{ kJ/mol}$ of heat. The $\Delta H$ is $-2,220\text{ kJ/mol}$.