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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 $$-2,220\text{ kJ/mol}$$.