Exothermic and endothermic reactions

The enthalpy of a reaction can be positive or negative, depending on $\sum H_{products}$ and $\sum H_{reactants}$.

Thus $\sum H_{products} > \sum H_{reactants}$: $\Delta_r H$ is positive, which signifies the reaction is endothermic. On the other hand, if $\sum H_{products} < \sum H_{reactants}$, $\Delta_r H$ is negative, which means that heat is released and the reaction is exothermic.

For example,

$\mathrm{N_2(g)} + 2\,\mathrm{O_2(g)} \longrightarrow 2\,\mathrm{NO_2(g)}, \quad \Delta_r H = 66.4\ \mathrm{kJ}\ \text{(endothermic)}$ …