Exothermic Reaction
An exothermic reaction releases energy, usually as heat or light, to its surroundings; burning fuel and rusting iron are exothermic.
Definition
An exothermic reaction releases energy, usually as heat, to its surroundings, so nearby temperatures rise and the products end up holding less stored chemical energy than the reactants did. The energy change is written as a negative enthalpy, $\Delta H < 0$, and combustion, most oxidation, neutralization, and many synthesis reactions fall into this category. On a reaction energy diagram the products sit below the reactants, and the activation energy of the reverse reaction equals the forward activation energy plus $|\Delta H|$; exothermicity favors spontaneity through $\Delta G = \Delta H - T\Delta S$, though a reaction with a large entropy decrease can still be exothermic yet nonspontaneous at high temperature.
Example
A campfire is exothermic, releasing heat and light as wood burns, and a hand warmer heats up because the iron inside is reacting with oxygen. Burning methane releases $890\text{ kJ}$ per mole, $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + 890\text{ kJ}$, and mixing hydrochloric acid with sodium hydroxide warms the solution as the two neutralize. The thermite reaction, $Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3$, releases $852\text{ kJ}$ per mole of $Fe_2O_3$, enough to melt the iron it produces (above $1538^\circ\text{C}$), which is why it is used to weld railroad rails in the field.
Key Insight
"Exo" means out, like "exit," so an exothermic reaction sends heat out, but that does not mean it starts itself: a match will not light on its own, since the initial spark gets the first molecules over the energy hill and the reaction's own heat then keeps it going. By Le Chatelier's principle, raising the temperature of an exothermic equilibrium actually shifts it back toward reactants, which is the dilemma behind the Haber process: it is exothermic, so low temperature favors ammonia, but low temperature also makes the reaction too slow, and the compromise runs at about $450^\circ\text{C}$ with a catalyst.