Exothermic and Endothermic Reactions

Every Chemical Transformation Exchanges Energy

When methane burns in air, the reaction releases heat: the flame heats a pot, a room, the surrounding air. When you dissolve ammonium nitrate in the water of a beaker, the opposite happens: the beaker becomes cold to the touch. In both cases, a chemical reaction is accompanied by an exchange of energy with the surroundings.

Two Opposite Behaviors

• A reaction is exothermic when it releases energy to the surroundings, most often in the form of heat. Combustion, acid-base neutralization reactions, and cellular respiration are exothermic.
• A reaction is endothermic when it absorbs energy taken from the surroundings. The dissolution of ammonium nitrate, photosynthesis, and the melting of ice are endothermic.

A Clue, Not Absolute Proof

The change in temperature of the surroundings is a practical clue: it increases for an exothermic reaction (the released energy warms the surroundings), it decreases for an endothermic reaction (energy is drawn from the surroundings, which cools down). But the real, rigorous indicator is the energy exchanged at constant pressure, called the reaction enthalpy, denoted ΔrH.

Common Mistake

Do not confuse "fast reaction" with "exothermic reaction": the speed of a reaction (its kinetics) and the direction of its energy exchange (its thermochemistry) are two completely independent notions. A reaction can be very slow and yet strongly exothermic (the rusting of iron, for example), or very fast and endothermic.