Chemical reactions either release heat or absorb it. Exothermic reactions release energy - they feel hot. Endothermic reactions absorb energy - they feel cold. Burning wood is exothermic. Cold packs for injuries are endothermic.

Every reaction has an enthalpy change (ΔH). Negative ΔH means exothermic. Positive ΔH means endothermic.

Why Energy Matters

Understanding energy helps us predict if reactions will happen spontaneously. We'll get to Gibbs free energy later, but it's all connected to thermochemistry.