Chemical energetics

Exothermic and endothermic reactions

Exothermic reactions release heat to the surroundings (temperature rises; $\Delta H$ negative), e.g. combustion, neutralisation. Endothermic reactions take in heat (temperature falls; $\Delta H$ positive), e.g. thermal decomposition, photosynthesis. Breaking bonds takes in energy; making bonds releases it. $\Delta H=\sum(\text{bonds broken})-\sum(\text{bonds made})$. Energy released into water: $Q=mc\Delta T$.