Thermochemistry: Why Reactions Give Off Heat or Absorb It

Thermochemistry studies how chemical reactions release or absorb heat energy.
Exothermic reactions give off heat—like burning wood or fireworks exploding.
Endothermic reactions absorb heat—like ice melting or photosynthesis.
Heat in reactions comes from breaking and forming chemical bonds.
If bonds release more energy when forming than they need to break, the reaction is exothermic.
If breaking bonds requires more energy than forming new ones releases, it’s endothermic.
Enthalpy (ΔH) measures the heat change of a reaction at constant pressure.
Thermochemistry helps design efficient fuels, engines, and industrial processes.
Cold packs and hand warmers are everyday examples of controlled thermochemical reactions.
From campfires to living cells, thermochemistry explains how energy powers change.