thermochemistry
/ thur-moh-KEM-iss-tree /
Thermochemistry is the branch of chemistry that keeps track of the heat reactions give off or take in. Every chemical change comes with an energy bill — burning, dissolving, freezing, neutralizing — and thermochemistry is the careful accounting of those bills. It is what lets us say how much heat a gram of sugar yields, how much energy a battery stores, or how much warmth a campfire delivers.
It applies the first law of thermodynamics to chemical reactions, centering on enthalpy as the natural measure of reaction heat at constant pressure. Its toolkit is built around a few sturdy ideas: measure heat with calorimetry, tabulate it as standard enthalpies of formation, combine reactions with Hess's law, and estimate from bond enthalpies. Because enthalpy is a state function, these tools always agree on the same answer.
Thermochemistry answers the practical question "how much heat?" — but, on its own, not the question "will it happen?". For that you also need entropy and free energy, which weigh the disorder of a change alongside its heat. Thermochemistry is the first, indispensable half of the energy story of chemistry.
The "calories" on a food label are a thermochemical measurement: they report how much heat the food releases when fully burned, telling you the chemical energy your body can draw from it.
Food calories are thermochemistry in everyday life — the heat content of what we eat.
Thermochemistry is a part of the broader subject of thermodynamics. It focuses specifically on the heat of chemical and physical changes (mostly enthalpy), whereas full thermodynamics also tackles entropy, free energy, and the direction in which changes are driven.