thermochemistry

Thermochemistry is the branch of chemistry that studies the heat evolved or absorbed during chemical reactions and phase changes. It focuses on the energy changes accompanying chemical bonds forming and breaking and is rooted in the first law of thermodynamics, which states that energy is conserved. The key quantity is the enthalpy change (ΔH) at constant pressure, which approximates the heat exchanged with the surroundings. When ΔH is negative, the process is exothermic; when positive, it is endothermic.

Calorimetry is used to measure heat changes. Calorimeters, including bomb calorimeters for combustion, determine q by monitoring temperature changes. For reactions at constant pressure, q = ΔH. Standard enthalpies of formation (ΔHf°) allow estimation of reaction enthalpies via Hess's law: ΔHrxn = sum of products ΔHf° − sum of reactants ΔHf°. Bond energies provide a rough, qualitative estimate from bond breaking and formation.

Phase transitions such as fusion and vaporization have characteristic enthalpies (latent heats). Thermochemistry also encompasses heats of reaction in solution, combustion, corrosion, metabolism, and polymerization, and is essential in fields from engineering to environmental science.

Applications include predicting reaction feasibility and energy requirements in industrial processes, understanding metabolic heat production, evaluating fuel efficiency, and modeling environmental phenomena. The discipline intersects with kinetics and thermodynamics, helping connect microscopic bond changes to macroscopic heat flows.