Thermochemical

Thermochemistry is the branch of chemistry that studies the heat energy involved in chemical reactions and physical transformations. It focuses on how much heat is released or absorbed during processes and how factors such as temperature, pressure, and phase changes influence energy changes. The concept of enthalpy, a state function representing heat content at constant pressure, is central to the field.

The central quantity is the enthalpy change, ΔH. If ΔH is negative, the process is exothermic and releases heat; if positive, it is endothermic and absorbs heat. Calorimetry is the primary experimental method for measuring heat changes, using devices called calorimeters to isolate heat transfer.

Common topics include standard enthalpies of formation, Hess’s law, and bond enthalpies. These tools allow estimation of reaction enthalpies from known data. Calorimetric methods include constant-pressure calorimetry (such as a coffee-cix calorimeter) and constant-volume calorimetry (bomb calorimeter). Standard conditions, typically 25°C and 1 atmosphere, provide reference values for many enthalpy measurements.

Thermochemistry applies to chemical reactions, phase transitions, dissolution processes, and biological reactions. It supports fields such as materials science, geology, environmental science, and biochemistry by predicting energy requirements for synthesis, energy release in combustion, and the thermal behavior of systems.

In relation to broader thermodynamics, enthalpy is a state function with ΔH = ΔU + Δ(PV). For ideal gases, the enthalpy change depends on temperature and on changes in the amount of gas present. The temperature dependence of ΔH is described by heat capacities, with ΔH approximately equal to the integral of Cp with respect to T at constant pressure.