Thermochemistry
The study of the energy transferred as heat during the course of chemical reactions is
called thermochemistry. Thermochemistry is a branch of thermodynamics because
a reaction vessel and its contents form a system, and chemical reactions result in the
exchange of energy between the system and the surroundings. Thus we can use
calorimetry to measure the energy supplied or discarded as heat by a reaction, and can
identify q with a change in internal energy if the reaction occurs at constant volume or
with a change in enthalpy if the reaction occurs at constant pressure. Conversely, if we
know ΔU or ΔH for a reaction, we can predict the heat the reaction can produce.