热化学（英语：Thermochemistry）是以热力学的观点研究化学，是化学的一支，也是物理学中热学在化学中的应用。

人类在生物质能（Biomass）上对热化学技术的应用，可说是从人类始祖懂得生火开始。经过100多万年的演化，热化学技术在生质能上的应用也纯粹从火的使用，衍生到目前的焚化、气化、热解、液化等等的工业技术的应用。

Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.