What is thermochemistry?

Thermochemistry is the study of energy transfer with regard to physical and chemical reactions.
Thermochemistry is a sub-branch of thermodynamics, the study of energy transfer. It developed from the study of heat engines (specifically steam engines), in the 18th and 19th centuries.
Two fundamental principles of thermochemistry are: (i) the energy change associated with any process is equal and opposite to the reverse process (due to Laplace), and (ii) the energy change for a series of stepwise processes or reactions is the same as that of the entire process (Hess’ law). (These follow fundamental laws of thermodynamics, the first being ” You can’t win “, and the second, ” You can’t break even either “. )
These are posed as laws rather than theories in that there has never been an observed physical or chemical process that has violated these laws. This means that when you hear of spectacular claims of free energy, you can dismiss them out of hand.