Heat can be released or absorbed during a chemical reaction. This provides a powerful method for studying chemical equilibrium by means of chemical thermodynamics. Thermodynamics is based on a few fundamental postulates, called the first, second, and third laws of thermodynamics. We will discuss these laws first, and then apply them to chemical equilibria.

The first law of thermodynamics; enthalpy

In addition to the macroscopic kinetic and potential energy that a body or system as a whole may possess, it also contains internal energy due to the kinetic and potential energy of its molecules or atoms. Increases in internal kinetic energy in the form of molecular motions are manifested as increases in the temperature of the system, while changes in the potential energy of the molecules are caused by changes in their relative configurations.

Let us suppose that a system of unit mass takes in a certain quantity of heat energy q (measured in joules). As a result, the system may do a certain amount of external work w (also measured in joules). The excess energy supplied to the system, over and above the external work done by the system, is q – w. Therefore, if there is no change in the macroscopic kinetic and potential energy of the system, it follows from the principle of conservation of energy that the internal energy of the system must increase by q – w.