It was first put forward in 1840 by the Swiss-born Russian chemist Germain Henri Hess (1802-50).

If reactants can be converted into products by a series of reactions, the sum of the heats of these reactions is equal to the heat of reaction for direct conversation from reactants to products.

The law can be used to obtain thermodynamic data that cannot be measured directly.

Sometimes called the law of constant heat summation and is a consequence of the law of conservation of energy.