Hess's law is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840.The law states that the total enthalpy change during the complete course of a reaction is same whether the reaction is made in one step or in several steps.
Hess's law is now understood as an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state function). It applies to the special case of paths consisting of chemical reactions (or changes of state) at constant temperature and pressure. Hess's law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation, that may be used as a basis to design complex syntheses.
Read more about Hess's Law: Definition, Example, Extension To Entropy and Free Energy, Applications
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