<P> Hess's law of constant heat summation, also known as Hess's law (or Hess' 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 chemical reaction is the same whether the reaction is made in one step or in several steps . </P> <P> 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). Reaction enthalpy changes can be determined by calorimetry for many reactions . The values are usually stated for processes with the same initial and final temperatures and pressures, although the conditions can vary during the reaction . 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 . </P> <P> Hess's law states that the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states . </P>

Applications of hess's law of constant heat summation