Hess’s Law states that the total enthalpy of a system stay the same throughout a chemical reaction whether said reaction is comprised of one step or of many steps. It is a common relationship in physical chemistry that describes the differences in reactants and products throughout chemical reactions. Hess’s Law was named after Germain Hess who published the law in 1840.
Hess’s Law is also known as the law of constant heat summation due to the law stating that the sum of the enthalpy of each reaction step should equal the heat absorbed or evolved even if the reaction takes place in only one step. Due to Hess’s Law, chemistry students can analytically find the total enthalpy of any chemical reaction.The law is especially helpful to calculate enthalpies of reactions that are difficult to measure chemically due to the reactions being difficult to control. Hess’s Law allows students and researchers to estimate the enthalpy of reactions by adding together all of the enthalpies of formations for all of the reactants. Each common reactant in the chemistry has been assigned an enthalpy of formation. To calculate the enthalpy of a reaction for any chemical reaction, it's necessary to add all of the enthalpies of formation of all of the reactants. The sum of the enthalpies of formation of the reactants is equal to the total enthalpy of the reaction.