Hess's Law

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Hess's law

[′hes·əz ‚lȯ]
(physical chemistry)
The law that the evolved or absorbed heat in a chemical reaction is the same whether the reaction takes one step or several steps. Also known as the law of constant heat summation.

Hess’s Law


a basic law of thermochemistry, according to which the thermal effect of a reaction depends solely on the initial and final states of the system and not on the intermediate states or the paths of transition.

Hess’s law was discovered by H. Hess (in Russian, G. I. Gess) in 1840 on the basis of experimental investigations. It is one of the forms of the law of conservation of energy as applied to chemical reactions, which was discovered later, and is related to processes occurring with constant volume or constant pressure. Hess’s law is widely used for calculating the thermal effect of a given process on the basis of experimental data relating to other processes (including processes that are virtually unattainable under the given conditions). Thus, for 298.15° K the heat of formation of carbon monoxide (ΔH0x kilocalories per mole [kcal/mole]) from graphite can be calculated, knowing that the heats of combustion ΔH0comb carbon monoxide and graphite to CO2 at this temperature are −282.99 and −393.32 kilojoules per mole (kJ/mole), or −67.635 and −94.051 kcal/mole, respectively. Examining two paths of the formation of CO2 from graphite during its direct combustion to CO2 and during the intermediate formation of CO, and knowing that according to Hess’s law the total thermal effect of both transformations should be identical, we find ΔH0x = −(94,051 - 67.635) = −26.416 kcal/mole (heat is discharged).