This law is given by a Russian Chemist, G. H. Hess, in 1840. The law is known after his name as Hess’s Law.
It states that:
“If a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature”.
In general, if enthalpy of an overall reaction A ➝ B along one route is ∆H and H1, H2, H3……
Representing enthalpies of reactions leading to same product, B along another route, if the reaction is taking place in three steps, then we have
∆H = H1 + H2 + H3
It can be represented as:
Example : Combustion of carbon dioxide can be done in two ways :
Path I: C(s) + O2(g) ➝ CO2(g) ∆H = -393.5 kJ
Thus, the heat of reaction in single step and that in two steps is same. This proves Hess’s Law.
Uses of Hess’s Law of constant Heat:
(i) Determination of transition : It helps in the determination of enthalpy of transition during allotropic modification.
(ii) Determination of enthalpy of formation : It helps in the determination of enthalpy of formation which cannot be determined experimentally e.g. it is not possible to calculate enthalpy of formation of CO experimentally, but can be calculated by Hess’s law.
(iii) Bond Energy : It may be defined as, “The quantity of heat evolved when a bond is formed between two free atoms in a gaseous state to form a molecular product in a gaseous state”. It is also known as enthalpy of formation of the bond. It may also be defined as, “The average quantity of heat required to break (dissociate) bonds of that type present in one mole of the compound”.
