Question

State and explain Hess's law of constant heat summation? Give its application.

Answer 1

It states that, “The amount of heat evolved or absorbed in a chemical change is the same whether the process takes place in one step or in several steps”.

(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.