Write the Hess's law of constant heat summation and explain giving example. Mention its uses also.
Answers
Hess's law of constant heat summation 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.
For example, carbon can be oxidised to either directly or in two different steps as given below:
I st method : C(s)+O_2→CO_2 (g)∆H= -94.3kcal
II method : C(s)+〖1/2 O〗_2 (g)→CO(g)∆H_1= -26.0kcal
CO(g)+〖1/2 O〗_2 (g)∆H_2= -68.3kcal
Some of its applications are as follows:
(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.