Question

State and explain Hess's law of constant heat summation by with a suitable example.

Answer 1

Hess's law of heat summation:- It states that "the heat change in a particular reaction is always constant and is independent of the manner in which the reaction takes place". In other words, the overall heat change in a chemical reaction carried out either at constant pressure (or at constant volume) is the same irrespective of that fact whether the reaction proceeds in one or several stages.

Example:- Let us consider the formation of CO₂ from C and O₂. The carbon might be burnt directly to CO₂. On the other hand, carbon might be burnt to carbon monoxide and then the latter burnt to carbon dioxide.

First procedure:- (in one step only)

C (s) + O₂ (g) → CO₂ (g) ΔH = 393.5 kJ

Second procedure

Adding

Thus, it is seen that the two-step procedure evolves the same amount of heat as the one step.