Question

State and explain Hess’s law of constant heat summation.

Answer 1

1. Hess's law of constant heat summation states that," The change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps".
2. The Hess's law is a direct consequence of fact that the enthalpy is a state function and so enthalpy change depends only on the initial and final states of the system and not on the path by which the reaction takes place.
   eg. The conversion of A to C can take place directly
   in a single step.
   A → C,    ΔH° = ΔH₁
3. The reaction can also take place in two steps for which ΔH°
   values are known.
   Step (1)     A → B,     ΔH° = ΔH₂
   Step (2)    B → C,     ΔH° = ΔH₃
                   ____________________________
   Overall     A → C,     ΔH° = ΔH_{2 +} ΔH₃
4.  According to Hess's law, ΔH₁ = ΔH_{2 +} ΔH_3.
   The sequence of steps is represented in the figure.