Hess's Law
Hess Law of Constant Heat Summation is also known as Second Law of Thermochemistry. The law states that the total heat change accompanying a chemical reaction is the same whether the reaction takes place in one step or multiple steps.
Example: Carbon dioxide can be formed directly from carbon and also from carbon via carbon monoxide. The heat change involved in both the process are found to be same.
Formation of CO2 directly-
C (s) + O2 (g) → CO2 (g) ΔH = – 94 K Cal.
Formation of CO2 Via Carbon Monoxide
C (s) + 1/2 O2 (g) → CO (g) ΔH1 = – 26.4 K Cal.
CO (g) + 1/2 O2 (g) → CO2 (g) ΔH2 = – 67.6 K Cal.
ΔH = ΔH1 + ΔH2
ΔH = –26.4 + (– 67.6) K Cal.
ΔH = – 26.4 – 67.6 K Cal.
ΔH = – 94 K Cal.
Application of Hess Law
Hess' law finds its application in determining the heat of changes for reactions for which experimental determination is not possible. Some important applications are given below
1. Calculation of Enthalpy of Formation
2. Calculation of Enthalpy of Allotropic Transformation
3. Calculation of Calorific Value
