Hess’s Law of Constant Heat Summation

Hess’s 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 more 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)   ;    DH   =   – 94 K Cal.

Formation of CO₂ Via Carbon Monoxide:

C (s)     +   ½ O₂ (g)  ––––––––>     CO (g)    ;         DH₁  =  – 26.4 K Cal.

CO (g)  +   ½ O₂ (g)  ––––––––>   CO₂ (g)   ;    DH₂  =   – 67.6 K Cal.

DH  =  DH₁  +  DH₂      

DH  =  –26.4 + (– 67.6) K Cal.

DH  =  – 26.4 – 67.6 K Cal.

DH  =  – 94 K Cal..