Wednesday, May 25, 2011

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.. 

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