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

Explanation

Hess's law of constant heat summation : The law states that the change in enthalpy for a reaction is the same whether the reaction takes place in one step or a series of steps.
The Hess's law is a direct consequence of the fact that the enthalpy is a state function and so the enthalpy change depends only on the initial and final states of the system and not on the path by which the reaction takes place.
Example :
`A rarr C, Delta H^(@)=DeltaH_(1)`
Directly in a single step.
Reaction can also take place in two steps.
`{:("Step I. " A rarrB","DeltaH^(@)=DeltaH_(2)),("Step II. " B rarr C","DeltaH^(@)=DeltaH_(3)),(bar("Overall " ArarrC"," DeltaH^(@)=DeltaH_(2)+DeltaH_(3))):}`
According to Hess's law,
`DeltaH_(1)=DeltaH_(2)+DeltaH_(3)`