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

Explanation

Hess.s law states that the total amount of heat evolved or absorbed in a chemical reaction is always same whether the reaction is carried out in one step (or) in several steps.
Illustration : This means that the heat of reaction depends only on the initial and final stages and not on the intermediate stages through which the reaction is carried out. Let us consider a reaction in which A gives D. The reaction is brought out in one step and let the heat of reaction be `triangleH`.
`A to D, triangleH`
Suppose the same reaction is brought-out in three stages as follows
`A to B, triangle H_(1)`
`B to C , triangle H_(2)`
`C to D , triangle H_(3)`

The net heat of reaction is `triangleH_1 + triangleH_2 +triangleH_3`
According to Hess law `triangleH=triangleH_1+triangleH_2+triangleH_3`
Ex : Consider the formation of `CO_2`. It can be prepared in two ways.
1) Direct method : By heating carbon in excess of `O_2.`
`C(s) + O_2(g) to CO_2(g): triangleH"= -393.5 kJ"`
2) Indirect method : Carbon can be converted into `CO_2,` in the following two steps.
`C(s)+1/2 O_(2) (g) to CO_(2) (g) , triangleH_(1)=-110.5kJ`
`CO(g)+1/2 O_(2) (g) to CO_(2) (g) , triangleH_(2)=-283.02kJ`
Total `triangleH=-393.52kJ (triangleH_(1)+triangleH_(2))`
The two `triangleH` values are same.