State and explain Hess's law.

Explanation

Hess.s law : It states that the tota amount of heat evolved or absorbed in a reaction is the same whether the reaction takes place in one step or in a number of steps. For example. When carbon burns to form `CO_(2)` the heat changes are given as below.
`C(s) + O_(2) (g) to CO_(2) (g) , Delta Hk = - 393.5 kJ`
This reactions occurs in one step. This reaction can be written in two steps.
(i)` C(s) + (1)/(2) O_(2) (g) to CO(g) , Delta H_(1) = - 110.5 kj`
(ii) `CO(g) + (1)/(2) O_(2) (g) to CO_(2) (g) , Delta H_(2) = - 283.0 kj`
In the first step, carbon burns to form carbon monoxide. Then in, second step, carbon monoxide burns to form carbon dioxide.
The total heat evoled during formation of `CO_(2)` will come out by adding equation (i) and (ii) `Delta H = Delta H_(1) + Delta H_(2) = (-110.5) + (-285.0) = - 393.5 kj`
The value of `Delta H` is ame whether the reaction takes place in one step or in two steps.