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

Explanation

Hess.s law : Energy changes remains constant wheather the reactions takes place in the single step or in several steps .
1. Formation of `CO_(2) : CO_(2)` can be formed either in one step or in two steps.
a) `C_(("graphite"))+O_(2)(g) to CO_(2)(g)`, `DeltaH=-393.5` KJ
b) `C_(("graphite"))+(1)/(2)O_(2)(g) to CO(g)`, `DeltaH=-110.5` kJ
`CO(g)+(1)/(2)O_(2)(g) to CO_(2)(g)`, `DeltaH=-283.5` kJ
Total `DeltaH=-393.52` kJ
Reaction . a . is completed in single step and reaction . b . is completed in two steps . But in both the cases energy changes remain, which proves Hess.s law.
II. Formation of `NH_(4)Cl(aq)` :
a) `NH_(3)(g)+H_(2)O(l) to NH_(3)(aq)`, `DeltaH=-35.1` kJ
`HCl(g)+H_(2)O(l) to HCl (aq)`, `DeltaH=-72.9` kJ
`NH_(3)(aq)+HCl(aq) to NH_(4)Cl(aq)`, `DeltaH=-51.5` kJ
Total `DeltaH=-159.5` kJ

b) `NH_(3)(g)+HCl(g) to NH_(4)Cl(g)`, `DeltaH=-176.1` kJ
`NH_(4)Cl(s)+H_(2)O(l) to NH_(4)Cl. (aq)`, `DeltaH=+16.3` kJ
Total `DeltaH=-159.8` kJ
In both reactions (a) and (b) , the heat of formation of `NH_(4)Cl` (aq.) is the same, which proves Hess.s law.
Uses of Hess.s law : It is used to determine
1) Heat of formation
2) Heat of reaction
3) Crystal lattice
4) Transition temperatures of allotropic forms.