State and explain “Hess law of constant heat summation” with example.

Hess law states that “the total enthalpy change in a reaction is the same whether the chemical reaction takes place in a single step or in several steps”. It is based on first law of thermodynamics.
Let us consider D is formed from A in two different paths.

$$\begin{gathered} \text{ PATH - I : }\mathrm{A}\to \mathrm{D},\mathrm{\Delta H}=\mathrm{Q} \\ \text{ PATH - II : }\mathrm{A}\to \mathrm{B},\mathrm{\Delta H}={\mathrm{q}}_1 \\ \mathrm{B}\to \mathrm{C},\mathrm{\Delta H}={\mathrm{q}}_2 \\ \mathrm{C}\to \mathrm{D},\mathrm{\Delta H}={\mathrm{q}}_3 \end{gathered}$$
According to Hess law, Q = q₁ + q₂ + q₃
Eg : CO₂ can be obtained from C and O₂(g) in two different ways.
PATH – I :
C_(graphite) + O₂ $\to$CO_2(g) : $∆$H = -393.52KJ
PATH - II :
C_(graphite) + ½O₂ $\to$CO_(g) ; $∆$H = -110.5KJ
CO_(g) + ½O_2(g) $\to$CO₂ : $∆$H =-283.02KJ
Total heat change in PATH-II = –393.52KJ.
Enthalpy change in both cases is same.