Explain Hess's law of constant heat …

ess'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 or a series of steps. The Hess's law can also be stated as the enthalpy change for a chemical reaction is the same regardless of the path by which the reaction occurs.

For example, consider following two paths for the preparation of methylene chloride
Path I :
CH_4​(g)+2Cl_2​(g)→CH_2​Cl_2​(g)+2HCl(g)   ΔH^₁0​=−202.3kJ

Path II :
CH4​(g)+Cl2​(g)→CH3​Cl(g)+HCl(g)ΔH20​=−98.3kJ
CH3​Cl(g)+Cl2​(g)→CH2​Cl2​(g)+HCl(g)ΔH30​=−104.0kJ
Adding two steps
CH_4​(g)+2Cl_2​(g)→CH₂​Cl₂​(g)+2HCl(g)         ΔH₂⁰=−202.3kJ
Thus whether we follow path I or path II, the enthalpy change of the reaction is same. ΔH₁^0​=ΔH₂⁰​+ΔH₃^0​=−202.3kJ