State Hess's law of constant heat summation and explain it with an example.

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Hess'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 :
$C H_{4} (g) + 2 C l_{2} (g) \to C H_{2} C l_{2} (g) + 2 H C l (g) Δ H_{1}^{0} = - 202.3 k J$
Path II :
$C H_{4} (g) + C l_{2} (g) \to C H_{3} C l (g) + H C l (g) Δ H_{2}^{0} = - 98.3 k J$
$C H_{3} C l (g) + C l_{2} (g) \to C H_{2} C l_{2} (g) + H C l (g) Δ H_{3}^{0} = - 104.0 k J$
Adding two steps
$C H_{4} (g) + 2 C l_{2} (g) \to C H_{2} C l_{2} (g) + 2 H C l (g) Δ H^{0} = - 202.3 k J$
Thus whether we follow path I or path II, the enthalpy change of the reaction is same. $Δ H_{1}^{0} = Δ H_{2}^{0} + Δ H_{3}^{0} = - 202.3 k J$

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