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 chloridePath I : \\\\(\\displaystyle CH_4(g) + 2Cl_2(g) \\rightarrow CH_2Cl_2(g) + 2HCl (g) \\: \\: \\: \\Delta H^0_1=-202.3 \\: kJ \\\\)Path II : \\\\(\\displaystyle CH_4(g) + Cl_2(g) \\rightarrow CH_3Cl(g) + HCl (g) \\: \\: \\: \\Delta H^0_2=-98.3 \\: kJ \\\\)\\\\(\\displaystyle CH_3Cl(g) + Cl_2(g) \\rightarrow CH_2Cl_2(g) + HCl (g) \\: \\: \\: \\Delta H^0_3=-104.0 \\: kJ \\\\)Adding two steps\\\\(\\displaystyle CH_4(g) + 2Cl_2(g) \\rightarrow CH_2Cl_2(g) + 2HCl (g) \\: \\: \\: \\Delta H^0=-202.3 \\: kJ \\\\)Thus whether we follow path I or path II, the enthalpy change of the reaction is same. \\\\(\\displaystyle \\Delta H^0_1=\\Delta H^0_2+\\Delta H^0_3=-202.3 \\: kJ \\\\)