Hess's Law of Heat Summation           
When thermochemical equations are added to give some new equation, their values of Ho are also added to give the Ho of the new equation.
The enthalpy change for a reaction is a state function. Its value is determined only by the enthalpies of the initial and final states of the chemical system, and not by the path taken by the reactant as they form the products. To appreciate the significance of this, let us consider again the combustion of carbon.
          C(s) + O2(g) -------> CO2(g) Ho = -393.5 kJ
This is only one possible way to make CO2.
The second pathway to CO2 involves two steps. The first is the combination of carbon with just enough oxygen to form carbon monoxide. Then, in the second step, this CO is burned in additional oxygen to produce CO2. Both steps are exothermic, and their thermochemical equations are:
             C(s) + ½O2(g) ------> CO(g) Ho = -110.5 kJ
         CO(g) + ½O2(g) ------> CO2(g) Ho = -283.0 kJ
Note please that if we add the amount of heat liberated in the first step to the amount released in the second, the total is the same as the heat given off by the one-step reaction that was described first.
                            (-110.5 kJ) + (-283.0) = -393.5 kJ