Since Enthalpy is a state function (independent of path) we are only concerned with the initial state of the reactants and the final state of the products. It does not matter if this happens in one step, or in multiple steps, the overall change in enthalpy will be the same. This means the enthalpy change for an overall chemical reaction can be calculated by manipulating a series of chemical reactions. Two examples of such a process are shown below.
5.6 Using Hess’s Law to Determine the Enthalpy of a Reaction Example Problem
5.6 Using Hess’s Law to Determine the Enthalpy of a Reaction Example Problem #2