What is Hess Law ? Explain.

We have already seen that heat changes in chemical reactions are equal to the difference in internal energy (∆E) or heat content (∆H) of the products and reactants, depending upon whether the reaction is studied at constant volume or constant pressure. 

Since ∆E and ∆H are functions of the state of the system, the heat evolved or absorbed in a given reaction must be independent of the manner in which the reaction is brought about. Thus it depends only on the initial state and final states of the system and not the manner or the steps in which the change takes place.

 This generalization is known as Hess’s Law and maybe stated as:  If a chemical change can be made to take place in two or more different ways whether in one step or two or more steps, the amount of total heat change is the same no matter by which the method the change is brought about.