Proof of Hess’s Law

Hess’s law can be proved on the basis of first law of thermodynamics, according to which heat can neither be created nor destroyed. Let us suppose Q₁ Joules is heat change when A changes to B directly and Q₂ Joules is when A changes B indirectly via C. Let us suppose Q₁ > Q₂. Now if we move from A to B directly and then come back via C, then (Q₁ – Q₂) will be the heat evolved. This means that when we move from A to B and then back from B to A certain amount of heat is created which is against first law of thermodynamics. Thus we conclude, that Q₁ cannot be more than nor less than Q₂, but are equal, thus verifies Hess’s Law.