Hess’s law of Constant Heat Summation

In 1940, Hess, in his law of constant summation, stated that the heat given out or taken in during a chemical reaction is always constant and it is independent of the route, time and any intermediate changes involved. i.e. the heat of reaction is the same whether the reaction occurs in one step or more steps.

It can be inferred that thermochemical equations or data can be added together or subtracted, as in algebraic equations.

To illustrate Hess’s law: The conversion of ice, H₂O_(s) to steam, H₂O_(g) goes through the intermediate liquid state H₂O_(l)

Given:

i. H₂O_[s]   → H₂O_[l]; ∆H₁ = +25.2 KJ mol^-1 . [eqn 1]