Hess's Law and Enthalpy Cycles

Many enthalpy changes cannot be measured directly. The enthalpy of formation of methane, for instance, cannot be found by combining carbon and hydrogen in a calorimeter because the reaction is slow and produces a mixture of products. Hess's Law provides the solution. It states that the total enthalpy change for a reaction is independent of the route taken, depending only on the initial and final states. This is a direct consequence of the law of conservation of energy: if a different amount of energy were released by one pathway than another, we could build a perpetual-motion machine by cycling between them.

Because enthalpy is a state function, we can construct enthalpy cycles that connect the reaction we want with reactions whose enthalpies we know. The practical rule is that the enthalp