Hess's Law

Hess's law is one of the most powerful tools in energetics. It allows us to calculate enthalpy changes for reactions that are difficult or impossible to measure directly. Once you understand how to construct enthalpy cycles, you can solve a huge range of problems.

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Statement of Hess's Law

Hess's law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.

This is a direct consequence of the law of conservation of energy. If you could get more (or less) energy by taking a different route from the same reactants to the same products, you would be creating (or destroying) energy, which violates the first law of thermodynamics.

In practical terms, this means that if a direct reaction is hard to measure, we can calculate its enthalpy change by finding an alternative route through intermediate steps whose enthalpy changes are known.

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Enthalpy Cycles Using Enthalpies of Formation

One of the most common applications is calculating the enthalpy of a reaction using standard enthalpies of formation.

The General Formula

For any reaction:

ΔH°reaction = Σ ΔH°f (products) − Σ ΔH°f (reactants)