Intermediate Topic 352 of 1102 Thermodynamics & Energetics
Hess's Law
The total enthalpy change of a reaction is the same however many steps it takes.
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In-depth explanation of Hess's Law
Hess's law states that the total enthalpy change for a reaction is the same whether it occurs in one step or in several intermediate steps, because enthalpy is a state function. This allows the enthalpy change of a reaction that is difficult to measure directly to be calculated by adding the known enthalpy changes of other reactions whose equations sum to the target. Steps may be reversed, which changes the sign of delta H, or multiplied by a factor, which multiplies delta H by the same factor. Hess's law underlies the use of formation data and bond energies in thermochemistry. It is one of the most practical tools for estimating energy changes in chemistry.
Going deeper: Hess's Law
Key ideas: Method: write the target equation, arrange the known equations so that species cancel, flip the sign of delta H when reversing and multiply when scaling, then add. Check that the final sum matches the target exactly, including states. Cycle diagrams help to avoid sign errors.
Common mistakes and connections: The most frequent mistakes are forgetting to reverse the sign when flipping and not scaling delta H when multiplying coefficients. Another is leaving intermediates uncancelled. Hess's law rests on enthalpy being a state function and is the engine behind formation data, bond enthalpy estimates and Born-Haber cycles.
Visualization of Hess's Law
C to CO2 (-393.5) equals C to CO (-110.5) plus CO to CO2 (-283.0). The total enthalpy change is the same for either route. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The enthalpy of formation of carbon monoxide can be found by combining the combustion of carbon to CO2 with the reverse of the combustion of CO to CO2.
Frequently asked questions about Hess's Law
What is Hess's Law?
The total enthalpy change of a reaction is the same however many steps it takes.
Is Hess's Law a beginner, intermediate or advanced topic?
Hess's Law is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Hess's Law?
Study these earlier topics in Thermodynamics & Energetics first: Enthalpy of Combustion, Enthalpy of Neutralization, Enthalpy of Solution.
What comes after Hess's Law?
The next topics in Thermodynamics & Energetics are Reaction Enthalpy from Formation Data and Bond Enthalpy Calculations.
Can I practise Hess's Law with quizzes?
Yes. The interactive lesson includes a self-check question on Hess's Law, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Hess's Law fits in Thermodynamics & Energetics
Hess's Law is topic 24 of 91 in Thermodynamics & Energetics.