Advanced Topic 371 of 1102 Thermodynamics & Energetics
Free Energy and Work
The decrease in Gibbs energy equals the maximum non-expansion work obtainable.
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In-depth explanation of Free Energy and Work
The change in Gibbs energy at constant temperature and pressure equals the maximum non-expansion work that can be obtained from a process, so minus delta G is the upper limit of useful work. Real devices lose some of this energy as heat because they operate irreversibly, and the difference between actual and maximum work measures the inefficiency. For electrochemical cells the relationship is delta G equals minus n F E, where n is the number of electrons transferred, F is the Faraday constant and E is the cell potential. This connection determines the voltage of batteries and fuel cells from thermodynamic data. It also explains why fuel cells are not limited by the Carnot efficiency that constrains heat engines.
Going deeper: Free Energy and Work
Key ideas: The most work you can extract at constant temperature and pressure, aside from expansion work, is minus delta G. For cells use w_max equals n F E. Real devices achieve less, and efficiency equals actual work divided by this maximum.
Common mistakes and connections: Do not confuse the Gibbs energy limit with the Carnot limit, since the latter applies only to engines using heat. This distinction explains why fuel cells can in principle exceed the efficiency of combustion engines, and why biological systems extract usable work from nutrients.
Visualization of Free Energy and Work
Delta G sets the maximum non-expansion work obtainable, as in fuel cells. For methane combustion about 885 of the 890 kJ/mol can be harvested. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A hydrogen fuel cell converts part of the Gibbs energy of hydrogen oxidation into electrical work, and this energy appears as its cell voltage.
Frequently asked questions about Free Energy and Work
What is Free Energy and Work?
The decrease in Gibbs energy equals the maximum non-expansion work obtainable.
Is Free Energy and Work a beginner, intermediate or advanced topic?
Free Energy and Work is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Free Energy and Work?
Study these earlier topics in Thermodynamics & Energetics first: Standard Free Energy of Formation, Free Energy and Equilibrium, Reaction Quotient and Free Energy.
What comes after Free Energy and Work?
The next topics in Thermodynamics & Energetics are Helmholtz Free Energy and Coupled Reactions.
Can I practise Free Energy and Work with quizzes?
Yes. The interactive lesson includes a self-check question on Free Energy and Work, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Free Energy and Work fits in Thermodynamics & Energetics
Free Energy and Work is topic 43 of 91 in Thermodynamics & Energetics.