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The Concept of Gibbs Energy
This page gives a brief description of the terms in the Free Energy Equation and
electrochemical irreversibility.
Free Energy (Gibbs Energy) and Work
The Gibbs Energy is defined as:
G = H - T.S or for a constant temperature process (where d is delta): dG = dH - T.dS but from the first law (SFEE): Q + W = dH Hence Wmax = dH - T.dS or Wmax = dG (where both Wmax and dG are negative) This applies for a chemical process at constant temperature and pressure, and is the maximum possible useful work or available energy, known as the "free energy". (For a thermal process at constant temperature and pressure Wmax=dG=0) |
Index of technical reviews |
dH = Hproducts - Hreactants
dS = Sproducts - Sreactants
Entropy increases from zero at 0K and a perfect crystal structure, as random vibration and disorder of atoms or molecules in the chemicals increases. The entropy term in the Gibbs energy equation arises from the difference in the molecular structure between the products and reactants. In general, more complex molecules have more degrees of freedom of motion, and hence greater entropy than simpler molecules.
Hence the entropy term in the Wmax equation does not represent irreversibility as the energy is not dissipated to the surroundings, but it does represent a reduction in the work potential of the reaction.
Hence the magnitude of the real work out is less than the Gibbs energy:
W < Wmax
Similar irreversibility occurs in the chemical combustion process, and when comparing the theoretical efficiencies of fuel cell systems with combustion engine systems, the efficiency of the combustion process should be included with the Carnot efficiency of the heat engine.
- S Zumdahl; Chemistry; Houghton Mifflin 1997
- Fuel cell general information and electrochemical reactionsUS Department of Energy
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