Chapter 3

# Thefugacityisdefinedin

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Unformatted text preview: s providing criteria for the spontaneity of processes occurring either  at constant V and T or at constant P and T, the Helmholtz and Gibbs Free  Energies have further significant meanings.  Let us consider first the case of the Helmholtz free energy.  The second law states that dS ≥ δq / T. Using the first law dU = δw + δq, we  get:  T dS ≥ dU ‐ δw which is equivalent to: δw ≥ dU ‐ T dS.  If the process occurs at constant temperature, then we can write:  δw ≥ dA or w ≥ DA  If we focus on a process whereby work is being done by the system   (w < 0). We now recall from the discussion of reversible vs. spontaneous  expansions of ideal gases that the maximum work of expansion is done by  the system when the expansion occurs reversibly.  This corresponds to the  equality between w and ΔA.  We therefore conclude that:  Δ A is the maximum work that can be done by a system at constant  temperature. This is actually why the symbol A was chosen for the  Helmholtz Free Energy. In German, A stands for “Arbeit”, which in English  means “work”.  Marand’s Notes: Chapter 3 ‐ The Second Law of Thermodynamics  120    Let us consider now the meaning of ΔG.  Using a similar approach, we write the first law in its most general form.  dU = δw + δwe + δq  where δw is the volume expansion work and δwe is any extra (non‐ expansion) work.  We now use the second law in the Clausius formulation.  dS ≥ δq /T  which...
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## This note was uploaded on 01/26/2014 for the course CHEM 3615 taught by Professor Aresker during the Spring '07 term at Virginia Tech.

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