The Case Of Zero Change In Gibbs Free Energy
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The Case Of Zero Change In Gibbs Free Energy

What is the ΔG (change in Gibbs free energy) when liquid water changes phase to vapor at 100oC and 1 atm. Molar enthalpy of vaporization is 9720 cal.

The strings:

S7P7A72 (Dynamic Equilibrium)

The math:
Pj Problem of Interest is of type equilibrium (zero Gibbs free energy). This problem can be cast as a change problem but it is not because the primary problem of interest is the equilibrium established when the change in Gibbs free energy is zero. It is in this sense that the primary problem of interest is of type equilibrium.

The equation of interest is:
Change in Gibbs free energy = enthalpy change - T(entropy change)

So, ΔG = ΔH - TΔS -------------(1) where T is temperature in degrees Kelvin.

ΔS = q/T = ΔH/T = 9720/373 cal/mole oK

So, ΔG = 9720 - 373(9720/373) = 0.

A system is in equilibrium when ΔG = 0 and has no usable energy to perform work.

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