Chapter5

# Chapter5 - Phase Equilibrium Pure Substance A substance has...

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Phase Equilibrium : Pure Substance A substance has a spontaneous tendency to change into the phase with the lowest molar Gibbs energy.

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Thermodynamics of Phase Equilibrium At given T and p, a system evolves spontaneously to the state of minimum Gibbs energy. Stable phase is that of minimum molar Gibbs energy G m . At 1 atm, ice is the stable phase of water below 0ºC, while above 0ºC but below 100ºC, liquid water is the stable phase, and above 100ºC, steam is the stable phase. At this pressure, ice and liquid water have the same value of G m at 0ºC, and coexist in equilibrium, while at 100ºC, liquid water and steam have the same G m , and coexist in equilibrium.
Variation of G with T and p dG = dU + d(pV) - d(TS) and with dU = TdS - pdV one has dG = - SdT + Vdp It follows that ( G m / p) T = V m and ( G m / T) p = - S m For each phase or ‘state of aggregation’ of a substance, there is a surface representing G m as a function of T and p.

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Variation of G with p at Constant T ( G m / p) T = V m Vapour stable at low pressures, liquid at intermediate pressures, and solid at high pressures . Transition when molar Gibbs energies are equal .
Gibbs Energy of a Perfect Gas ( G m / p) T = V m = RT/p . Integrating at constant temperature from standard pressure p° to desired pressure p, G m = G m °(T) + RT ln(p/p°)

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Variation of G with T at Constant p ( G m
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## This note was uploaded on 02/23/2011 for the course MATH 112 taught by Professor Ritadubey during the Spring '11 term at Amity University.

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Chapter5 - Phase Equilibrium Pure Substance A substance has...

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