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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: gingi85 on January 03, 2008, 06:17:03 AM

Title: Clapeyron's Equation and Phase Transitions
Post by: gingi85 on January 03, 2008, 06:17:03 AM

Clapeyron's Equation States:

dP/dT = Δ_trsS/Δ_trsV

According to the text book I am reading, in order to determine the slope of the phase boundaries on the phase diagram we use

Δ_trsS = Δ_trsH/T

and substitute it in to the Clapeyron's Equation.

However, this equation for the entropy of a phase transition is only true under a constant pressure (dP=0). How then can we use it in our situation where pressure is not constant?

Am I missing something?

Title: Re: Clapeyron's Equation and Phase Transitions
Post by: FeLiXe on January 03, 2008, 10:13:11 AM

i would say that the constant pressure refers to the transition. so you have the transition at various constant p's. and at every pressure this equation holds

I don't know if S=H/T even requires constant p. it requires equilibrium conditions and then p and V are constant anyway.

actually i did this with fixed volume. you change tempurates and write down the eqilibrium pressures. that way you can construct the vapor pressure curve