CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Lecture Summary, 2 March 2012


Phase diagrams:  The slope of a phase diagram is defined by the Clapeyron equation:

    dP/dT = deltaH(transition)/(T(transition)*deltaV(transition))

This is a completely general equation for a transition between any two phases.  If the transition involves the gas phase, we can estimate that V(g) >> V(other phase), and so deltaV(transition) ~ V(g).  This allows us to plug the ideal gas law into the Clapeyron equation and rearrange to find the Claussius-Clapeyron equation:

    dP/dT = PdeltaH(vap)/(nRT^2)

While this equation is no longer exact, these simplifying assumptions have allowed us to formulate an equation that is very convenient to solve certain problems.  This equation can be rearranged to the following:

   ln(P2/P1) = -(deltaH/R)*(1/T2 - 1/T1)