CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 12 March  2015


Phase transitions:  A few definitions:

Phase: a form of matter that is uniform in its chemical and physical composition.  Thatdoes not mean it has to be pure, just homogeneous.

Phase transition:  the conversion of one phase to another, which occurs atcharacteristic temperatures and pressures and which can be expressed on a phase diagram.

Phase diagram:  plot of pressure (y-axis) vs. temperature (x-axis) that showsregions of thermodynamic space at which various phases are stable.  Phase transitions occur at the boundaries between two stablephases.

We drew a couple generic phase diagrams and definied some interestingcharacteristics, such as the triple point and the critical point.  There are many examples of phase diagrams in your text book.   

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)
 
Finally, here is a link to the ice skating question we discussed in class.