# Phase Diagrams - Density?

1. Feb 4, 2009

### meganw

1. The problem statement, all variables and given/known data

"Which is the denser phase, monoclinic or rhombic sulfur?

2. Relevant equations

Density = Mass/Volume...

3. The attempt at a solution

How can you tell density from a phase diagram??

Thanks!! =)

-Megan

2. Feb 4, 2009

### Mapes

As pressure increases, denser phases are favored. (Also: as temperature increases, phases with higher entropy are favored. Now you know how to rank phases by entropy also.)

The reason is that at constant temperature and pressure, the phase with the lowest Gibbs free energy G is energetically favored, and $G=U+PV-TS$.

3. Feb 4, 2009

### meganw

What is u?

and why do you have PV in the G equation?

I thought the one two equations were:

G=-RTln(K)

and

G=H-TS

4. Feb 4, 2009

### Mapes

U is internal energy; enthalpy $H=U+PV$. The other equation $\Delta G^0=-RT\ln K$ applies to reactions at equilibrium. Gibbs free energy pops up in a lot of places because it's the parameter that Nature seeks to minimize at constant temperature and pressure. Also, reactions under these conditions are at equilibrium when the Gibbs free energy of the products equals that of the reactants.