Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Phase Diagrams - Density?

  1. Feb 4, 2009 #1
    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!! =)

  2. jcsd
  3. Feb 4, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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 [itex]G=U+PV-TS[/itex].
  4. Feb 4, 2009 #3
    What is u?

    and why do you have PV in the G equation?

    I thought the one two equations were:



  5. Feb 4, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    U is internal energy; enthalpy [itex]H=U+PV[/itex]. The other equation [itex]\Delta G^0=-RT\ln K[/itex] 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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook