AP Notes, Outlines, Study Guides, Vocabulary, Practice Exams and more!

Gibb's Free Energy

Gibbs free energy - free energy

  • G < 0 >> spontaneous reaction
  • G = 0 >> reaction at equilibrium
  • G > 0 >> nonspontaneous reaction (reverse reaction could be spontaneous)
  • G = H - TS
  • DG =DH - T (DS)
  • always negative when H negative, S positive
  • always positive when H positive, S negative

Find the conditions surrounding the stretching of a rubber band.  

  • needs energy to stretch rubber band
    • DG is positive (nonspontaneous event)
  • rubber band becomes more orderly when stretched
    • DS is negative
  • release of heat when rubber band is stretched
    • DH is negative

Calculate DG (in kJ) for Mg(s) + 1/2O2 >> MgO(s) at 300K  

  • Given:
    • DHf of MgO = -601.8
    • DS° of Mg(s) = 32.7 J
    • DS° of O2 = 205 J
    • DS° of MgO(s) = 26.9 J
  • DS = 26.9 - (1/2 x 205 + 32.7) = -108.3 J = -0.1083 kJ
  • DG = -601.8 - 300(-0.1083) = -569.31 kJ

nonstandard free energy - value differs from standard value at different conditions

  • DG = G° + RT lnQ
    • R = constant (8.314 J/mol-K)
    • Q = equilibrium constant
  • at equilibrium,G° = -RT lnKeq
  • G < 0 >> Keq > 1
  • G = 0 >> Keq = 1
  • G > 0 >> Keq< 1

Calculate the Keq for 2NO2 (g) >> N2O4 (g) at 25°C  

  • Given:
    • DHf of NO2 = 33.84 kJ
    • DHf of N2O4 = 9.66 kJ
    • DS° of NO2 = 240.4 J
    • DS° of N2O4 = 304.3 J
    • R = 8.314
  • DS = 304.3 - 2(240.4) = -176.5 J = -0.1765 kJ
  • DH = 9.66 - 2(33.84) = -58.02 kJ
  • DG = -58.02 - (273+25)( -0.1765) = -5.423 kJ = -5423 J
  • DG = -RT lnKeq
  • -5423 = -(8.314)(298) lnKeq
  • lnKeq = 2.1888
  • Keq = e2.1888 = 8.92
Subject: 
Chemistry
Subject X2: 
Chemistry

Need Help?

We hope your visit has been a productive one. If you're having any problems, or would like to give some feedback, we'd love to hear from you.

For general help, questions, and suggestions, try our dedicated support forums.

If you need to contact the Course-Notes.Org web experience team, please use our contact form.

Need Notes?

While we strive to provide the most comprehensive notes for as many high school textbooks as possible, there are certainly going to be some that we miss. Drop us a note and let us know which textbooks you need. Be sure to include which edition of the textbook you are using! If we see enough demand, we'll do whatever we can to get those notes up on the site for you!