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