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Free Energy and Pressure

Free Energy and Pressure. Sections 16.7- 16.9. Free Energy. G o = H o - TS o System at const T and P will proceed spontaneously to lower its free energy This depends on equilibrium. How does pressure affect?. Δ H is not pressure dependant Δ S is pressure dependant. For a Gas.

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Free Energy and Pressure

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  1. Free Energy and Pressure Sections 16.7- 16.9

  2. Free Energy • Go = Ho - TSo • System at const T and P will proceed spontaneously to lower its free energy • This depends on equilibrium

  3. How does pressure affect? • ΔH is not pressure dependant • ΔS is pressure dependant

  4. For a Gas • Slarge vol > Ssmall vol • Slow press > Shigh press • Because S depends on P, we can look at eqn:

  5. G = Go + RTln(Q) • Q = rxn quotient • R = gas law const 8.31 J/Kmol • T = temp (in K) • Go =free energy at 1atm

  6. Try it. • Calculate G at 700 K • C(s,graph)+H2O(g)->CO(g)+H2(g) • PH2O = .85 atm • PCO = 1.0 x 10-4 atm • PH2 = 2.0 x 10-4 atm

  7. So, G? • Tells us whether products or reactants are favored • does not mean that the rxn will go to completion • Tells us rxn will go to equilibrium

  8. Towards Equilibrium • defined as when forward and reverse rates are equal • or lowest value of free energy

  9. Case 1: Go = 0 • Free energy of reactants and products are equal • System is at equilibrium

  10. Case 2: Go < 0 • Goprod < Goreact • System will adjust right • K will be greater than 1

  11. Case 3: Go > 0 • Goreact < Goprod • System will adjust left (toward reactants) • K < 1

  12. C(s,graph)+H2O(g)->CO(g)+H2(g) • Determine the direction of the rxn if: • T = 700K, Go = 92 kJ • PH2O= .67atm, PCO= .23 atm, PH2 = .51 atm

  13. Cr2O3(s)+2Al(s) -> Al2O3(s) + 2Cr(s) • Use the tables to calc Go • Then calculate the K value

  14. Temp and K • Go = -RTln(K) • Go = Ho - TSo

  15. Free Energy and WORK • Free energy tells us if a rxn will proceed spontaneously • maximum possible useful work obtainable from a process at const temp and pressure

  16. wmax = G • G represents energy that is free to do work • or minimum energy needed to make rxn occur

  17. Remember • This is a hypothetical amount of work • A real pathway wastes energy

  18. Homework • P 816 • 55, 57, 61, 63

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