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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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Free Energy and Pressure

Sections 16.7- 16.9


Free Energy

  • Go = Ho - TSo

  • System at const T and P will proceed spontaneously to lower its free energy

  • This depends on equilibrium


How does pressure affect?

  • ΔH is not pressure dependant

  • ΔS is pressure dependant


For a Gas

  • Slarge vol > Ssmall vol

  • Slow press > Shigh press

  • Because S depends on P, we can look at eqn:


G = Go + RTln(Q)

  • Q = rxn quotient

  • R = gas law const8.31 J/Kmol

  • T = temp (in K)

  • Go =free energy at 1atm


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


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


Towards Equilibrium

  • defined as when forward and reverse rates are equal

  • or lowest value of free energy


Case 1: Go = 0

  • Free energy of reactants and products are equal

  • System is at equilibrium


Case 2: Go < 0

  • Goprod < Goreact

  • System will adjust right

  • K will be greater than 1


Case 3: Go > 0

  • Goreact < Goprod

  • System will adjust left (toward reactants)

  • K < 1


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


Cr2O3(s)+2Al(s) -> Al2O3(s) + 2Cr(s)

  • Use the tables to calc Go

  • Then calculate the K value


Temp and K

  • Go = -RTln(K)

  • Go = Ho - TSo


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


wmax = G

  • G represents energy that is free to do work

  • or minimum energy needed to make rxn occur


Remember

  • This is a hypothetical amount of work

  • A real pathway wastes energy


Homework

  • P 816

  • 55, 57, 61, 63


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