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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

Free Energy and Pressure

Sections 16.7- 16.9

free energy
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
How does pressure affect?
  • ΔH is not pressure dependant
  • ΔS is pressure dependant
for a gas
For a Gas
  • Slarge vol > Ssmall vol
  • Slow press > Shigh press
  • Because S depends on P, we can look at eqn:
g g o rtln q
G = Go + RTln(Q)
  • Q = rxn quotient
  • R = gas law const 8.31 J/Kmol
  • T = temp (in K)
  • Go =free energy at 1atm
try it
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
slide7
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
Towards Equilibrium
  • defined as when forward and reverse rates are equal
  • or lowest value of free energy
case 1 g o 0
Case 1: Go = 0
  • Free energy of reactants and products are equal
  • System is at equilibrium
case 2 g o 0
Case 2: Go < 0
  • Goprod < Goreact
  • System will adjust right
  • K will be greater than 1
case 3 g o 0
Case 3: Go > 0
  • Goreact < Goprod
  • System will adjust left (toward reactants)
  • K < 1
c s graph h 2 o g co g h 2 g
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
cr 2 o 3 s 2al s al 2 o 3 s 2cr s
Cr2O3(s)+2Al(s) -> Al2O3(s) + 2Cr(s)
  • Use the tables to calc Go
  • Then calculate the K value
temp and k
Temp and K
  • Go = -RTln(K)
  • Go = Ho - TSo
free energy and work
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
w max g
wmax = G
  • G represents energy that is free to do work
  • or minimum energy needed to make rxn occur
remember
Remember
  • This is a hypothetical amount of work
  • A real pathway wastes energy
homework
Homework
  • P 816
  • 55, 57, 61, 63
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