Free Energy and Pressure

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# Free Energy and Pressure - PowerPoint PPT Presentation

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

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 const 8.31 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