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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

• Go = Ho - TSo

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

• This depends on equilibrium

• ΔH is not pressure dependant

• ΔS is pressure dependant

• 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

• 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

• 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

• 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

• Go = -RTln(K)

• Go = Ho - TSo

• 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

• This is a hypothetical amount of work

• A real pathway wastes energy

• P 816

• 55, 57, 61, 63