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Free Energy and PressurePowerPoint Presentation

Free Energy and Pressure

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### Free Energy and Pressure

Sections 16.7- 16.9

Free Energy

- Go = Ho - TSo
- 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 - TSo

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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