Gibb's free energy

1
Thermodynamics
Will a reaction occur spontaneously?
2

• Laws of Thermodynamics
• 1st Law the energy in the universe is constant.
• 2nd Law for a spontaneous process, there will
  be an increase in entropy.
• 3rd Law the entropy of a perfect crystal at 0 K
  is zero.

3
If ?Suniverse is then reaction is
spontaneous ?Suniv ?Ssurr ?Ssys ?Ssurr
-?H/T for an exothermic process for an
endothermic process ?Ssys ( for a system
becoming more random) Ex (s) ?(l) or (l)
?(g) or less ? more
gaseous particles
4
?Suniv ?Ssurr ?Ssys ?Suniv - ?G /
T ?Ssurr -?H/T -?G / T -?H / T ?Ssys So,
at standard state conditions ?Go ?Ho - T
?Sosys
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Units on the State Functions
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Predict Spontaneity ?G ?H - T ?Ssys If ?H is
(-) and ?S is () ?G will be (-) at all
temperatures system will proceed spontaneously
at any temperature
7
Predict Spontaneity ?G ?H - T ?Ssys If ?H is
() and ?S is (-) ?G () at all temperatures
the system will NOT proceed spontaneously at
any temperature
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• Predict Spontaneity
• ?G ?H - T ?Ssys
• If ?H(-) and ? S(-)
• ? G (-) at low temperatures
• the system will NOT proceed spontaneously at
  high temperatures

9

• Predict Spontaneity
• ?G ?H - T ?Ssys
• If ?H() and ? S()
• ? G (-) at high temperatures
• the system will NOT proceed spontaneously at low
  temperatures

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Summary of Spontaneity ?G ? H - T? S ?H
?S ?G Spont. - -
yes - no -
- ? - at low T
? - at high T
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• ?G 0 for a system at equilibrium
• Such as normal melting or boiling
• Since, ?Go ? Ho - T? So
• At equilibrium T ?Ho/ ?So
• A system will proceed spontaneously
• if ?Go is (-)
• See example 16.5 in book

12
State Functions follow Hess Law ?GO
?n?GO(p) - ?n?GO(r) ?SO Sn ?So (p) - Sn ?So
(r) ?Ho Sn ?Ho (p) - Sn ?Ho (r)
13
Dependance of Free energy on Pressure

• As a reaction proceeds the free energy of the
  system changes until equilibrium is established
• ?G ?Go RT ln Q
• (at equilibrium ?G 0 and Q K)
• Therefore ?Go - RT ln K
• if ?Go 0 then (K 1) if ?Go is then
  (Klt1) if ?G is - (Kgt1)

14

• Since, ?Go - RT ln K
• And ?Go ? Ho - T? So
• Then, ? Ho - T? So - RT ln K
• Or, ln K -? Ho/RT ? So / R

15

• ln K -? Ho/R (1/T) ? So / R
• y m x
  b
• A plot ln K vs. 1/T
• gives a line with slope -? Ho/R
• ln K
  y-int ? So/R
• 1/T (K-1)

16

• The most work that can be obtained from a system
  is ?G
• (energy is lost from any system so all real
  systems are irreversible)
• Maximum work obtained from any system
• w ?G