Predicting Spontaneous Change

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Predicting Spontaneous Change
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Spontaneous Change

• One that occurs by itself under specified
  conditions, without an ONGOING input of energy
• Burning wood
• Freezing water at -1 ?C
• Cup breaking
• If a reaction (at a specific temperature and
  pressure) is spontaneous in one direction, it is
  NOT spontaneous in the opposite.

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Spontaneous Reactions

• Spontaneity has nothing to do with time

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Diamond vs. Graphite
diamond
graphite
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How Does Spontaneity Relate to Chemical
Equilibria?

• Consider a reaction (it is balanced)
• A(g) B(g) C(g)
  K 10
• If you start a reaction with
• AB 1 M and C 7 M
• Which way will the reaction shift to get to
  equilibrium?

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How Does Spontaneity Relate to Energy and Heat?

• Consider the reaction shown below
• H2O(l) H2O(g) ?H
  -44.0 kJ

The sign of ?H alone cant predict spontaneity!!
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Must Be Something Else Involved

• One feature that ALL spontaneous processes share
  is an increase in the degree of freedom or
  disorder of the universe (system surroundings)
• ENTROPY (S)
• Solid ? liquid ? gas ?Sgt0
• Salt liquid ? ions in solution ?Sgt0
• A(g) B(g) C(g)
  ?Sgt0
• ?Sgt0, Entropy is increasing.

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Predicting the SIGN of ?S

• Temperature
• Increase temperature, ?Sgt0 (increase in
  entropy)
• Phase Change
• ?Sgt0 (increases) from solid?liquid?gas
• Usually ?Sgt0 (increases), when dissolving a
  solid in a liquid.
• ?Slt0 (decreases), when dissolving a gas in a
  liquid.
• Size and Complexity
• In general, ?Sgt0 (increases) with increasing
  size and complexity for substances of the same
  phase.

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Comparison of the entropy of solids, liquids and
gases
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Entropy decreases when a gas is dissolved in a
liquid.
?S(O2(g)) 205.0 J/mol?K
?S(O2(aq)) 110.9 J/mol?K
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Entropy increases with increasing molecular
complexity.
?S 211 J/mol?K
240 J/mol?K
304 J/mol?K
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2nd Law of Thermodynamics

• Entropy is always increasing for spontaneous
  reactions
• ?Suniv ?Ssys ?Ssurr gt 0
• Calculating ?S is just like calculating ?H
• ?Srxn SnSproducts SnSreactants
• Appendix B has the ?S values
• Problems 20.33, 20.35, 20.37, 20.39

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Gibbs Free Energy

• Way to relate ?H and ?S
• ?G ?H-T?S
• ?G is called Gibbs Free Energy
• Measure of the spontaneity of a reaction
• ?Glt0 reaction is spontaneous
• ?Ggt0 reaction is nonspontaneous
• ?G0 reaction is at equililbrium

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Spontaneity, or the sign of ?G, depends on sign
?H, ?S and sometimes the temperature of the
system.
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Free Energy and Equilibria

• Gibbs Free Energy is related to K
• ?G -RT ln K

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Pop Quiz 10 Part A.
At ordinary pressure, water freezes below 273 K
and melts above 273 K. Explain how both of
these reactions can be spontaneous. Specifically,
what are the qualitative values (, -, 0) of ?H
and ?S for each reactions below? H20 (s) ? H20
(l) above 273 K ?H _____ ?S ______ -T
?S______ ?G______ H20 (l) ? H20 (s) below
273 K ?H _____ ?S ______ -T ?S______
?G______