Spontaneity, entropy and free energy

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Chapter 16

• Spontaneity, entropy and free energy

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Spontaneous

• A reaction that will occur without outside
  intervention.
• We cant determine how fast.
• We need both thermodynamics and kinetics to
  describe a reaction completely.
• Thermodynamics compares initial and final states.
• Kinetics describes pathway between.

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Thermodynamics

• 1st Law- the energy of the universe is constant.
• Keeps track of thermodynamics doesnt correctly
  predict spontaneity.
• Entropy (S) is disorder or randomness
• 2nd Law the entropy of the universe increases.

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Entropy

• Defined in terms of probability.
• Substances take the arrangement that is most
  likely.
• The most likely is the most random.
• Calculate the number of arrangements for a system.

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• 2 possible arrangements
• 50 chance of finding the left empty

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• 4 possible arrangements
• 25 chance of finding the left empty
• 50 chance of them being evenly dispersed

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Gases

• Gases completely fill their chamber because there
  are many more ways to do that than to leave half
  empty.
• Ssolid ltSliquid ltltSgas
• there are many more ways for the molecules to be
  arranged as a liquid than a solid.
• Gases have a huge number of positions possible.

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DS

• DS S final - S initial
• If DS is positive, then the system has become
  more disordered.
• If DS is negative, then the system has become
  less disordered, more ordered.

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Entropy

• Solutions form because there are many more
  possible arrangements of dissolved pieces than if
  they stay separate.
• 2nd Law
• DSuniv DSsys DSsurr
• If DSuniv is positive the process is spontaneous.
• If DSuniv is negative the process is only
  spontaneous in the opposite direction.

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Example

• Predict whether the DS is likely to be positive
  or negative for each of the following reactions.
• H2CCH2(g) Br2(g) ? CH2BrCH2Br(l)
• 2C2H6(g) 7O2(g) ? 4CO2(g) 6H2O(l)

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Temperature and Spontaneity

• Entropy changes in the surroundings are
  determined by the heat flow.
• An exothermic process is favored because by
  giving up heat the entropy of the surroundings
  increases.
• The size of DSsurr depends on temperature
• DSsurr -DH/T

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Gibb's Free Energy

• Being able to look at enthalphy (H) and
  entropy(S) together to see if a reaction is
  spontaneous
• DGDH-TDS
• If DG 0 the process is spontaneous
• If DG 0 the process at equilibrium
• If DG 0 the process is nonspontaneous

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Lets Check

• For the reaction H2O(s) H2O(l)
• DSº 22.1 J/K mol DHº 6030 J/mol
• Calculate DG at 10ºC and -10ºC
• Look at the equation DGDH-TDS
• Spontaneity can be predicted from the sign of DH
  and DS.