SPONTANEITY

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SPONTANEITY

• SPONTANEOUS REACTIONS naturally favor the
  formation of products at the specified
  conditions.
• give substantial amounts of products at
  equilibrium.
• NONSPONTANEOUS REACTIONS do not favor the
  formation of products at specified conditions.
• do not give substantial amounts of products at
  equilibrium.

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SPONTANEITY

• Spontaneity and reaction rate ARE NOT RELATED.
• Spontaneity depends upon existing conditions
  something might be spontaneous at one point, and
  not spontaneous under a different set of
  conditions.

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Exothermic reactions are not necessarily
spontaneous!!

• It is tempting to associated spontaneity with an
  enthalpy change namely when heat is released,
  something will tend to be spontaneous. This
  however is only part of the story
• EXAMPLE a melting ice cube at 25oC is an
  endothermic, but spontaneous process.

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SPONTANEITY

• ALL SPONTANEOUS REACTIONS RELEASE FREE ENERGY!!

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FREE ENERGY

• FREE ENERGY the energy available to do work.
• Not all energy associated with a system can be
  used to do work.
• Comprised of two things
• Amount of energy (enthalpy, ?H)
• Order/disorder of the energy (entropy)

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ENTROPY

• ENTROPY the disorder of a system, particularly
  the order of the energy of a system (loosely
  speaking).
• Energy is released when entropy increases. IT
  TAKES ENERGY TO ORGANIZE SOMETHING.
• Nature has a tendency to cause the disorder of
  energy, or the degree of disorder in itself, to
  increase.

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RIDDLE

• How do you unscramble an egg?
• Feed it to the chicken, however you still will
  not get the whole egg back some of the original
  energy will have been lost as it degraded to a
  more disordered form.

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NATURES TENDENCIES

• Moves in the direction of decreasing free energy
  (lower energy states)
• Moves in the direction of increasing disorder
  (increasing entropy)

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This does not mean that entropy can never
decrease for a single event but this decrease
will be at a cost of increased entropy somewhere
else.

• Spilled bucket of pennies
• Mt. St. Helens

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ENTROPY

• Entropy is given the symbol S and has units of
  J/K.
• So is a measure of entropy of a substance at
  standard temperature and pressure (STP).
• A perfect crystal at absolute zero is considered
  perfectly organized and therefore has no entropy.
• We can assign values to the entropy of substances
  relative to this imaginary perfect crystal.
• For a reaction, ?So So products So reactants

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EXAMPLE Find ?So for the following

• CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (l)
• Standard entropy values (Sos) are from Table
  18.2, page 515
• 186.2 J/(K-mol CH4)
• 205.0 J(K-mol O2)
• 213.6 J/(K-mol CO2)
• 69.94 J/(K-mol H20)
• ?So (1 mol CO2)(213.6 J/(K-mol CO2) (2 mol
  H2O)(69.94 J/K-mol H2O) - (1 mol CH4)(186.2
  J/K-mol CH4) (2 mol O2)(205.0 J/K-mol O2)
  -242.5 J/K

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ENTROPY

• Sgasses gt Sliquids gt Ssolids

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BACK TO SPONTANEITY

• ALL SPONTANEOUS REACTIONS RELEASE FREE ENERGY!!
• The free energy change associated with a reaction
  depends upon both the
• change in enthalpy
• change in entropy.

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Gibbs Free Energy Change ?G

• ?G ?H (T?S)
• In the above equation, T must be stated in
  Kelvins, not in Celsius or Fahrenheit.
• To convert to Kelvins, add 273.15o to the
  Celsius temperature.
• Here, ?H has units of kJ/mol ?S has units of
  J/(K-mol)
• ?G is energy that is available to do useful work.

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Using ?G to predict spontaneity

• ?G ?H (T?S)
• If ?G is negative then the reaction will be
  spontaneous.
• If ?G is positive, the reaction will not be
  spontaneous.

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?G and the Big Picture

• We measure ?G for the system that we are
  investigating. Everything else is part of the
  surroundings, or the universe. The free energy
  that is associated with a change is released or
  absorbed into the universe. This means that if
• free energy is absorbed by the system, then the
  energy of the universe decreases.
• free energy is released by the system, then the
  energy of the universe increases.

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?G and Spontaneity

• ?G means energy is absorbed by the system and
  the reaction/condition is nonspontaneous
• - ?G means energy is released and the
  reaction/condition is spontaneous.

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EXAMPLE Is this reaction spontaneous?

• CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (l)
• Hos -74.86 kJ/(mol CH4) 0.0 kJ/(mol O2)
  -393.5 kJ/(mol CO2) -285.8 kJ/(mol H20)
• Standard heats of formation values are from Table
  A.6, page 787. (NOTE UNITS ARE IN KILOJOULES,
  NOT JOULES LIKE ENTROPY UNITS)
• ?Ho (1 mol CO2)(-393.5 kJ/(K-mol CO2) (2 mol
  H2O)(-285.8 kJ/K-mol H2O) - (1 mol
  CH4)(-74.86 kJ/K-mol CH4) (2 mol O2)(0.0
  kJ/K-mol O2) -892.24 kJ/K
• ?G ?H (T?S) -892.24 (273.1525)(-.2425)
  -819.9386 kJ/K therefore spontaneous

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?G Depends on ?H and T?S?G ?H (T?S)

• CASE 1 ?G is always (-) always spontaneous
  free energy released by system.
• CASE 2 ?G is (-) when T is large, positive when
  T is small (melting ice).
• CASE 3 ?G is (-) when T is small, positive when
  T is large.
• CASE 4 ?G is always () always
  non-spontaneous free energy absorbed by system.

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APPLETS

• http//wps.prenhall.com/esm_hillpetrucci_genchem_4
  /0,8603,1081286-,00.html
• Go to Chapter 17 ? e-media activities?
• Gibbs Free Energy
• Free Energy Change vs. Temperature