AP Chemistry Chapter 17

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AP Chemistry Chapter 17

• Spontaneity of Reaction

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Spontaneous reactions

• What does that mean?
• Some occur without any help
• Others require some help
• No help ice cube melting
• Help wood burning

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• If a reaction is spontaneous under a certain set
  of conditions, the reverse reaction must be
  nonspontaneous
• In any spontaneous change, the amount of free
  energy available decreases toward zero as the
  process proceeds towards equilibrium.

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Reactions tend to be spontaneous when

• it leads to lower energy -?H
• But not always!!!!
• Also tend to be spontaneous if the reaction
  results in an increase in randomness
• Entropy S
• Greater entropy more random the system is. ?S
  increase in entropy ?Sgt0
• - ?S decrease in entropy ?Slt0

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• Page 448 Example 17.1 Predict sign of ?S

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• In general, nature tends to move spontaneously
  from more ordered to more random state
• Entropy increases in the order
• s ltl lt g
• Increasing temperature of a substance increased
  its entropy

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Third Law of Thermodynamics

• A completely ordered pure crystalline solid has
  an entropy of zero at 0K

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?S for reactions

• Pg. 450 Table of Standard Entropies
• Used to calculate the standard entropy change,
  ?So, for reactions.
• ?So ? So products ?So reactants
• Must remember to multiply by the number of moles
  from balanced equation

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• Note that So is a positive quantity for both
  compounds and elements can be negative for ions
  in solutions

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• Reactions which So is positive tend to be
  spontaneous, at least at high temperatures.
• H2O(s) ? H2O(l) ( ?S gt 0)
• H2O(l) ? H2O(g) (?S gt 0)
• Fe2O3(s) 3H2(g) ? 2Fe(s) 3H2O(g) (?S gt 0)
• All of these reactions are endothermic (?Hgt0)
• They become spontaneous at high temperatures

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• A reaction that results in an increase in the
  number of moles of gas is accompanied by an
  increase in entropy.
• If the number of moles of gas decreases, ?S is a
  negative quantity

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• Elements have nonzero standard entropies
• Standard molar entropies of pure substances are
  always positive quantities
• Aqueous ions may have negative So values

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• Among substances of similar structure and
  physical state, entropy usually increases with
  molar mass
• Molecule becomes more complex, more ways for the
  atoms to move about with respect to one another
  (higher entropy)

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• Pg. 451 sample
• Example 17.2

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Second Law of Thermodynamics

• In a spontaneous process, there is a net increase
  in entropy, taking into account both system and
  surroundings.
• ?Suniverse ?Ssystem ?Ssurroundings gt 0
• spontaneous

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Gibbs Free Energy G

• Two quantities affect reaction spontaneity
• enthalpy, H and entropy, S
• Put them together in a way that the signs will
  give us a clue
• G H TS
• T kelvin temp

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• ?G for a reaction at constant temp and
  pressure, represents that portion of the total
  energy change that is available to do useful work
  is a state property
• Depends only on the nature of products and
  reactants and the conditions (temp/pressure/concen
  tration), not on the path by which the reaction
  is carried out

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• - ?G spontaneous
• ?G not spontaneous (reverse is
• 
  spontaneous
• ?G 0 system is at equilibrium (no tendency for
  reaction to occur in either direction)

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?G measure of the driving force of a reaction

• Reaction, at constant pressure and temperature,
  go in such a direction as to decrease the free
  energy of the system
• Products have lower free energy, reaction will go
  in that direction
• Reactants have lower free energy, reaction will
  go in that direction (means the reverse rxn
  spontaneous)

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Gibbs-Helmholtz equation

• ?G ?H - T?S
• To make ?G negative
• Negative value for ?H (exothermic)
• Positive value for ?S (less ordered)

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Gibbs-Helmholtz equation

• Valid under all conditions but we will apply it
  only under standard conditions
• Meaning gases are at one atmosphere partial
  pressure
• Ions or molecules in solution are at one molar
  concentration
• ?G standard free energy change

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• ?Go ?Ho - T?So
• now we can use the tables in the book
• If ?Go is negative spontaneous at standard
  conditions
• If ?Go is positive nonspontaneous at standard
  conditions
• ?G 0 system is at equilibrium at standard
  conditions

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Calculation of ?G at 25oCFree Energies of
Formation!!

• Make sure units are correct
• Use ?H is kJ, convert ?S for J/K to kJ/K
• Pg. 455 Example 17.3
• Pg. 456 Example 17.4, 17.5

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• Chart on pg. 458