Chapter 15: Equilibrium

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Chapter 15 Equilibrium
Chemistry 1062 Principles of Chemistry II Andy
Aspaas, Instructor
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Chemical equilibrium

• Most reactions are reversible they can react in
  the forward or reverse direction
• kf forward rate constant, kr reverse rate
  constant
• In the forward reaction, the rate decreases over
  time due to the consumption of A
• But, in the reverse direction, the rate increases
  due to buildup of B
• Eventually the system reaches equilibrium, where
  the forward rate equals the reverse rate

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Equilibrium quantities

• If initial amounts of reactants are known, and
  the quantity of any substance at equilibrium is
  known, the quantities of the other substances can
  be calculated
• Use nx as the change in moles from starting to
  equilibrium states, where n is the coefficient
  for that particular substance (-nx for reactants
  since they decrease, nx for products since they
  increase)

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Equilibrium quantities
CO(g) H2O(g) CO2(g) H2(g)
Initial 1.00 mol 1.00 mol 0 mol 0 mol
Change -x -x x x
Equilibrium 1.00-x 1.00-x x x 0.43 mol
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The equilibrium constant

• Since the forward rate the reverse rate at
  equilibrium, kfA krB
• Rearranging gives (kf / kr) (B / A)
• This quotient is the equilibrium constant, Kc
• In general, for the following reaction, at
  equilibrium,

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Determining equilibrium constant

• Convert any chemical quantities into molar
  concentration, since thats what is used in Kc
  equation
• Use the method with initial, change, and
  equilibrium values as before
• Substitute the values into the Kc equation
• If two reactions are summed (ex. elementary
  reactions) their respective Kc values are
  multiplied

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Reaction quotient

• Reaction quotient, Qc same as equilibrium
  constant, except represents an instant in time
  where the reaction is not necessarily at
  equilibrium
• If Qc gt Kc, there is an overabundance of
  products, and the reaction will go to the left to
  reach equilibrium
• If Qc lt Kc, there is an overabundance of
  reactants, and the reaction will go to the right
• If Qc Kc, the reaction is at equilibrium

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Le Chateliers Principle

• How will the concentrations of products and
  reactants be affected by addition of extra
  product or reactant, or changing pressure or
  temperature?
• Le Chateliers Principle if a system at
  equilibrium is disturbed by a change in
  temperature, pressure, or a concentration, the
  system shifts in equilibrium composition to
  counteract the change

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Concentration

• When extra reactant is added or when product is
  removed, Qc lt Kc, and the reaction will shift
  more to the right to counteract the change
• When extra product is added, or when reactant is
  removed, Qc gt Kc, and the reaction will shift
  more to the left to counteract the change
• After the shift, the system is restored back to
  Kc, but the composition may be different

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Pressure change in equilibrium

• A pressure change in a gaseous reaction can
  change the composition of the equilibrium mixture
• A pressure increase (which increases the partial
  pressure of gaseous components) will cause the
  reaction to shift to reduce the total number of
  gas molecules in the system
• Pressure increases caused by introduction of an
  inert gas does not affect the equilibrium, since
  partial pressures remain the same

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Effect of temperature change

• Temperature effects equilibrium in two ways
• An increase in temperature will cause forward and
  reverse reaction rates to increase, and
  equilibrium will be reached faster
• The equilibrium constant itself will change
• In an exothermic reaction (?H -), view heat as
  a product
• When heat is added (temperature increased) the
  equilibrium will shift towards reactants
• The opposite is true for endothermic reactions

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Catalysis

• Catalysts increase the rates of reactions, but
  have no effect on the equilibrium concentration
  of the mixture
• The equilibrium is reached more quickly
• Helpful for reactions with a large equilibrium
  constant, but are too slow to be run at room
  temperature.