Shifting Equilibria

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Shifting Equilibria

• Le Chateliers Principle If a stress is applied
  to a system at equilibrium, the equilibrium
  shifts in such a way as to reduce the stress.
• Lets examine the possible stresses.
• Concentration Changes
• Suppose that the N2O4 (g) ? 2 NO2(g) reaction
  is at equilibrium.
• Now, add some additional N2O4 to the flask.

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• The denominator in the reaction quotient Q
  increases, so now Q lt Kc.
• Therefore, the reaction shifts to the right to
  make more products until equilibrium is once more
  attained.
• Now, what if we instead had removed some N2O4
  from the flask?
• The denominator has decreased, so Q gt Kc.
• The reaction shifts to the left to reach
  equilibrium.

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• Similar considerations apply to the products.
• Pressure (or Volume) Changes
• A decrease in total system volume increases the
  partial pressures (and molarities) of all gaseous
  species present.
• However, the side of the reaction with the
  greater number of moles of gases will be affected
  to a greater extent in the mass-action expression
  (remember that the number of moles becomes an
  exponent).

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• Ex In the above reaction, if the total volume
  is halved, then the molarities of both N2O4 and
  NO2 are doubled.
• Since Q gt Kc , the reaction shifts to the left.
• A volume increase would have the opposite
  effect.
• Summary Higher pressure favors the side of the
  reaction with fewer moles of gases.

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Figure 13.9The Effect of Decreased Volume on the
Ammonia Synthesis Equilibrium
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• Temperature Change add or remove heat
• Endothermic heat is a reactant ( ?H)
• Exothermic heat is a product ( ?H)
• Ex. 2 CO(g) O2(g) ? 2 CO2(g) heat
• Raise temp, add heat (product)
• Reaction shifts to the left.
• Lower temp, remove heat (product)
• Reaction shifts to the right.
• Of course, these predictions are reversed for an
  endothermic reaction.

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• Catalysts Can they shift equilibria?
• Suppose a catalyst is added to speed up the
  forward reaction with the goal of having a higher
  ratio of products/reactants at equilibrium
• If the energy of the transition state is
  lowered, the activation energy for the reverse
  reaction is also lowered. The reverse reaction
  also speeds up.
• Therefore, catalysts allow a reaction to reach
  equilibrium faster, but can not shift the
  position of equilibrium.