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Le Chatelier's Principle for AP Chemistry (page 2)

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By — McGraw-Hill Professional
Updated on Feb 9, 2011

Changes in Temperature

Changing the temperature changes the value of the equilibrium constant. It also changes the amount of heat in the system and can be treated as a concentration effect. To treat it this way, one must know which reaction, forward or reverse, is exothermic (releasing heat).

One last time, let's consider the Haber reaction:

The formation of ammonia is exothermic (liberating heat), so the reaction could be written as:

If the temperature of the reaction mixture were increased, the amount of heat would be increased and the equilibrium would shift to the left to consume the added heat. In doing so, the concentration of nitrogen and hydrogen gases would increase and the concentration of ammonia gas would decrease. If you were in the business of selling ammonia, you would probably want to operate at a reduced temperature, in order to shift the reaction to the right.

Consider the following equilibrium (endothermic as written), and predict what changes, if any, would occur if the following stresses were applied after equilibrium was established.

  1. add CO2
  2. remove CO2
  3. add CaO
  4. increase T
  5. decrease V
  6. add a catalyst

Answers:

  1. Left—the equilibrium shifts to remove some of the excess CO2.
  2. Right—the equilibrium shifts to replace some of the CO2.
  3. No change—solids do not shift equilibria unless they are totally removed.
  4. Right—endothermic reactions shift to the right when heated.
  5. Left—a decrease in volume, or an increase in pressure, will shift the equilibrium toward the side with less gas.
  6. No change—catalysts do not affect the position of an equilibrium.

Practice problems for these concepts can be found at:

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