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Le Chatelier’s Principle Chapter 11
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Le Chatelier’s Principle states that a system at equilibrium will stay at equilibrium UNLESS it is acted upon externally. If there is an external action, then the equilibrium has to be re-established.
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Let us examine the following reaction: aA + bB cC + dD
At equilibrium, there is a certain amount of A, B, C, D present. These amounts will remain unchanged UNLESS some external stress is applied. Let’s see how equilibrium is affected when a stress is applied.
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aA + bB cC + dD Addition of some B
If you add more B, then there will be more molecules of B able to react. The forward reaction will be favored and equilibrium will move towards the right. With time, there will be less A, more B, more C and more D present than at the original equilibrium.
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aA + bB cC + dD Removal of some A
If you remove some A this will cause the reaction to shift towards the left (reactant side). The amount of A will go down, the amount of B will go up and the amount of C and D will also go down.
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aA + bB cC + dD Increasing or Decreasing the pressure
Pressure for a gas depends on the number of gas particles present. An increase or decrease in pressure will have an effect on equilibrium only when there is a different number of gas particles on either side of the balanced equation.
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An increase in pressure will favor the side with the fewer gas particles.
A decrease in pressure will favor the side with the larger number of gas particles.
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aA + bB cC + dD + Energy Increasing or Decreasing the temperature For an exothermic reaction, an increase in temperature will favor the reverse reaction. (equilibrium shifts to the left) A decrease in temperature will have the opposite effect.
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aA + bB + Energy cC + dD For an endothermic reaction, an increase in temperature will favor the forward reaction. (equilibrium shifts to the right) A decrease in temperature will have the opposite effect
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Pressure (due to decreased volume): increase in pressure favors side with fewer molecules. shift right Catalysts: do not influence the reaction
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