Le Chatelier's principle

The equilibrium shift for the examples in these cards are just for the example, i.e. an increase in pressure will not always cause the equilibrium to shift to the right, but it is so for the example given.

Chemical Ideas 7.1

OCR Salters B F332

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Le Chatelier's principle

  • The position of the equilibrium can be altered by changing the concentration of solutions, the pressure of gases or the temperature.
  • If a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible.
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Concentration

Increasing reactants:

  • Equilibrium shifts to the right

Increasing products:

  • Equilibrium shifts to the left

Decreasing reactants:

  • Equilibrium shifts to the left

Decreasing products:

  • Equilibrium shifts to the right
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Pressure

Increasing pressure:

  • Equilibrium shifts to the side with fewer gas molecules
  • Example:
    • CO(g) + 2H2(g)  ⇄  CH3OH(g)
    • 3 molecules 1 molecule
    • Equilibrium shifts to the right

Decreasing pressure:

  • Equilibrium shifts to the side with more gas molecules
  • Example:
    • CH4(g) + H2O(g)  ⇄  CO(g) + 3H2(g)
    • 2 molecules  4 molecules
    • Equilibrium shifts to the right
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Temperature

Increasing temperature:

  • Equilibrium shifts in the direction of the endothermic reaction

Decreasing temperature:

  • Equilibrium shifts in the direction of the exothermic reaction

Example:

  • 2NO2(g)   ⇄  N2O4(g)
    • Forward reaction is exothermic
    • Backward reaction is endothermic
  • Temperature increases, equilibrium shifts to the left
  • Temperature decreases, equilibrium shifts to the right
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Catalysts

  • Catalysts do not change the position of the equilibrium, just the rate at which the equilibrium is established.
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