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Introduction to Le Châtelier’s Principle

Chemistry: IB Diploma Higher Level, topic 5.4.1

Le Châtelier’s Principle

Le Châtelier’s principle is a powerful tool for predicting the effect of physical changes on an equilibrium.

Definition

Definition

  • Le Châtelier’s principle is simple but powerful.
  • Le Châtelier’s principle states that when a change is imposed on an equilibrium, the equilibrium position will shift to oppose the change.
What changes?

What changes?

  • The change Le Chatelier refers to can be as a result of:
  • Change in temperature.
  • Change in pressure.
  • Change in concentration.
  • Change in pH.
Equilibrium position

Equilibrium position

  • The equilibrium position describes how much of each chemical you have.
  • If you have 50% reactants and 50% products, that is one position.
  • If you have 75% reactants and 25% products, that is another.
Equilibrium shifts

Equilibrium shifts

  • A system at equilibrium is in a state of dynamic balance, with forward and reverse reactions taking place at equal rates.
  • If an equilibrium system is subjected to a change in conditions that affects these reaction rates differently (a stress), then the rates are no longer equal.
  • The system is now not at equilibrium.
  • The system will subsequently experience a net reaction in the direction of greater rate (a shift) that will re-establish the equilibrium.
Le Chatelier's principle in practice

Le Chatelier's principle in practice

  • If you change a system by heating it, increasing the pressure, or changing the concentration of a reactant, the proportion of reactants and products will change to:
  • Reduce the temperature.
  • Reduce the pressure.
  • Change the concentration of a reactant.
An example

An example

  • Say you have a reversible reaction, and the forwards reaction is exothermic (gives out heat).
  • If you cool the reaction down (e.g. by submerging it in an ice bath), the equilibrium position shifts to the right (i.e. the forwards reaction increases).
  • This will counteract the change and return the temperature of the system back towards its original value.
  • This increases the proportion of products relative to reactants.

Test yourself on Introduction to Le Châtelier’s Principle

  • What is Le Chatelier's Principle?

  • Which of the following are changes to which Le Chatelier's Principle applies?

  • What will happen to the equilibrium position of an exothermic reaction if it is cooled down?

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