Effect of Concentration on Equilibrium

The effect of concentration on equilibrium

If the concentration of a substance is changed, the equilibrium will shift to minimise the effect of that change.

• If the concentration of a \(\color{blue}{\textbf{reactant}}\) is increased the equilibrium will shift in the direction of the reaction that uses the reactants, so that the reactant concentration decreases. The forward reaction is favoured.

• The forward reaction is also favoured if the concentration of the \(\color{red}{\textbf{product}}\) is decreased, so that more product is formed.

• If the concentration of a \(\color{blue}{\textbf{reactant}}\) is decreased the equilibrium will shift in the direction of the reaction that produces the reactants, so that the reactant concentration increases. The reverse reaction is favoured.

• The reverse reaction is also favoured if the concentration of the \(\color{red}{\textbf{product}}\) is increased, so that product is used.

For example, in the reaction between sulfur dioxide and oxygen to produce sulfur trioxide:

\(\color{blue}{\text{2SO}_{2}\text{(g)}} + \color{blue}{\text{O}_{2}\text{(g)}} \leftrightharpoons \color{red}{\text{2SO}_{3}\text{(g)}}\)

• If the \(\color{blue}{\text{SO}_{2}}\) or \(\color{blue}{\text{O}_{2}}\) concentration was increased:

  • Le Chatelier's principle predicts that equilibrium will shift to decrease the concentration of reactants.

  • Increasing the rate of the forward reaction will mean a decrease in reactants.

  • So some of the sulfur dioxide or oxygen is used to produce sulfur trioxide.

  • Equilibrium shifts to the right. That is, when a new equilibrium is reached (when the rate of forward and reverse reactions are equal again), there will be more product than before.

    Fact:

    When the concentration of reactants is increased, the equilibrium shifts to the right and there will be more product than before. There will also be more reactants than before (more reactants were added). Once equilibrium has been reestablished (the rate of the forward and reverse reactions are equal again), \(\text{K}_{\text{c}}\) will be the same as it was before the change to the system. This concept is explained in more detail later in this tutorial.

• If the \(\color{blue}{\text{SO}_{2}}\) or \(\color{blue}{\text{O}_{2}}\) concentration was decreased:

  • Le Chatelier's principle predicts that the equilibrium will shift to increase the concentration of reactants.

  • Increasing the rate of the reverse reaction will mean an increase in reactants.

  • So some sulfur trioxide would change back to sulfur dioxide and oxygen to restore equilibrium.

  • Equilibrium shifts to the left. That is, when a new equilibrium is reached there will be less product than before.

• If [\(\color{red}{\text{SO}_{3}}\)] decreases:

  • Le Chatelier's principle predicts that the equilibrium will shift to increase the concentration of products.

  • Increasing the rate of the forward reaction will mean an increase in products.

  • So some sulfur dioxide or oxygen is used to produce sulfur trioxide.

  • Equilibrium shifts to the right. That is, when a new equilibrium is reached there will be more product than before.

• If [\(\color{red}{\text{SO}_{3}}\)] increases:

  • Le Chatelier's principle predicts that the equilibrium will shift to decrease the concentration of products.

  • Increasing the rate of the reverse reaction will mean a decrease in products.

  • So some of the sulfur trioxide would change back to sulfur dioxide and oxygen to restore equilibrium.

  • Equilibrium shifts to the left. That is, when a new equilibrium is reached there will be less product than before.