• The?reaction quotient,?Q, is the ratio of products and reactants for a reaction that has?NOT?yet reached equilibrium
• The expression for?Q?is very similar to?K_c:

• It is a useful concept because the size of?Q?can tell us how far a reaction is from equilibrium and in which direction the reaction proceeds
• For Example,
  • If?Q = Kc?then the reaction is?at equilibrium, no net reaction occurs
  • If?Q < Kc?the reaction?proceeds to the right?in favour of the products
  • If?Q > Kc?the reaction?proceeds to the left?in favour of the reactants

• Using values of the concentrations of the substances present we can work out if a reaction is at equilibrium or not, as the following example shows:

Answer:

The reaction quotient expression is

Reaction mixture 1:

In this mixture?Q >> K_c, so Q has to decrease to reach K_c. This means the reaction must be moving to the left, in order to reach equilibrium, so the reactants are favoured

Reaction mixture 2:

In this mixture, the value of?Q = Kc, so the reaction is at equilibrium

Reaction mixture 3:

In this mixture?Q < K_c, so Q has to increase to reach K_c. This means the reaction must be moving to the right, in order to reach equilibrium, so the products are favoured