IB Chemistry SL

Revision Notes

7.1.4 The Reaction Quotient

The Reaction Quotient

  • 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 Kc:

Reaction Quotient Expression, downloadable IB Chemistry revision notes

  • 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:

Worked Example

The equilibrium constant for the following reaction:

COI2 (g) ⇌ CO (g) + I2 (g)

is 5.1 x 10-2 at 298 K

Deduce whether the following reaction mixture concentrations represent a reaction at equilibrium and for those not at equilibrium indicate the direction is proceeding:

Reaction Quotient-Worked Example Question, downloadable IB Chemistry revision notes

Answer:

The reaction quotient expression is

Reaction mixture 1:

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

 

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