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The Reaction Quotient (HL) (HL IB Chemistry)

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The Reaction Quotient

What is the reaction quotient?

  • The reaction quotient, Q, is calculated using the same equation as the equilibrium constant expression, but with non-equilibrium concentrations of reactants and products
  • The expression for Q is therefore the same as K

For more information on the equilibrium constant expression, see our revision notes on The Equilibrium Law

  • 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 = K then the reaction is at equilibrium, no net reaction occurs
    • If Q < K the reaction proceeds to the right in favour of the products
    • If Q > K the reaction proceeds to the left in favour of the reactants
  • Using concentration values 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 reaction below is 5.1 x 10-2 at 298 K.

COI2 (g) ⇌ CO (g) + I2 (g)

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

Reaction mixture [COI2 (g)] [CO (g)] [I2 (g)]
1 0.012 0.050 0.050
2 0.020 0.032 0.032
3 0.150 0.025 0.025

Answer:

  • The reaction quotient expression is:

Q space equals space fraction numerator open square brackets CO space open parentheses straight g close parentheses close square brackets open square brackets straight I subscript 2 space open parentheses straight g close parentheses close square brackets over denominator open square brackets COI subscript 2 space open parentheses straight g close parentheses close square brackets end fraction

  • For reaction mixture 1:

Q space equals space fraction numerator 0.050 space cross times space 0.050 over denominator 0.012 end fraction space equals space 0.21

    • In this mixture Q >> K, so Q has to decrease to reach K

    • This means the reaction must be moving to the left, in order to reach equilibrium, so the reactants are favoured

  • For reaction mixture 2:

size 14px Q size 14px space size 14px equals size 14px space fraction numerator begin mathsize 14px style 0.032 space cross times space 0.032 end style over denominator begin mathsize 14px style 0.020 end style end fraction size 14px space size 14px equals size 14px space size 14px 0 size 14px. size 14px 051

    • In this mixture, the value of Q = K, so the reaction is at equilibrium

  • For reaction mixture 3:

Q space equals space fraction numerator 0.025 space cross times space 0.025 over denominator 0.150 end fraction space equals space 0.0042

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

Exam Tip

The value of Q is not a fixed value as it can be measured at any time but the value of K  is constant at a given temperature.

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Caroline

Author: Caroline

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.