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The Reaction Quotient (HL) (DP IB Chemistry): Revision Note

Caroline Carroll

Written by: Caroline Carroll

Reviewed by: Philippa Platt

Updated on

The reaction quotient

What is the reaction quotient?

  • The reaction quotient, Q, uses the same expression as K, but with concentrations that may not be at equilibrium

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

  • By comparing Q and K, we can tell:

    • If the reaction is at equilibrium

    • Or which direction it will shift to reach equilibrium

  • 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

Examiner Tips and Tricks

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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    Author
    Reviewer
    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics & Chemistry Subject Lead

    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.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener