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First teaching 2014

Last exams 2024

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Equilibrium Constant Relationships (DP IB Chemistry: SL)

Revision Note

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Equilibrium Constant Relationships

  • In the previous section we saw that the concentrations of the substances are raised to the power of the coefficients from the balanced equation
  • This means the Kc expression is dependent on a specific equation
  • For example, take the reaction between nitrogen and hydrogen to make ammonia

½N2(g)    +    1½H2(g)  ⇌     NH3(g)

  • The Kc expression for this reaction is:

Kc expression for NH3, downloadable IB Chemistry revision notes

  • If you double the stoichiometry the equation becomes

N2(g)    +    3H2(g)  ⇌ 2NH3(g)

  • The new Kc expression for this reaction is then:

Kc expression for NH3 doubled, downloadable IB Chemistry revision notes

  • What is the relationship between these two Kc values? You can probably see that when we double the coefficient the new Kc is the square of the original value:

Kc relationships squared, downloadable IB Chemistry revision notes

 

  • If we reverse the equation:

2NH3(g) ⇌ N2(g)    +    3H2(g)

  • Kc  becomes the reciprocal of the original Kc value:

Kc expression for NH3 reversed, downloadable IB Chemistry revision notes 
  • Test your understanding in the following example:

Worked example

Kc for 2NO2 (g) + F2 (g) ⇌ 2NO2F (g) is 7.1 × 1032What is Kc for the following reaction, at the same temperature?

NO2F (g) ⇌  NO2 (g) + ½F2 (g)

Answer:

The correct option is B.

    • The original equation has been reversed and halved, so the Kvalue must be square rooted and inverted to obtain the reciprocal

Examiner Tip

You must use square brackets in equilibrium constant expressions as they have a specific meaning, representing concentrations. In an exam answer you would lose the mark if you used round brackets

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Stewart

Author: Stewart

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Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.