The Effect of Changing Concentration (AQA GCSE Chemistry)

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The Effect of Changing Concentration

  • Le Chatelier's Principle can be used to predict the effect of changes in concentration on systems in equilibrium
  • The following table summarises how a concentration change alters the position of equilibrium:

Effect of Concentration Changes on an Equilibrium Table

Effect of Concentration on Equilibrium, downloadable IGCSE & GCSE Chemistry revision notes

Worked example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride:

ICl (l) + Cl2 (g) ⇌ ICl3 (s)

dark brown            yellow

Question: Predict the effect of a change in the concentration of ICl or Cl2 on the position of equilibrium.

Answer

  • An increase in the concentration of ICl or Cl2 causes the equilibrium to shift to the right, so more of the yellow solid, ICl3, is formed. The reaction would be increasingly yellow!
    • This is because when the concentration of a reactant increases, the equilibrium moves to oppose the change and create more product from the excess reactant
  • A decease in the concentration of ICl or Cl2 causes the equilibrium to shift to the left, so less of the yellow solid, ICl3, is formed. The reaction would be increasingly brown!
    • This is because when the concentration of a reactant decreases, the equilibrium moves to oppose the change and create more reactant from the now in excess product

Exam Tip

Changing the concentration of either the reactants or the products pushes the system away from equilibrium. The system responds to bring itself back to the equilibrium state by restoring the position of equilibrium. This means opposing the change.

You can think of this like a grumpy toddler trying to do exactly the opposite of what is done to them!

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Stewart

Author: Stewart

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 Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.