The Effect of Changing Concentration (AQA GCSE Chemistry: Combined Science)
Revision Note
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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
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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