The Effect of Changing Concentration

Higher tier only

How does concentration affect equilibrium?

If the concentration of a reactant increases:
- The equilibrium will shift to the right
- This is to reduce the effect of increasing the concentration of a reactant
- This makes more product / increases the yield
If the concentration of a reactant decreases:
- The equilibrium will shift to the left
- This is to reduce the effect of decreasing the concentration of a reactant
- This makes less product / decreases the yield

Worked example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride:

ICl (l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown yellow

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

Answer:

An increase in the concentration of ICl or Cl₂ causes the equilibrium to shift to the right (product)
- So, more of the yellow solid, ICl₃, is formed, which means that 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 decrease in the concentration of ICl or Cl₂ causes the equilibrium to shift to the left (reactant)
- So, less of the yellow solid, ICl₃, is formed, which means that 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

Examiner 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!

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

Revision Note