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Calculating Equilibrium Concentrations

Chemistry: IB Diploma Higher Level, topic 5.6.4

Calculating Equilibrium Concentrations

You can calculate the concentrations or partial pressures of substances in a reversible reaction at equilibrium if you have some information about the equilibrium.

What you need to know

What you need to know

  • To complete calculations with the equilibrium constant, you need to know three of the following:
  • The balanced equation for the reaction.
  • The initial concentrations of the substances.
  • The equilibrium concentrations of the reactants or products.
  • The equilibrium constant (K).
Example question

Example question

  • Iodine molecules react reversibly with iodide ions to produce triiodide ions.
  • I2(aq) + I−(aq) ⇌\rightleftharpoons I3−(aq)
  • A solution has concentrations of I2 and I− both equal to 1.000 × 10−3 M before the reaction gives an equilibrium.
  • At equilibrium, the concentration of I2 is 6.61 × 10−4 M.
  • What is the equilibrium constant for the reaction?
Solutions - step 1

Solutions - step 1

  • The equilibrium constant for this reaction can be calculated using
  • Kc=[I3−][I2]  [I−]K_c = \large\frac{[I_3^-]}{[I_2]\; [I^-]}
  • Equilibrium concentrations are needed for all the reactants and products to calculate the equilibrium constant (K).
Solution - step 2

Solution - step 2

  • You are told the initial concentrations of the reactants and the equilibrium concentration of the product in the question.
  • Create a table to show the initial concentrations.
Solution - step 3

Solution - step 3

  • As the reaction reaches equilibrium:
  • The reactants each lose a concentration of xx.
  • The product gains a concentration of xx.
  • Fill in another row of the table to show the change in concentrations.
Solution - step 4

Solution - step 4

  • Add another row which shows the concentrations of the reactants and products at equilibrium in terms of xx.
Solution - step 5

Solution - step 5

  • You know from the question that the concentration of I2 at equilibrium is 6.61 × 10−4 M. So:
  • 1 × 10−3 − xx = 6.61 × 10−4
  • You can work out xx = 3.39 × 10−4 M
Solution - step 6

Solution - step 6

  • You can then update the equilibrium concentrations in your table by substituting in the values of xx.
Solution - step 7

Solution - step 7

  • Finally, substitute the equilibrium concentrations into the equilibrium constant (K) expression and solve to find K.
  • Kc=[I3−][I2]  [I−]K_c = \large\frac{[I_3^-]}{[I_2]\; [I^-]}
  • Kc=3.39×10−46.61×10−4  ×  6.61×10−4K_c = \large\frac{3.39\times10^{-4}}{6.61\times10^{-4}\;\times\;6.61\times10^{-4}} = 776

Test yourself on Calculating Equilibrium Concentrations

  • If a reaction is reversible, when can it be said to have reached equilibrium?

Practise this topic for free