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Calculating the Equilibrium Constant

Chemistry: IB Diploma Higher Level, topic 5.6.1

Calculating the Equilibrium Constant

The equilibrium constant (KcK_c or KpK_p) can be calculated using experimental methods.

Experimental methods

Experimental methods

  • The concentrations of the reactants and products at equilibrium can be used to calculate KcK_c.
  • The partial pressures of the reactants and products at equilibrium can be used to calculate KpK_p.
Example

Example

  • The reversible reaction I2(aq) + I−(aq) ⇌\rightleftharpoons I3−(aq) is at equilibrium.
  • The reaction quotient for this reaction is Qc=[I3−][I2]  [I−]Q_c = \large\frac{[I_3^-]}{[I_2]\; [I^-]}
  • If you know the concentrations of each reactant and product at equilibrium, you can substitute in the values and calculate KcK_c.
  • Remember QcQ_c ≡ KcK_c at equilibrium.
Example cont.

Example cont.

  • Kc=[I3−][I2]  [I−]K_c = \large\frac{[I_3^-]}{[I_2]\; [I^-]} with the concentrations of each substance at equilibrium.
  • 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 the Equilibrium Constant

  • Which of the following is the correct KcK_c expression for the below reaction?

    N2(g)+O2(g)⇌2NO(g)

  • Which of the following is the correct KpK_p expression for the below reaction?

    2H2O(g)⇌H2(g)+O2(g)

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