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Properties of the Equilibrium Constant

Chemistry: IB Diploma Higher Level, topic 5.6.3

Properties of the Equilibrium Constant

The equilibrium constant (K) tells you to what extent reactants form products when a reaction is at equilibrium. There are some useful properties of the equilibrium constant (K):

Reverse reactions

Reverse reactions

  • The equilibrium constant (K) can be inverted to find for the equilibrium constant (K) for the reverse reaction.
  • E.g. K = 3 for a reaction.
  • The reverse of the reaction has K = 13\frac{1}{3}
K is inverted

K is inverted

  • K is inverted for the reverse of a reaction because the reactants and the products switch in the reaction.
  • Switching the reactants and the products flips the fraction used to calculate K.
  • A  ⇌  B      A\;\rightleftharpoons\;B\;\;\; →       Kc=[B][A]  \;\;\;K_c = \large\frac{[B]}{[A]}\; at equilibrium.
  • B  ⇌  A      B\;\rightleftharpoons\;A\;\;\; →       Kc=[A][B]  \;\;\;K_c = \large\frac{[A]}{[B]}\; at equilibrium.
Stoichiometric coefficients

Stoichiometric coefficients

  • Changing the stoichiometric coefficients in an equation changes the value of the equilibrium constant.
  • Multiplying the stoichiometric coefficients in an equation by a factor of xx results in the equilibrium constant (K) being raised to the power of xx.
  • A  ⇌  B      A\;\rightleftharpoons\;B\;\;\; →       Kc=[B][A]  \;\;\;K_c = \large\frac{[B]}{[A]}\; at equilibrium.
  • xA  ⇌  xB      xA\;\rightleftharpoons\;xB\;\;\; →       Kc′=[B]x[A]x\;\;\;K_{c'} = \large\frac{[B]^x}{[A]^x} =Kcx   = {K_c}^x\; at equilibrium.
Adding equilibrium equations

Adding equilibrium equations

  • When you add two or more equilibrium equations together, you form an overall equation.
  • If A ⇌\rightleftharpoons B and B ⇌\rightleftharpoons C, you can write one equilibrium where A ⇌\rightleftharpoons C.
Equilibrium constants

Equilibrium constants

  • A ⇌\rightleftharpoons B has the equilibrium constant K1 = [B][A]\large\frac{[B]}{[A]}
  • B ⇌\rightleftharpoons C has the equilibrium constant K2 = [C][B]\large\frac{[C]}{[B]}
  • You can work out the equilibrium constant for the overall reaction.
  • A ⇌\rightleftharpoons C has the equilibrium constant K3 = [C][A]\large\frac{[C]}{[A]}
The reaction quotient

The reaction quotient

  • The expression for the equilibrium constant (K) is the same as the expression for the reaction quotient (Q).
  • All the properties of the equilibrium constant (K) also apply to the reaction quotient (Q) because the expressions are the same.

Test yourself on Properties of the Equilibrium Constant

  • Reaction 1: 2Mg + O2 → 2MgO

    Reaction 2: 4Mg + 2O2 → 4MgO

    K for reaction 1 is 4. What is K for reaction 2?

  • Reaction 1: Mg + ½O2 → MgO

    Reaction 2: 4Mg + 2O2 → 4MgO

    K for reaction 2 is 16. What is K for reaction 1?

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