Colourful Solutions > The extent of chemical change > The equilibrium law (SL)

The equilibrium law describes how the position of equilibrium depends on the concentrations of the reactants and the products of the equilibrium reaction.

Syllabus reference R2.3.2

Reactivity 2.3.2 - The equilibrium law describes how the equilibrium constant, K, can be determined from the stoichiometry of a reaction.

  • Deduce the equilibrium constant expression from an equation for a homogeneous reaction.

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The equilibrium expression

The concentrations of reactants and products are constant at equilibrium. This means that their ratio can be expressed as a simple mathematical formula:

For any reaction of the form:

The equilibrium expression is given by:

Where:

  • [A] = the concentration of component A,
  • [B] = the concentration of component B, etc.

Kc is called the equilibrium constant with respect to the concentration of the reactants and products (that's why there is a subscript 'c' following the letter traditionally used for a constant value, 'K'.

NOTE In the above example, the coefficients of the equation appear in the equilibrium expression as powers to which the concentrations of the reactants and products are raised.

Example:Show the equilibrium expression, with respect to concentrations, for the following reversible reaction:

H2(g) + I2(g) ⇋ 2HI(g)

Kc is given by the product concentrations raised to the power of their coefficients, divided by the reactants concentrations raised to the power of their coefficients:

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Solids and liquids

Only those substances that appear in the equilibrium in a form that can be expressed in terms of concentration can appear in the equilibrium expression. This means to say that solids NEVER appear and liquids only appear if they are part of a homogeneous system.

Example

Show the expression for the equilibrium that exists between iron, water, hydrogen and iron(III) oxide at elevated temperatures:

2Fe(s) + 3H2O(g) ⇋ Fe2O3(s) + 3H2(g)

Inspection of the equation reveals that both iron and iron oxide are in the solid states, i.e. they do not have a concentration. This means that they cannot appear in the equilibrium expression, therefore:

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