The equilibrium constant KcEdexcel A-Level Chemistry: Revision notes

Section 1

The equilibrium constant expression

For a reversible reaction at equilibrium, Kc relates the equilibrium concentrations of the species in the equation. For the general homogeneous equilibrium $aA + bB \rightleftharpoons cC + dD$ :

$K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

Square brackets mean concentration in mol dm⁻³ at equilibrium. The concentration of each product is raised to the power of its balancing coefficient and multiplied together on the top line. The reactants are treated the same way on the bottom line.

Key termsKcequilibrium concentration

Section 2

Writing an expression step by step

Write the balanced equation with state symbols.
Put the products on top and the reactants underneath.
Raise each concentration to the power of its coefficient.
Leave out pure solids (see below).

Examples

$\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ : $K_c = \frac{[\text{NH}_3]^2}{[\text{N}_2][\text{H}_2]^3}$
$\text{Fe}^{3+}(aq) + \text{SCN}^-(aq) \rightleftharpoons [\text{Fe(SCN)}]^{2+}(aq)$ : $K_c = \frac{[\text{Fe(SCN)}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]}$

Key termsbalanced equationcoefficient

Common mistake

Do not add concentrations, and do not use initial concentrations. Multiply, with products on the top line.

Section 3

Homogeneous and heterogeneous systems

In a homogeneous equilibrium every species is in the same phase (all gases, or all in solution). Every species appears in the expression.

In a heterogeneous equilibrium the species are in more than one phase. The concentration of a pure solid is constant, so it is incorporated into Kc and is left out of the expression.

$\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)$ : $K_c = [\text{CO}_2]$
$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightleftharpoons \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$ : $K_c = \frac{[\text{H}_2]^4}{[\text{H}_2\text{O}]^4}$

Key termshomogeneousheterogeneouspure solid

Section 4

Changing the way the equation is written

The powers in Kc are the coefficients in the equation as written, so the expression depends on the equation.

Reverse the equation: products and reactants swap, so the new constant is the reciprocal, $K_c' = \frac{1}{K_c}$ .
Multiply the coefficients by n: every power is multiplied by n, so the new constant is $K_c^{\,n}$ . Doubling gives $K_c^2$ ; halving gives $\sqrt{K_c}$ .

This is why a value of Kc must always be quoted with the equation it refers to.

Key termsreciprocal

Exam tip

Check the equation you are given, not the one you remember. A halved or reversed equation changes the expression.

Must know

Kc uses equilibrium concentrations, products over reactants, each to the power of its coefficient
Pure solids are left out; gases and aqueous species are included
Reversing an equation inverts Kc; multiplying by n raises it to the power n
Quote the equation with any value of Kc

That's the notes covered.

Carry on to the next subtopic.

Exam questions on The equilibrium constant Kc

Iron(III) ions react with thiocyanate ions in aqueous solution to form a blood-red complex ion: Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq). A student studies the equilibrium mixture.
The equation is rewritten as 2Fe³⁺(aq) + 2SCN⁻(aq) ⇌ 2[Fe(SCN)]²⁺(aq). Deduce the expression for Kc for this equation and state how it is related to Kc for the original equation.2 marks
Calcium carbonate decomposes when heated strongly in a sealed container: CaCO₃(s) ⇌ CaO(s) + CO₂(g). A student investigates this heterogeneous equilibrium.
Deduce the expression for Kc for the equilibrium 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g).2 marks
Methanol is manufactured from carbon monoxide and hydrogen: CO(g) + 2H₂(g) ⇌ CH₃OH(g). All three substances are gases.
Deduce the expression for Kc for the formation of methanol. Deduce the expression for Kc' for the decomposition of methanol, CH₃OH(g) ⇌ CO(g) + 2H₂(g), and state how Kc' is related to Kc.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).