Write Kc for aA + bB \rightleftharpoons cC + dD.

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

Write Kc for N2(g) + 3H2(g) \rightleftharpoons 2NH3(g).

Kc = \frac{[NH3]^2}{[N2][H2]^3}

What does the square-bracket symbol [X] mean?

The concentration of X in mol dm⁻³ at equilibrium.

What is a homogeneous equilibrium?

One in which all species are in the same phase.

What is a heterogeneous equilibrium?

One in which the species are in more than one phase, such as a solid and a gas.

Why are pure solids left out of Kc expressions?

The concentration of a pure solid is constant, so it is incorporated into the value of Kc.

How is Kc for a reversed equation related to the original Kc?

It is the reciprocal: Kc' = \frac{1}{Kc}.

How does Kc change if every coefficient in the equation is doubled?

The new constant is the square of the original: Kc^2.

How does Kc change if every coefficient is halved?

The new constant is the square root of the original: \sqrt{Kc}.

Why must Kc be quoted with its equation?

The powers in the expression depend on the coefficients as written, so different forms of the equation have different expressions and values.

The equilibrium constant KcEdexcel A-Level Chemistry: Flashcards

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What does the Kc expression contain?: Equilibrium concentrations of the species in the balanced equation, products on the top line and reactants on the bottom, each raised to the power of its coefficient.
Write Kc for $aA + bB \rightleftharpoons cC + dD$ .: $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$
Write Kc for $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ .: $K_c = \frac{[\text{NH}_3]^2}{[\text{N}_2][\text{H}_2]^3}$
What does the square-bracket symbol [X] mean?: The concentration of X in mol dm⁻³ at equilibrium.
What is a homogeneous equilibrium?: One in which all species are in the same phase.
What is a heterogeneous equilibrium?: One in which the species are in more than one phase, such as a solid and a gas.
Why are pure solids left out of Kc expressions?: The concentration of a pure solid is constant, so it is incorporated into the value of Kc.
Write Kc for $\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)$ .: $K_c = [\text{CO}_2]$
Write Kc for $\text{C}(s) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}(g) + \text{H}_2(g)$ .: $K_c = \frac{[\text{CO}][\text{H}_2]}{[\text{H}_2\text{O}]}$
How is Kc for a reversed equation related to the original Kc?: It is the reciprocal: $K_c' = \frac{1}{K_c}$ .
How does Kc change if every coefficient in the equation is doubled?: The new constant is the square of the original: $K_c^2$ .
How does Kc change if every coefficient is halved?: The new constant is the square root of the original: $\sqrt{K_c}$ .
Why must Kc be quoted with its equation?: The powers in the expression depend on the coefficients as written, so different forms of the equation have different expressions and values.

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).