Chemistry • Year 12 • Module 5 • Lesson 9

Writing Keq Expressions

Recall and apply the core rules for writing equilibrium constant expressions: products over reactants, stoichiometric exponents, and excluding pure solids and pure liquids.

Build • Recall & Vocab

1. Match each term to its definition

Write the correct term from the word bank into the right-hand column. Each term is used once. 10 marks (1 each)

Word bank: equilibrium constant (K_eq) • stoichiometric coefficient • homogeneous equilibrium • heterogeneous equilibrium • activity • molar concentration • reciprocal relationship • multiplier relationship • equilibrium position • temperature

#	Definition	Your term
1.1	The ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficient.
1.2	The number written in front of a species in a balanced equation; used as the exponent in a K_eq expression.
1.3	An equilibrium in which all reactants and products are in the same phase.
1.4	An equilibrium in which reactants and products exist in more than one phase.
1.5	The thermodynamic quantity equal to 1 for pure solids and pure liquids, which is why they are excluded from K_eq expressions.
1.6	Amount of substance per unit volume (mol L^-1); the quantity used for aqueous and gas species in K_eq.
1.7	When an equation is reversed, the new K_eq equals 1 divided by the original K_eq.
1.8	When all coefficients in an equation are multiplied by n, the new K_eq equals the original K_eq raised to the power n.
1.9	The relative amounts of reactants and products present at equilibrium; changes with concentration, pressure, and temperature — but not with catalysts.
1.10	The only factor that changes the numerical value of K_eq.

Stuck? Revisit the lesson formula panel and Key Terms grid.

2. True or false — with correction

Circle T or F. If false, write the corrected version on the line below. 8 marks (1 T/F + 1 correction where needed)

2.1 For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the correct K_eq expression is K_eq = [NH₃]² / ([N₂][H₂]³). T / F

2.2 CaCO₃(s) ⇌ CaO(s) + CO₂(g) has the K_eq expression: K_eq = [CaO][CO₂] / [CaCO₃]. T / F

2.3 Adding a catalyst to a system at equilibrium increases the value of K_eq. T / F

2.4 If K_eq = 3.2 × 10⁸, the equilibrium mixture consists almost entirely of products. T / F

Stuck? Check the lesson’s rules for exclusion of pure solids/liquids, magnitude guide, and temperature dependence.

3. Write the K_eq expression for each reaction

Complete the table. In the ‘Excluded species’ column, write any species omitted from the expression and the reason why. Leave blank if nothing is excluded. 12 marks (2 each)

Equilibrium equation	K_eq expression	Excluded species & reason
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
CaCO₃(s) ⇌ CaO(s) + CO₂(g)
CH₃COOH(aq) ⇌ CH₃COO^-(aq) + H⁺(aq)
Fe₂O₃(s) + 4H₂(g) ⇌ 3Fe(s) + 4H₂O(g)
AgCl(s) ⇌ Ag⁺(aq) + Cl^-(aq)
CO₂(g) + C(s) ⇌ 2CO(g)

Stuck? Apply the two-step flowchart: (1) write products/reactants each raised to their coefficient, then (2) remove any (s) species and any pure liquid acting as solvent.

4. Fill in the blanks — K_eq rules in context

Use the word bank to complete the paragraph. Each word is used once. 8 marks (1 each)

Word bank: numerator • denominator • stoichiometric • excluded • temperature • catalyst • one • aqueous

For any reversible reaction at equilibrium, the K_eq expression places product concentrations in the ___________ and reactant concentrations in the ___________. Each concentration is raised to the power of its ___________ coefficient as written in the balanced equation. Pure solids and pure liquids are ___________ because their thermodynamic activity is always equal to ___________. Gas-phase species (g) and ___________ species (aq) are both included in K_eq. The numerical value of K_eq changes only when ___________ changes; adding a ___________ shifts neither the equilibrium position nor the value of K_eq.

Stuck? Re-read the lesson formula panel and Card 1 (rules for writing K_eq) and Card 5 (temperature dependence).

5. What does each quantity tell us?

Answer each in 1–2 sentences using precise lesson terms. 6 marks (2 each)

5.1 What does the magnitude of K_eq tell us about the composition of the equilibrium mixture?

5.2 What does K_eq = 1/K_eq(original) mean in practice when you reverse a chemical equation?

5.3 Why is water excluded from the K_eq expression for CH₃COOH(aq) ⇌ CH₃COO^-(aq) + H⁺(aq)?

Stuck? Re-read Cards 2–4 of the lesson, focusing on heterogeneous equilibria examples and the magnitude guide.

Answers — Do not peek before attempting

Q1 — Term-definition matches

1.1 equilibrium constant (K_eq) • 1.2 stoichiometric coefficient • 1.3 homogeneous equilibrium • 1.4 heterogeneous equilibrium • 1.5 activity • 1.6 molar concentration • 1.7 reciprocal relationship • 1.8 multiplier relationship • 1.9 equilibrium position • 1.10 temperature.

Q2 — True / false with correction

2.1 True. K_eq = [NH₃]² / ([N₂][H₂]³) is correct: products numerator, stoichiometric exponents, all are gases.

2.2 False. Correction: K_eq = [CO₂]. CaCO₃ and CaO are both pure solids (activity = 1) and must be excluded. The only species contributing to K_eq is CO₂(g).

2.3 False. Correction: A catalyst does not change K_eq. It lowers the activation energy for both forward and reverse reactions equally, speeding up the attainment of equilibrium but leaving the equilibrium constant value unchanged.

2.4 True. K_eq = 3.2 × 10⁸ is much greater than 1 (much greater than 10³), so products are strongly favoured and the mixture consists almost entirely of products.

Q3 — K_eq expression table

Row 1 (2SO₂ + O₂ ⇌ 2SO₃, all gases): K_eq = [SO₃]² / ([SO₂]²[O₂]). No exclusions.

Row 2 (CaCO₃(s) ⇌ CaO(s) + CO₂(g)): K_eq = [CO₂]. Excluded: CaCO₃ and CaO — both pure solids, activity = 1.

Row 3 (CH₃COOH(aq) ⇌ CH₃COO^-(aq) + H⁺(aq)): K_eq = [CH₃COO^-][H⁺] / [CH₃COOH]. No exclusions (all aqueous; water is the solvent and not shown).

Row 4 (Fe₂O₃(s) + 4H₂(g) ⇌ 3Fe(s) + 4H₂O(g)): K_eq = [H₂O]⁴ / [H₂]⁴. Excluded: Fe₂O₃ and Fe — pure solids. Note: H₂O here is a gas (not solvent), so it IS included.

Row 5 (AgCl(s) ⇌ Ag⁺(aq) + Cl^-(aq)): K_eq = [Ag⁺][Cl^-]. Excluded: AgCl — pure solid, activity = 1. (This expression is also the K_sp for AgCl.)

Row 6 (CO₂(g) + C(s) ⇌ 2CO(g)): K_eq = [CO]² / [CO₂]. Excluded: C — pure solid, activity = 1.

Q4 — Cloze paragraph

numerator; denominator; stoichiometric; excluded; one; aqueous; temperature; catalyst.

Q5 — Function recall

5.1 K_eq much greater than 1 (typically >10³) means products are strongly favoured; K_eq close to 1 means both reactants and products are significant; K_eq much less than 1 (typically <10^-3) means reactants are strongly favoured and very little product is present at equilibrium.

5.2 The reciprocal relationship means that if the forward reaction has K_eq = K, the reverse reaction has K_eq = 1/K. Practically: a large K for the forward reaction becomes a small K for the reverse — a reaction that strongly favours products in the forward direction strongly favours reactants in the reverse direction.

5.3 Water acts as the solvent in aqueous reactions. As a pure liquid solvent, its thermodynamic activity is defined as 1 and its concentration does not change measurably during the reaction. It is therefore excluded from K_eq (and K_a) expressions for reactions in aqueous solution.