Equilibria — AS & A Level Chemistry (9701) questions by topic
Kp and Kw calculations sit alongside qualitative shifts here: predicting how adding NaQ affects [H+] and pH for a weak acid, calculating Kp for the hydrogen-iodine equilibrium, and describing dynamic equilibrium and lattice structures in diamond and graphite.
Calculate, deduce, identify, state and explain lead 53 questions from 2023 to 2025 papers, spanning 1 to 19 marks, where a single mole-fraction slip changes the whole Kp value. Instant marking after each equilibrium calculation catches a squared Kw arithmetic error or an unsuitable significant-figure gradient reading immediately, rather than letting it distort a graph-based enthalpy calculation later in the same paper.
What examiners look for
- A common mistake was not appreciating that Kw = [H+]^2, leading to arithmetic errors when squaring the answer or giving incorrect indices.
- Examiners flag graph-reading errors: omitting the power of 10^-3 in x-coordinates, misreading values from the graph, or giving a gradient to an unsuitable number of significant figures.
- Reports note some candidates confusing graphs of rate against time with concentration against time.
- A recurring criticism is a less precise explanation such as 'the reaction moves backwards as it's exothermic', without linking the shift to the equilibrium position.
- Strong scripts correctly stated that an increase in temperature produced an increase in Kw, and most plotted all points with a sensible best-fit line that clearly ignored the anomaly.
May–June 2025, Paper 11
An aqueous solution X contains substance HQ which behaves as a weak acid. HQ(aq) ⇌ H^+(aq) + Q^-(aq) The soluble salt NaQ is added to X. What happens to the [H^+(aq)] and the pH…
Answer this question and get it marked →Dinitrogen tetroxide, N2O4, decomposes reversibly. N2O4 ⇌ 2NO2 ΔH = +58.0 kJ mol^-1 An equilibrium mixture of N2O4 and NO2 gases is placed in a closed container under standard…
Answer this question and get it marked →A reaction involving ammonium ions is shown. NH4^+ + OH^- ⇌ NH3 + H2O Four statements about this reaction are listed: 1 The ammonium ions are reduced. 2 In the reverse reaction,…
Answer this question and get it marked →May–June 2025, Paper 12
The reaction between sulfur dioxide and oxygen has the equation shown. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔH = –197 kJ mol^-1 1.0 mol of SO2 and 1.0 mol of O2 are placed in a closed…
Answer this question and get it marked →A reaction occurs when ammonium chloride is added to calcium hydroxide. What is the role of the ammonium ions in this reaction? A acid B base C oxidising agent D reducing agent
Answer this question and get it marked →May–June 2025, Paper 13
The reversible reaction shown is in equilibrium at a temperature of 450 °C. H2 + I2 ⇌ 2HI ΔH = –9.5 kJ mol^-1 The table shows the equilibrium concentrations: H2 = 1.02 mol dm^-3,…
Answer this question and get it marked →May–June 2025, Paper 14
Methanol, CH3OH, is made industrially from carbon monoxide and hydrogen in the equilibrium reaction shown. CO(g) + 2H2(g) ⇌ CH3OH(g) ΔH = –100 kJ mol^-1 Which statement about this…
Answer this question and get it marked →A nitrogen–hydrogen mixture, initially in the mole ratio of 1:3, reaches equilibrium with ammonia when 50% of the nitrogen has reacted. The total final pressure is p. N2 + 3H2 ⇌…
Answer this question and get it marked →May–June 2025, Paper 22
Diamond and graphite are both crystalline solids made from carbon atoms. Graphite conducts electricity. Diamond does not conduct electricity. (i) Name the type of lattice…
Answer this question and get it marked →May–June 2025, Paper 24
H2(g) and I2(g) react to form HI(g) in a reversible reaction: H2(g) + I2(g) ⇌ 2HI(g). In three separate experiments, a student combines different amounts of two or more of these…
Answer this question and get it marked →May–June 2025, Paper 41
Chromium(III) hydroxide, Cr(OH)3, is only slightly soluble in water. The value of the solubility product, Ksp, of Cr(OH)3 is 1.0 × 10−33 at 298 K. (i) Complete the expression for…
Answer this question and get it marked →May–June 2025, Paper 42
HClO is a weak acid. HClO ⇌ H⁺ + ClO⁻ Write the expression for Ka for this equilibrium and identify the conjugate acid–base pairs.
Answer this question and get it marked →May–June 2025, Paper 43
Chromium(III) hydroxide, Cr(OH)3, is only slightly soluble in water. The value of the solubility product, Ksp, of Cr(OH)3 is 1.0 × 10⁻³³ at 298 K. (i) Complete the expression for…
Answer this question and get it marked →May–June 2025, Paper 44
Nickel iodate, Ni(IO3)2, is a sparingly soluble salt. (i) Write an expression for Ksp of Ni(IO3)2. State the units of Ksp. (ii) The solubility of Ni(IO3)2 is 2.30×10⁻² mol dm⁻³.…
Answer this question and get it marked →May–June 2024, Paper 11
An alcohol, ROH, reacts reversibly with ethanoic acid to produce an ester. ROH(l) + CH3COOH(l) <= CH3COOR(l) + H2O(l) 3.0 mol of ROH, 2.0 mol of ethanoic acid and 1.0 mol of water…
Answer this question and get it marked →Graphs can be drawn to show the percentage of ammonia at equilibrium when nitrogen and hydrogen are mixed at different temperatures and pressures. Which diagram correctly…
Answer this question and get it marked →May–June 2024, Paper 12
Which reaction has an equilibrium constant, Kp, that has no units? A H2(g) + I2(g) <= 2HI(g) B 3H2(g) + N2(g) <= 2NH3(g) C 2NO2(g) <= N2O4(g) D 2SO2(g) + O2(g) <= 2SO3(g)
Answer this question and get it marked →May–June 2024, Paper 13
Hydrogen is produced industrially from methane as shown in the equation. CH4(g) + H2O(g) <= CO(g) + 3H2(g) deltaH(standard) = +205 kJ mol^-1 Which conditions give the highest…
Answer this question and get it marked →Gas X dissociates on heating to set up the following equilibrium. X(g) <= Y(g) + Z(g) A quantity of gas X is heated at constant pressure, p, at a certain temperature. The…
Answer this question and get it marked →May–June 2024, Paper 21
May–June 2024, Paper 22
Describe what is meant by dynamic equilibrium.
Answer this question and get it marked →May–June 2024, Paper 41
(i) State what is meant by partition coefficient, K(pc). (ii) The partition coefficient, K(pc), for a compound X between carbon disulfide, CS2, and water is 10.5. 1.85 g of X is…
Answer this question and get it marked →May–June 2024, Paper 42
Aqueous solutions of methanoic acid, HCOOH, and propanoic acid, CH3CH2COOH, are mixed together. An equilibrium is set up between two conjugate acid-base pairs. (i) Define…
Answer this question and get it marked →May–June 2024, Paper 43
(i) State what is meant by partition coefficient, K(pc). (ii) The partition coefficient, K(pc), for a compound X between carbon disulfide, CS2, and water is 10.5. 1.85 g of X is…
Answer this question and get it marked →October–November 2024, Paper 11
Lithium reacts with nitrogen at room temperature to form solid Li₃N. Three vessels of equal volume are connected by taps 1 and 2. The left-hand vessel is evacuated, the middle…
Answer this question and get it marked →When 0.20 mol of hydrogen gas and 0.15 mol of iodine gas are heated at 723 K until equilibrium is established, the equilibrium mixture contains 0.26 mol of hydrogen iodide. The…
Answer this question and get it marked →October–November 2024, Paper 12
When some solid Ca₅(PO₄)₃OH is added to a beaker of water, an equilibrium is set up. Ca₅(PO₄)₃OH(s) ⇌ 5Ca²⁺(aq) + 3PO₄³⁻(aq) + OH⁻(aq) Which compound, when added to the…
Answer this question and get it marked →Gaseous hydrogen and gaseous iodine react to form gaseous hydrogen iodide. H₂(g) + I₂(g) ⇌ 2HI(g) In an experiment, 2.0 mol of hydrogen and 2.0 mol of iodine are placed in a…
Answer this question and get it marked →October–November 2024, Paper 13
Lithium reacts with nitrogen at room temperature to form solid Li₃N. Three vessels of equal volume are connected by taps 1 and 2. The left-hand vessel is evacuated, the middle…
Answer this question and get it marked →When 0.20 mol of hydrogen gas and 0.15 mol of iodine gas are heated at 723 K until equilibrium is established, the equilibrium mixture contains 0.26 mol of hydrogen iodide. The…
Answer this question and get it marked →October–November 2024, Paper 41
Disodium phosphate, (Na+)2(HPO4 2-), reacts with an acid to form monosodium phosphate, Na+(H2PO4 -). (a)(i) Identify the ions that are a conjugate acid-base pair in this reaction,…
Answer this question and get it marked →October–November 2024, Paper 42
(a) The pH of a saturated solution of calcium hydroxide is 12.35 at 298 K. (a)(i) Show that the concentration of hydroxide ions in a saturated solution of calcium hydroxide is…
Answer this question and get it marked →October–November 2024, Paper 43
Disodium phosphate, (Na+)2(HPO4 2-), reacts with an acid to form monosodium phosphate, Na+(H2PO4 -). (a)(i) Identify the ions that are a conjugate acid-base pair in this reaction,…
Answer this question and get it marked →February–March 2023, Paper 12
The table shows the partial pressures in an equilibrium mixture formed by the Haber process. 3H2(g) + N2(g) ⇌ 2NH3(g) substance partial pressure / kPa --- --- nitrogen 7000…
Answer this question and get it marked →A reversible reaction is shown. 2NOCl(g) ⇌ 2NO(g) + Cl2(g) ΔH = +77.0 kJ mol⁻¹ Which change in conditions will move the position of equilibrium to the right and increase the value…
Answer this question and get it marked →February–March 2023, Paper 42
Hypophosphorous acid is an inorganic acid. The conjugate base of hypophosphorous acid is H2PO2⁻. Give the formula of hypophosphorous acid.
Answer this question and get it marked →May–June 2023, Paper 11
For which equilibrium do both of the equilibrium constants Kc and Kp have no units? A H2(g) + I2(g) ⇌ 2HI(g) B N2(g) + 3H2(g) ⇌ 2NH3(g) C N2O4(g) ⇌ 2NO2(g) D SO2(g) + ½O2(g) ⇌…
Answer this question and get it marked →May–June 2023, Paper 12
In which equilibrium will an increase in pressure at constant temperature increase the yield of the products on the right-hand side of the equation? A CaCO3(s) <= CaO(s) + CO2(g)…
Answer this question and get it marked →Hydrogen iodide is added to an evacuated reaction vessel. The vessel is sealed and warmed. A decomposition reaction occurs. Hydrogen and iodine are formed. Some hydrogen iodide…
Answer this question and get it marked →May–June 2023, Paper 13
Which mixture will react to form exactly one mole of water? volume 2.00 mol dm^-3 H2SO4 / cm3 volume 1.00 mol dm^-3 NaOH / cm3 --- --- --- A 250 500 B 250 1000 C 500 500 D 500 1000
Answer this question and get it marked →Sulfur dioxide reacts with oxygen as shown. 2SO2(g) + O2(g) <= 2SO3(g) delta H = -197 kJ mol^-1 Which change will move the equilibrium position to the right side and change the…
Answer this question and get it marked →Solutions X and Y both have a concentration of 0.10 mol dm^-3. A fixed volume of solution X is added to a conical flask, and solution Y is added from a burette to the conical…
Answer this question and get it marked →May–June 2023, Paper 52
Use Fig. 2.1 to calculate the hydrogen ion concentration of pure water at 45 degrees C. pH = -log[H+(aq)].
Answer this question and get it marked →October–November 2023, Paper 11
Nitrogen dioxide, NO2, exists in equilibrium with dinitrogen tetroxide, N2O4: 2NO2(g) <= N2O4(g), delta-H = -57 kJ mol-1. Which conditions give the greatest percentage of N2O4(g)…
Answer this question and get it marked →When an equimolar mixture of H2 and I2 react, the mole fraction of HI in the final mixture is x. What is the equilibrium constant, Kp, for the reaction? A: x^2 / (1-x)^2. B: x^2 /…
Answer this question and get it marked →October–November 2023, Paper 12
0.200 mol of sulfur dioxide and 0.200 mol of oxygen are placed in a 1.00 dm3 sealed container. The gases are allowed to react until equilibrium is reached: 2SO2 + O2 <= 2SO3. At…
Answer this question and get it marked →The diagram shows a gas syringe with a free-moving piston containing gaseous hydrogen, gaseous iodine and gaseous hydrogen iodide at equilibrium: H2(g) + I2(g) <= 2HI(g). Three…
Answer this question and get it marked →October–November 2023, Paper 13
Nitrogen dioxide, NO2, exists in equilibrium with dinitrogen tetroxide, N2O4. 2NO2(g) <= N2O4(g) delta H = -57 kJ mol^-1 Which conditions give the greatest percentage of N2O4(g)…
Answer this question and get it marked →When an equimolar mixture of H2 and I2 react, the mole fraction of HI in the final mixture is x. What is the equilibrium constant, Kp, for the reaction? A x^2 / (1 - x)^2 B x^2 /…
Answer this question and get it marked →October–November 2023, Paper 22
NO and NO2 react at 25 degrees C to give N2O3 as shown in the equation. NO(g) + NO2(g) <= N2O3(g) delta H = -7.2 kJ mol^-1 The reaction is reversible and reaches equilibrium in a…
Answer this question and get it marked →October–November 2023, Paper 41
Define Kw mathematically by completing the expression Kw = ...
Answer this question and get it marked →October–November 2023, Paper 42
Benzoic acid, C₆H₅COOH, is a weak acid; the Ka of benzoic acid is 6.31 × 10⁻⁵ mol dm⁻³ at 298 K. A 1.00 dm³ buffer solution is made at 298 K containing 1.00 g of C₆H₅COOH and a…
Answer this question and get it marked →October–November 2023, Paper 43
Define Kw mathematically by completing the expression Kw = ...
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