Chemical reactivity

Practice

Recall it first, then use it — testing yourself builds the memory, where rereading the notes above only feels easier.

Recall and application shuffled together — you have to work out what each item wants before you can answer it, which is the part the exam actually tests. Both kinds feed the same spacing schedule, so anything you miss comes back sooner.

Say the answer out loud before flipping — that attempt is where the learning happens, even when you get it wrong.

Exam-style only. Instant feedback, shuffled each attempt, best score saved on this device.

What actually comes up

Three questions, every year, each blending real applied contexts (pāua shells, industrial gas manufacture, fertiliser production) with the three strands: rates, equilibrium, and acids & bases. Writing and using the Kc EXPRESSION — from a balanced equation, or backwards from an expression to the equation — appears in all three of the last three years, and is not covered anywhere else on this page except the new Kc section above.

Across 4 years

The pattern — what to prepare

The 2025 paper ↗ Marking schedule ↗

Question One — Ocean acidification equilibrium

H₂CO₃(aq) + H₂O(ℓ) ⇌ HCO₃⁻(aq) + H₃O⁺(aq), Kc = 4.30 × 10⁻⁷.

• 1a(i) Write the equilibrium constant expression for this reaction. Achieved
• 1a(ii) The value of the equilibrium constant is 4.30 × 10⁻⁷. Explain what this indicates about the ratio of products and reactants. Merit
• 1a(iii) Predict the impact on the position of equilibrium as ocean temperature increases, given ΔrH = −3.85 kJ mol⁻¹ — describe the effect on the forward and reverse reactions and explain any change to Kc. Excellence
• 1b(iii) Compare the pH and conductivity of hydrochloric acid (strong, pH 1.0) and carbonic acid (weak, pH 4.7) at equal 0.10 mol L⁻¹ concentration, with definitions, equations and an explanation of each. Excellence

Question Two — Pāua shells and shell-building enzymes

Calcium carbonate pāua shells react with ocean acid; carbonic anhydrase catalyses shell formation.

• 2a(ii) Two pāua shells of different total surface area (110 cm² vs 220 cm²) are compared. Explain which shell is more vulnerable to ocean acidification, referring to collision theory and rate of reaction. Merit
• 2b(ii) Discuss how the carbonic anhydrase enzyme affects the rate of shell formation, referring to the role of a catalyst, collision theory, and activation energy. Excellence
• 2c(iii) 0.10 mol L⁻¹ solutions of Ca(OH)₂ (pH 13.2), CaCl₂ (pH 7) and Ca(CH₃COO)₂ (pH 8) are given. Discuss the differences in pH, including definitions of acids and bases, how proton transfer links to pH, and each compound's ability to transfer protons. Excellence

Question Three — Manufacturing PCl₅ and NOCl

PCl₃(g) + Cl₂(g) ⇌ PCl₅(g); Cl₂(g) + 2NO(g) ⇌ 2NOCl(g).

• 3a(ii) Explain why producing PCl₅ is favoured when the reaction is transferred to a vessel with a much smaller volume — describe the changes occurring, explain equilibrium principles, and state what happens to the position of equilibrium as pressure increases. Excellence
• 3b(i) Calculate the equilibrium constant at 300 °C given [PCl₃] = 0.016 mol L⁻¹, [Cl₂] = 0.021 mol L⁻¹, [PCl₅] = 0.00013 mol L⁻¹. Merit
• 3c The formation of NOCl(g) is favoured by lower temperature, yet the reaction is run at 95 °C rather than 25 °C — identify whether the forward reaction is exothermic or endothermic, explain the disadvantage of higher temperature using equilibrium concepts, and explain the advantage using collision theory and rate principles. Excellence

The 2024 paper ↗ Marking schedule ↗

Question One — Extracting aluminium from bauxite

Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂(aq) + H₂O(ℓ), graph of Al₂O₃ mass vs time.

• 1a(ii) Explain the effect that increasing NaOH concentration from 0.5 to 2 mol L⁻¹ would have on the rate of reaction, referring to the mass of Al₂O₃, each section of the line, and collision theory. Merit
• 1c Using collision and particle theories, explain why a catalyst would help increase the rate of production of NaAlO₂ — including an energy profile diagram. Excellence

Question Two — Sulfuric acid manufacture

2SO₂(g) + O₂(g) ⇌ 2SO₃(g), Kc = 32.7 at 25 °C.

• 2a(ii) Calculate [O₂] at equilibrium given [SO₂] = 0.17 mol L⁻¹ and [SO₃] = 0.50 mol L⁻¹. Merit
• 2b(i) Given [SO₂] = 0.530, [O₂] = 0.710, [SO₃] = 0.620 mol L⁻¹, use a calculation to explain why this reaction is NOT at equilibrium. Excellence
• 2c(i) For SO₃(g) + H₂O(g) ⇌ H₂SO₄(g), identify and describe the effect on the position of equilibrium when water vapour is added, and when NaOH(aq) is added. Merit
• 2c(ii) Explain why running this reaction under high pressure is beneficial to the manufacturing process. Excellence

Question Three — Conjugate pairs and conductivity

HSO₄⁻(aq) + CO₃²⁻(aq) ⇌ SO₄²⁻(aq) + HCO₃⁻(aq); 0.1 mol L⁻¹ solutions of NH₃, HNO₃, NH₄Cl.

• 3a(ii) Write the Kc expression for the reaction of HSO₄⁻ and CO₃²⁻. Achieved
• 3a(iii) The Kc value increases when temperature rises from 25 °C to 50 °C. Explain whether the forward reaction is exothermic or endothermic. Merit
• 3b(i) Explain the pH of 0.1 mol L⁻¹ NH₃, HNO₃ and NH₄Cl solutions, choosing a pH band for each, classifying each substance, and giving equations. Excellence
• 3b(ii) Discuss the conductivity of the three equal-concentration solutions, explaining the requirements for conduction and referencing the equations from (b)(i). Excellence

The 2022 paper ↗ Marking schedule ↗

Question One — Hydrogen iodide equilibrium and strong/weak acids

H₂(g) + I₂(g) ⇌ 2HI(g), ΔrH = +53.0 kJ mol⁻¹; HI is a strong acid, HF is a weak acid.

• 1a(i) Write the Kc expression for the hydrogen iodide equilibrium. Achieved
• 1a(ii) Calculate Kc at 490 °C given [H₂] = [I₂] = 0.105 mol L⁻¹ and [HI] = 0.711 mol L⁻¹. Merit
• 1a(iii) Explain what would happen to the value of Kc if the temperature of the equilibrium mixture was increased. Excellence
• 1b(i) Write equations to show HI(g) and HF(g) reacting with water. Achieved
• 1b(ii) Solutions of HI(aq) and HF(aq) are made to the same concentration. Explain how the pH and electrical conductivity of the two solutions would differ. Merit

Question Two — Methanol manufacture, equilibrium graph, and a catalyst

CO(g) + 2H₂(g) ⇌ CH₃OH(g); a concentration-time graph shows equilibrium (Section A) then CO added (Section B); zinc oxide catalyses the reaction.

• 2a(i) Explain how the graph shows the system is at equilibrium throughout Section A, referring to the rates of the forward and reverse reactions. Achieved
• 2a(ii) Explain, using equilibrium principles and referring to the graph, how the system responds to restore equilibrium after CO is added at the start of Section B. Merit
• 2a(iii) Using equilibrium principles, explain why carrying out the reaction at high pressure is advantageous in the manufacture of methanol. Excellence
• 2b(i) State the role of zinc oxide, ZnO(s), in the reaction. Achieved
• 2b(ii) Add a line to an energy diagram to show how it would differ with zinc oxide added. Achieved
• 2b(iii) Explain how zinc oxide increases the rate of reaction, referring to collision theory and activation energy. Merit

Question Three — pH calculations, identifying unknowns, and temperature's effect on rate

HCl at pH 2.4; 0.450 mol L⁻¹ LiOH; three unknown 0.1 mol L⁻¹ solutions (NaOH, NaCl, Na₂CO₃) with pH 11.6, 13.0, 7.0; Na₂CO₃ then reacted with HCl at 25 °C and 50 °C.

• 3a(i) Calculate [H₃O⁺] in a solution of HCl with pH 2.4. Achieved
• 3a(ii) Calculate the pH of a 0.450 mol L⁻¹ solution of LiOH. Merit
• 3b(i) Identify solutions A, B and C as NaOH, NaCl or Na₂CO₃ from their measured pH values. Achieved
• 3b(ii) Fully explain how each solution was identified, using relevant equations. Excellence
• 3c Explain the effect of increased temperature (25 °C vs 50 °C) on the rate of the Na₂CO₃/HCl reaction, referring to collision theory, both the rate and total volume of CO₂ produced, and the graph lines. Excellence

The 2023 paper ↗ Marking schedule ↗

Question One — Antacid tablets in stomach acid

NaHCO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(ℓ); a graduated CO₂-volume-vs-time graph, sections A and B.

• 1b(i) With reference to collision theory, explain why the steepness of the line changes as the reaction proceeds (sections A and B). Merit
• 1b(ii) Explain why the students' room-temperature (20 °C) experiment would not give a good indication of reaction time in a human stomach (37 °C), referring to collision theory. Excellence
• 1c(ii) Compare and contrast the pH and electrical conductivity of hydrochloric acid (strong) and propanoic acid (weak) solutions of equal concentration. Excellence

Question Two — Lactase and hydrogen gas from methane

Lactose + water ⇌ galactose + glucose, catalysed by lactase; CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g).

• 2a(iii) Explain how the rate of reaction is altered by the presence of the lactase enzyme, referring to collision theory and activation energy. Merit
• 2b(i) Write the Kc expression for the steam-methane reforming reaction. Achieved
• 2b(ii) Using equilibrium principles, explain why removing H₂(g) as it forms is advantageous for producing hydrogen gas. Merit
• 2b(iii) At higher temperatures, the value of Kc increases. Explain, using equilibrium principles, whether the forward reaction is endothermic or exothermic. Merit

Question Three — Amphiprotic species and a Kc-to-equation puzzle

HCO₃⁻(aq) + H₂O(ℓ) ⇌ CO₃²⁻(aq) + H₃O⁺(aq); a Kc expression for CF₄/CO₂/COF₂ = 2.00 at 1000 °C.

• 3a Identify the two acid/base conjugate pairs in the reaction of hydrogen carbonate with water. Achieved
• 3b(iv) A 0.1 mol L⁻¹ sodium ethanoate solution has pH 8.88; an equal-concentration KOH solution has a significantly higher pH. Justify the difference, with relevant equations. Excellence
• 3c(i) Given the Kc expression Kc = [COF₂]² ÷ ([CF₄][CO₂]) = 2.00 at 1000 °C, write the balanced equation for this reaction (all species gases). Merit
• 3c(ii) If [CF₄] = [CO₂] and [COF₂] = 0.105 mol L⁻¹ at equilibrium, calculate [CF₄]. Excellence
• 3c(iii) Explain what effect, if any, increasing the pressure of the system would have on the position of equilibrium. Merit

Kc values, concentrations and pH figures are transcribed exactly from the exam papers; some 2023 diagram/graph-based sub-parts (drawing a second line on a rate or energy-profile graph) are described in the context field rather than reproduced, since they cannot be rendered as text.

Past exams & resources

⏱ Timed opens the paper and starts a countdown matching the exam length. Official NZQA PDFs. If a link 404s, that year’s paper may sit under a slightly different code: browse the NZQA subject page.