AQA A-Level Chemistry: Transition metals (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.2.5, Transition metals, of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. The whole section is marked “A-level only”, so none of it is AS content. Inorganic chemistry is listed under A-level Paper 1, and Paper 3 can assess any content. Use only the data given in each question.

Before you start, you may want the study guide or the revision notes. The AQA A-Level Chemistry hub and printable checklist cover the rest of the course, and the free diagnostics show where to focus.

Questions

1. Define the terms ligand, complex and co-ordination number. [3]

2. Use electron configurations to explain why copper shows transition metal properties but zinc does not. [2]

3. This question is about ligand substitution.

(a) Write an equation for the reaction of [Co(H₂O)₆]²⁺ with excess ammonia, and explain why the co-ordination number does not change. [2] (b) Write an equation for the reaction of [Cu(H₂O)₆]²⁺ with concentrated hydrochloric acid. State the change in shape and explain the change in co-ordination number. [3] (c) With excess aqueous ammonia, [Cu(H₂O)₆]²⁺ forms [Cu(NH₃)₄(H₂O)₂]²⁺. State its co-ordination number and why this is called incomplete substitution. [2]

4. EDTA⁴⁻ reacts with hexaaquacopper(II) ions:

[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O

For this question take ΔH = −5.0 kJ mol⁻¹ and ΔS = +150 J K⁻¹ mol⁻¹. Explain the signs of ΔH and ΔS, calculate ΔG at 298 K, and state what it shows. [5]

5. This question is about shapes of complexes.

(a) [Co(NH₃)₄Cl₂]⁺ is octahedral. Name the type of isomerism it shows and give the Cl–Co–Cl angle in each isomer. [2] (b) State the co-ordination number of [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ and the type of stereoisomerism it shows. [2] (c) State the shape and bond angle of the silver complex in Tollens’ reagent. [1]

6. Haemoglobin contains haem.

(a) Name the type of bond between oxygen and iron in haemoglobin, and give the oxidation state of iron. [2] (b) Explain why carbon monoxide is toxic. [1]

7. A complex ion absorbs light most strongly at 510 nm. h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹, L = 6.02 × 10²³ mol⁻¹.

(a) Calculate ΔE for one ion. [2] (b) Convert your answer to kJ mol⁻¹. [1] (c) Explain why [Co(H₂O)₆]²⁺ and [CoCl₄]²⁻ have different colours. [2]

8. A student finds the concentration of copper(II) ions by colorimetry. Standards give a straight line through the origin; a 0.0200 mol dm⁻³ standard has absorbance 0.150.

(a) State how the student should choose the filter. [1] (b) Why is the colorimeter zeroed with a blank? [1] (c) The unknown has absorbance 0.390. Calculate its concentration. [2] (d) Suggest why the unknown must be treated with the same reagents as the standards. [1]

9. Zinc and dilute acid are added to a solution of ammonium vanadate(V).

(a) Give the formula and colour of the vanadium species in oxidation states +5 and +2. [2] (b) Write a half-equation for the reduction of VO₂⁺ to VO²⁺, and an overall equation for its reduction by zinc. [2] (c) Suggest why the flask is loosely stoppered to keep air out once V²⁺ forms. [1]

10. 2.016 g of hydrated ethanedioic acid, H₂C₂O₄·xH₂O, is dissolved and made up to 250 cm³. 25.0 cm³ portions are acidified, warmed and titrated with 0.0200 mol dm⁻³ KMnO₄. The mean titre is 32.00 cm³. Mr of H₂C₂O₄ = 90.0; Mr of H₂O = 18.0.

(a) Write the ionic equation for the reaction. [1] (b) Explain why no indicator is added. [1] (c) Calculate the value of x. [5] (d) Explain why hydrochloric acid is not used to acidify the mixture. [1]

11. This question is about catalysis by transition metals.

(a) Distinguish between heterogeneous and homogeneous catalysts. [2] (b) Write two equations to show how V₂O₅ acts as a catalyst in the Contact process, and explain the role of variable oxidation state. [3] (c) Explain why the uncatalysed reaction of S₂O₈²⁻ with I⁻ is slow, and write two equations to show how Fe²⁺ catalyses it. [3] (d) In the reaction of C₂O₄²⁻ with acidified MnO₄⁻, identify the autocatalyst and describe how the rate changes from the start of the reaction. [2]

12. Heterogeneous catalysts are often spread on a support and can be poisoned.

(a) Give the purpose of a support medium. [1] (b) Explain how poisoning affects a catalyst and why it matters to a manufacturer. [2]

Answers

1. Ligand: a molecule or ion that donates a pair of electrons to form a co-ordinate bond with a transition metal [1]. Complex: a central metal atom or ion surrounded by ligands [1]. Co-ordination number: the number of co-ordinate bonds to the central metal atom or ion [1]. [3] Examiner insight: “Number of ligands” scores nothing for co-ordination number, because bidentate ligands make two bonds each.

2. Cu²⁺ is [Ar] 3d⁹, an incomplete d sub-level [1]; Zn²⁺ is [Ar] 3d¹⁰ (and Zn is 3d¹⁰ 4s²), so no atom or ion has an incomplete d sub-level [1]. [2] Examiner insight: You must refer to the ions; quoting only the atoms’ configurations does not explain copper’s behaviour.

3. (a) [Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O [1]; NH₃ and H₂O are similar in size and both uncharged [1] (b) [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O [1]; octahedral to tetrahedral [1]; Cl⁻ is larger, so only four fit, and the co-ordination number falls from 6 to 4 [1] (c) 6 [1]; only four of the six water ligands are replaced [1] Examiner insight: Check that the complex charge balances in each equation; a wrong charge on [CuCl₄]²⁻ normally loses the equation mark.

4. Particles increase from 2 to 7 [1], so ΔS is positive [1]. ΔH is small because similar numbers of similar co-ordinate bonds are broken and made [1]. ΔG = −5.0 − (298 × 0.150) = −49.7 kJ mol⁻¹ [1]. ΔG is negative, so the reaction is feasible: the chelate effect is entropy-driven [1]. [5] Examiner insight: Forgetting to divide ΔS by 1000 gives a huge ΔG; the method mark may still be allowed, but the answer mark is lost.

5. (a) cis–trans isomerism [1]; 90° (cis) and 180° (trans) [1] (b) 6 [1]; optical isomerism [1] (c) Linear, 180° [1] Examiner insight: In (b) “3” is wrong: en is bidentate, so count six co-ordinate bonds.

6. (a) Co-ordinate (dative covalent) bond [1]; +2 [1] (b) CO replaces the oxygen co-ordinately bonded to Fe(II), so less oxygen is transported [1] Examiner insight: “CO binds to haemoglobin” alone is not enough; the mark needs the idea of replacing O₂ at Fe(II).

7. (a) ΔE = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(510 × 10⁻⁹) [1] = 3.90 × 10⁻¹⁹ J [1] (b) 3.90 × 10⁻¹⁹ × 6.02 × 10²³ = 235 kJ mol⁻¹ [1] (c) Different ligand and co-ordination number change ΔE between d-electron ground and excited states [1], so a different wavelength is absorbed and different light is transmitted [1] Examiner insight: An error carried from (a) into (b) is normally allowed as error carried forward, so show the conversion clearly.

8. (a) Use the colour of light the solution absorbs most [1] (b) So that absorbance due to the solvent and cuvette is removed, measuring only the copper(II) ions [1] (c) Gradient = 0.150/0.0200 = 7.50 [1]; c = 0.390/7.50 = 0.0520 mol dm⁻³ [1] (d) The coloured species and its absorbance depend on the ligand, so it must match the calibration [1] Examiner insight: In (c) the unit mol dm⁻³ is needed with the number for the final mark.

9. (a) VO₂⁺, yellow (+5) [1]; V²⁺, violet (+2) [1] (b) VO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O [1]; 2VO₂⁺ + 4H⁺ + Zn → 2VO²⁺ + Zn²⁺ + 2H₂O [1] (c) V²⁺ is readily oxidised by oxygen in air back to higher oxidation states [1] Examiner insight: VO₂⁺ and VO²⁺ are easily swapped; a wrong formula loses the mark even if the colour is right.

10. (a) 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂ [1] (b) MnO₄⁻ is purple and Mn²⁺ almost colourless, so the first permanent pink shows the end point [1] (c) n(MnO₄⁻) = 0.0200 × 32.00/1000 = 6.40 × 10⁻⁴ mol [1]; n(C₂O₄²⁻) = 6.40 × 10⁻⁴ × 5/2 = 1.60 × 10⁻³ mol [1]; in 250 cm³: 1.60 × 10⁻² mol [1]; Mr = 2.016/0.0160 = 126.0 [1]; x = (126.0 − 90.0)/18.0 = 2 [1] (d) MnO₄⁻ would oxidise Cl⁻ to chlorine, so the titre would be too large [1] Examiner insight: Using a 1 : 5 or 5 : 2 ratio here is a common slip; examiners can still credit later steps by error carried forward, but not the ratio mark.

11. (a) Heterogeneous: different phase from reactants, reaction at active sites on the surface [1]. Homogeneous: same phase, reaction through an intermediate species [1] (b) SO₂ + V₂O₅ → SO₃ + V₂O₄ [1]; V₂O₄ + ½O₂ → V₂O₅ [1]; V is reduced from +5 to +4 and reoxidised, so the catalyst is regenerated [1] (c) Both ions are negative and repel, so activation energy is high [1]; S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺ [1]; 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ [1] (d) Mn²⁺ [1]; slow at first, speeds up as Mn²⁺ forms, then slows as reactants are used up [1] Examiner insight: For (b) and (c) the two equations must add up to the overall reaction; if the catalyst does not cancel, expect to lose a mark.

12. (a) Maximises the surface area of catalyst and minimises the amount (cost) needed [1] (b) Impurities block the active sites, reducing efficiency [1]; the catalyst must be cleaned or replaced, or the feed purified, which adds cost [1] Examiner insight: “Poison stops the catalyst” is too vague; name the active sites being blocked.

Where marks are usually lost

• Counting ligands instead of co-ordinate bonds when asked for co-ordination number.
• Writing tetrahedral complexes with six Cl⁻ ligands, or missing the overall charge.
• Explaining the chelate effect with enthalpy alone.
• Giving cis and trans angles the wrong way round.
• Using λ in nm in hc/λ.
• Mixing up the MnO₄⁻ ratios: 1 : 5 for Fe²⁺, 2 : 5 for C₂O₄²⁻ and H₂O₂.
• Forgetting to scale from the titrated portion to the full volumetric flask.
• Writing catalyst equations in which the catalyst is not regenerated.

Next steps

• Recap with the revision notes.
• Re-read any weak section in the study guide.
• See the AQA A-Level Chemistry hub and the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards (AQA). Section 3.2.5 Transition metals (A-level only).