A Level Chemistry: Transition Elements — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Work through these before looking at the answers. Write full answers — the marks are for the reasoning, not the conclusion.

Related: Transition Elements revision notes

---

Section A — short answer

1. Define a transition element. [2]

2. Explain why zinc is not classed as a transition element. [2]

3. Give the full electron configuration of (a) Cr and (b) Fe²⁺. [2]

4. State the shape and bond angle of [CuCl₄]²⁻ and explain why it differs from [Cu(H₂O)₆]²⁺. [3]

5. Explain why [Zn(H₂O)₆]²⁺ is colourless. [2]

---

Section B — structured

6. Aqueous copper(II) sulfate is pale blue.

(a) Give the formula of the complex ion responsible. [1]

(b) Excess concentrated ammonia is added and the solution turns deep blue. Identify the new complex ion and name the type of reaction. [2]

(c) Explain, in terms of electron transitions, why the colour changes. [4]

(d) Excess concentrated hydrochloric acid is added to a separate sample instead. State the observation and explain the change in coordination number. [3]

7. EDTA⁴⁻ is a hexadentate ligand.

(a) Explain what is meant by hexadentate. [1]

(b) Write an equation for the reaction of [Cu(H₂O)₆]²⁺ with EDTA⁴⁻. [1]

(c) Beyond this syllabus’s 28.1/28.2 outcomes — background only, not examined in this form at 9701. This substitution (the chelate effect) is thermodynamically favourable even though the enthalpy change is close to zero. Explain why.

8. Manganate(VII) ions oxidise ethanedioate ions in acidic solution.

(a) Deduce the oxidation number of manganese in MnO₄⁻. [1]

(b) The reaction is autocatalytic — it speeds up as it proceeds. Suggest which species acts as the catalyst and explain, in terms of a property of transition elements, why it can do so. [3]

---

Section C — further chemistry

9. Aqueous sodium hydroxide is added to orange potassium dichromate(VI) solution, and the resulting solution is then treated with excess dilute sulfuric acid.

(a) State the colour change observed at each stage. [2]

(b) Name the two chromium(VI) species involved, and explain what causes the interconversion between them. [2]

10. [Ni(CN)₄]²⁻ is square planar, unlike most four-coordinate complexes.

(a) State its coordination number. [1]

(b) Give one other metal ion that commonly forms square planar, rather than tetrahedral, complexes. [1]

11. A solution of [Cu(H₂O)₆]²⁺ is treated with excess aqueous ammonia; a separate sample is treated instead with excess concentrated hydrochloric acid.

(a) State what is observed in each case. [2]

(b) Explain the overall pattern in terms of relative ligand strength and coordination number. [2]

---

Answers

1. A d-block element [1] that forms one or more stable ions with a partially filled d subshell [1]. The wording carries both marks. “An element in the d block” alone is not enough.

2. Zinc forms only Zn²⁺ [1], which has a 3d¹⁰ configuration — a full, not partially filled, d subshell [1].

3. (a) Cr = 1s²2s²2p⁶3s²3p⁶3d⁵4s¹ [1] — the half-filled d subshell is more stable than 3d⁴4s². (b) Fe²⁺ = 1s²2s²2p⁶3s²3p⁶3d⁶ [1]. The 4s electrons are lost first. 3d⁴4s² is the standard wrong answer.

4. Tetrahedral, 109.5° [1]. Chloride ions are larger than water molecules [1], so only four fit around the central ion instead of six [1].

5. Zn²⁺ is d¹⁰ — the d subshell is full [1], so no d–d electron transition is possible and no visible light is absorbed [1].

6. (a) [Cu(H₂O)₆]²⁺ [1].

(b) [Cu(NH₃)₄(H₂O)₂]²⁺ [1]; ligand exchange (ligand substitution) [1]. Only four of the six water ligands are replaced — writing [Cu(NH₃)₆]²⁺ is a common error.

(c) Ligands cause the d orbitals to split into two energy levels separated by ΔE [1]. An electron absorbs a photon of energy exactly equal to ΔE and is promoted [1]. The complementary colour to that absorbed is transmitted [1]. Ammonia is a stronger-field ligand than water, so ΔE is larger and a different frequency is absorbed [1].

(d) Solution turns yellow-green [1]. The complex becomes [CuCl₄]²⁻, so the coordination number falls from 6 to 4 [1], because chloride ligands are larger and fewer can fit around the copper ion [1].

7. (a) It forms six dative covalent bonds to the central metal ion, using six lone pairs [1].

(b) [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O [1].

(c) (Background, not examined at 9701 in this form.) Seven particles are produced from two, so there is a large increase in entropy — ΔS is positive. Since ΔG = ΔH − TΔS and ΔH ≈ 0, ΔG is negative and the reaction is feasible. This is the chelate effect: it is an entropy effect, not one driven by comparative bond strength. It is included here as useful background, but Cambridge 9701’s 28.1/28.2 outcomes do not require this explanation.

8. (a) +7 [1].

(b) Mn²⁺ [1]. Transition elements have variable oxidation states [1], so the catalyst can be oxidised and reduced in turn, providing an alternative route of lower activation energy [1].

9. (a) Orange turns yellow on adding NaOH [1]; adding excess H₂SO₄ turns it back to orange [1].

(b) The species are dichromate(VI), Cr₂O₇²⁻ (orange) and chromate(VI), CrO₄²⁻ (yellow) [1]. OH⁻ shifts the equilibrium between them towards CrO₄²⁻, and excess H⁺ shifts it back towards Cr₂O₇²⁻ [1].

10. (a) 4 [1].

(b) Pt²⁺ (e.g. in cisplatin) [1].

11. (a) With excess ammonia, pale blue [Cu(H₂O)₆]²⁺ turns deep blue as [Cu(NH₃)₄(H₂O)₂]²⁺ forms [1]. With excess concentrated hydrochloric acid, pale blue [Cu(H₂O)₆]²⁺ turns yellow-green as the tetrachlorocuprate(II) ion, [CuCl₄]²⁻, forms [1].

(b) NH₃ is a stronger ligand than H₂O, so in excess it displaces water from the coordination sphere while the coordination number stays at 6, giving the octahedral (distorted) ammine complex [1]. Cl⁻ is a larger ligand than H₂O, so a high concentration of Cl⁻ instead drives a change in coordination number from 6 to 4, giving the tetrahedral [CuCl₄]²⁻ ion — a mass-action effect from the very high chloride concentration, rather than Cl⁻ being intrinsically a stronger ligand than water [1].

---

Where marks are usually lost

• Defining a transition element without “partially filled”.
• Writing Fe²⁺ as 3d⁴4s².
• Giving [Cu(NH₃)₆]²⁺ instead of [Cu(NH₃)₄(H₂O)₂]²⁺.
• Saying the colour seen is the colour absorbed — it is the complementary one.
• Explaining the chelate effect by enthalpy rather than entropy.
• Assuming all four-coordinate complexes are tetrahedral — small ligands on certain ions (notably Ni²⁺ and Pt²⁺ with CN⁻) give square planar geometry instead.
• Predicting ligand exchange from a single fixed “ligand strength” order without regard to concentration. Displacement is governed by which complex has the larger stability constant (Kstab) under the conditions used — concentrated Cl⁻ displacing water to form [CuCl₄]²⁻ is a mass-action effect from very high chloride concentration, not evidence that Cl⁻ is intrinsically a stronger ligand than water or ammonia.