AQA A-Level Chemistry Paper 1, 2017: Question 7

Coordination chemistry basics

• Recognising complex ions vs counter-ions in a formula
• Ligand definition: lone pair donation to a metal ion (coordinate bond)
• Ligand substitution in Cu²⁺ complexes (H₂O ⇌ NH₃ / Cl⁻)

Colours & electron configuration

• Linking colour to d–d transitions (partially filled d subshell)
• Why d¹⁰ ions/complexes are colourless (no d–d transitions)

Exam skills

• Writing correct ionic equations with charges and balanced ligands
• Using the mark scheme wording (e.g. “covalent” vs “dative”) when asked about specific bonds
• Naming/identifying precipitates from qualitative tests

✅ Correct answer (1/1)

Covalent

O–H in H₂O is a covalent bond.

🧠 Exam technique

• Read precisely: “between oxygen and hydrogen” refers to the bonds inside each water molecule.
• Answer with the simplest accepted term. Here, just covalent scores.

❌ Common errors (explicitly not allowed)

• Saying dative covalent or coordinate (the mark scheme says “Do not allow”).
• Describing the Cu–O bond instead of the O–H bond.

💡 Key knowledge

In [Cu(H₂O)₆]Cl₂ , water acts as a ligand to Cu²⁺ via a lone pair on oxygen (that Cu–O interaction is coordinate), but the question isn’t asking about that.

✅ Correct answer (1/1)

The Cl⁻ ions are not donating a lone pair to Cu²⁺, so they do not form a coordinate (dative) bond to Cu²⁺ in this compound (they are counter-ions).

💡 Key knowledge

• Ligand = species that forms a coordinate bond by donating a lone pair to the metal ion.
• In [Cu(H₂O)₆]Cl₂ , the complex ion is [Cu(H₂O)₆]²⁺ and the two Cl⁻ are outside the brackets → counter-ions.

🧠 Exam technique (how to secure the mark)

• Use the bracket rule: inside square brackets = ligands; outside = ions balancing charge.
• Phrase it like the mark scheme: “does not donate lone pair / does not form coordinate bond (to Cu²⁺)”.

❌ Common errors

• Writing “Cl⁻ is ionic” with no mention of lone pair donation/coordinate bonding (too vague).
• Adding incorrect charges (the mark scheme allows missing charges, but penalises incorrect ones).

✅ Correct answers (2/2)

Ionic equation (1 mark):

Colour of solution B (1 mark): Deep blue (also accept: royal blue / dark blue).

💡 Key knowledge

• Excess NH₃ causes ligand substitution: NH₃ replaces 4 H₂O ligands to form [Cu(NH₃)₄(H₂O)₂]²⁺ .
• Water is released as ligands are replaced (hence +4H₂O on the product side).

🧠 Exam technique (equation marks)

• Balance ligands like “molecules”: 6 water ligands on the left become 2 on the right, so 4 H₂O must be produced.
• Keep the charge the same on both complex ions: Cu stays as Cu²⁺ here.
• Square brackets: the mark scheme says missing brackets aren’t penalised, but including them makes your meaning clear.

❌ Common errors & examiner insight

• Wrong colour: the deep blue is the key observation for the tetraammine complex.
• Wrong stoichiometry: using 6NH₃ or making 6H₂O without checking ligand counts.
• Writing an acid–base equation only (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻) and missing the substitution chemistry.

Mark scheme note: alternative stepwise routes via a hydroxide intermediate are allowed, but the simplest full-mark route is the single substitution equation above.

✅ Correct answer (1/1)

CuCO₃ (copper(II) carbonate)

The mark scheme also allows “copper carbonate”. It may also allow an acceptable formula for basic copper carbonate.

💡 Key knowledge

• Carbonate ions often form insoluble carbonates with 2+ metal ions.
• Cu²⁺ + CO₃²⁻ → CuCO₃(s) (commonly blue-green).

🧠 Exam technique

• When asked to “identify”, give a name and/or formula. Formula is safest for accuracy.
• Make sure oxidation state matches the ion present: copper is Cu²⁺ here.

❌ Common errors

• Using the wrong oxidation state (e.g. Cu₂CO₃).
• Confusing carbonate precipitate with hydroxide precipitate (Cu(OH)₂ is pale blue, not typically described as blue-green here).

✅ Correct answers (2/2)

Identity of reagent D (1 mark): HCl (hydrochloric acid)

Ionic equation (1 mark):

Also accepted by the mark scheme: using HCl explicitly, e.g. [Cu(H₂O)₆]²⁺ + 4HCl → [CuCl₄]²⁻ + 6H₂O + 4H⁺

💡 Key knowledge

• High [Cl⁻] shifts equilibrium towards the tetrachlorocuprate(II) complex [CuCl₄]²⁻ , which is yellow-green.
• This is ligand substitution: Cl⁻ replaces H₂O ligands.

🧠 Exam technique (what examiners reward)

• Write the complex ions with correct charge: both are 2−/2+ as shown.
• Balance ligands: 6 waters in, 0 waters bound in [CuCl₄]²⁻, so 6H₂O out.
• Reagent identity: “hydrochloric acid” or “HCl” is enough; concentration is ignored.

❌ Common errors & mark scheme guidance

• Using NaCl/KCl but then not providing a correct chloride-substitution equation. (The mark scheme allows “a soluble chloride salt” in principle, but your equation must match.)
• Incorrect complex formula: [CuCl₄]⁻ (wrong charge) or wrong number of ligands.
• Forgetting water molecules entirely (loses the equation mark if not balanced).

Examiner insight: marks are awarded independently—if you name HCl correctly but your equation is wrong, you can still get 1/2, and vice versa.

✅ Full-mark explanation (2/2)

• The metal ion is d¹⁰ (has a full 3d subshell).
• So there are no d–d transitions; it is colourless / cannot absorb visible light, so colorimetry cannot determine concentration.

Mark scheme wording: “(3)d¹⁰ or has full (3)d (sub)shell/orbital” + “colourless/cannot absorb (frequencies of) visible light”.

💡 Key knowledge (the logic chain)

• Colorimetry relies on absorption of visible light → needs a coloured species.
• Many transition metal ions are coloured because they have a partially filled d subshell.
• d¹⁰ systems have no available d–d transition within the split d orbitals → no visible absorption → colourless.

🧠 Exam technique

• Make two distinct points (2 marks): (1) electron configuration (d¹⁰ / full 3d), (2) consequence for visible absorption/colour.
• State the consequence explicitly: “cannot absorb visible light” → “colourless” → “colorimetry not possible”.

❌ Common errors

• Writing the wrong principal quantum number (must be 3d for first-row transition metals in A-level context; the mark scheme penalises incorrect n).
• Only saying “it’s colourless” without linking to d¹⁰ / full 3d (would score 1/2 at best).
• Vague phrasing like “it doesn’t absorb light” without specifying visible (the mark scheme wants visible frequencies).

✅ What to write to score full marks

• 07.1: Covalent
• 07.2: Cl⁻ not donating lone pair / no coordinate bond to Cu²⁺ (counter-ion)
• 07.3: [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O; deep blue
• 07.4: CuCO₃
• 07.5: HCl; [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O
• 07.6: d¹⁰ / full 3d; colourless / cannot absorb visible → colorimetry not usable

🧠 Final examiner-style tips

• Use the brackets to decide what is (and isn’t) a ligand.
• When writing complex equations, balance: (i) ligands, (ii) charge, (iii) atoms.
• For “colour” marks, use standard AQA accepted descriptors (deep/royal/dark blue; yellow-green).

❌ High-frequency mark losers

• Calling the O–H bond “dative/coordinate”.
• Unbalanced ligand substitution equations (missing waters or wrong numbers of ligands).
• Electron configuration point missing or incorrect in the colorimetry explanation.