Edexcel A-Level Chemistry Paper 3, June 2019: Question 4

✅ Correct Answers

• Test 1 Gas: ammonia / NH₃
• Test 1 Cation: ammonium / NH₄⁺
• Test 2 Cation: cobalt(II) / Co²⁺ / [Co(H₂O)₆]²⁺
• Test 2 Complex Ion: [CoCl₄]²⁻
• Test 3 Precipitate: barium sulfate / BaSO₄
• Test 3 Anion: sulfate((VI)) / SO₄²⁻

💡 Key Knowledge

• Test 1: Warming any ammonium salt with NaOH releases alkaline ammonia gas, which turns damp red litmus paper blue.
• Test 2: Adding concentrated HCl provides excess chloride ligands, driving a ligand exchange reaction that turns pink aqueous cobalt(II) blue.
• Test 3: Acidified barium chloride is the standard test for sulfate ions, producing a dense white precipitate of BaSO₄ .

❌ Common Errors

• Swapping names and formulae (e.g., writing the gas as ammonium instead of ammonia).
• Omitting charge signs on ions (e.g., writing CoCl4 instead of [CoCl₄]²⁻ ).
• Failing to include the essential oxidation state in the name of cobalt(II).

🧠 Exam Technique

• If both a name and a formula are requested or allowed, make sure they match. If one is incorrect, the mark may be lost.
• Square brackets are required for complex ions, though examiners sometimes allow minor bracket omissions if charges are clear. Always include charges explicitly.

✅ Correct Answer

(NH₄)₂Co(SO₄)₂ (or any valid combination of the three ions that yields a neutral formula, e.g., (NH₄)₂SO₄·CoSO₄ )

💡 Key Knowledge

• Compound C contains two cations ( NH₄⁺ and Co²⁺ ) and one anion ( SO₄²⁻ ).
• To achieve electrical neutrality, the total positive charge (+1 from NH₄⁺ and +2 from Co²⁺ = +4) must balance the negative charge from the sulfate ions ( 2 × SO₄²⁻ = -4).

🧠 Exam Technique

This is a Follow-Through (TE) mark based on your answers in part (a)(i). Even if you misidentified an ion earlier, as long as your formula correctly balances the charges of the ions you stated, you will gain the mark!

✅ Correct Answer

NH₄⁺ + OH⁻ → NH₃ + H₂O

💡 Key Knowledge

• The test involves the reaction between the ammonium cation ( NH₄⁺ ) from compound C and hydroxide ions ( OH⁻ ) from aqueous sodium hydroxide.
• This is a classic proton-transfer (acid-base) reaction where NH₄⁺ acts as an acid, donating a proton to OH⁻ .

❌ Common Errors

Including sodium spectator ions ( Na⁺ ) in the ionic equation without cancelling them out correctly, or failing to balance hydrogen and oxygen atoms.

🧠 Exam Technique

State symbols are not required for this question ("State symbols are not required"), so save time by omitting them, but ensure stoichiometry and charges are strictly balanced.

✅ Correct Answer

Ligand exchange (Accept: ligand substitution or ligand replacement)

💡 Key Knowledge

• In Test 2, water ligands coordinated to cobalt(II) are replaced by chloride ligands:
  [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O
• Because one type of ligand is swapped for another, it is classed as ligand exchange.

❌ Common Errors

Writing vague terms like "ligand change", or combining terms incorrectly (e.g., stating "reduction/ligand exchange" when no redox reaction occurs).

✅ Correct Answer

To react with / remove any carbonate, sulfite, or sulfate(IV) ions that would otherwise form a white precipitate with barium chloride, giving a false positive.

💡 Key Knowledge

• Barium ions ( Ba²⁺ ) form white precipitates with carbonate ( CO₃²⁻ ) and sulfite ( SO₃²⁻ ) ions, just as they do with sulfate ( SO₄²⁻ ).
• Adding dilute HCl first reacts away these interfering ions (e.g., 2H⁺ + CO₃²⁻ → H₂O + CO₂ ), ensuring that any white precipitate formed upon adding BaCl₂ is uniquely due to sulfate.

❌ Common Errors

Vague statements like "to remove impurities" or "to clean the test tube" without specifying which interfering anions are being removed.

🧠 Exam Technique

Always name the specific ions being prevented from interfering (carbonate or sulfite) to secure the mark. Mentioning that it prevents false-positive precipitates is key.