AQA A-Level Chemistry Paper 3, 2024: Question 21
1 mark · Easy difficulty · Multiple Choice
Identify which statement about chloride ions is correct (they form a cobalt(II) complex with a tetrahedral shape).
Practise this questionQuestion
Question text
21 Which statement about chloride ions is correct?
[1 mark]
A They are oxidised by concentrated sulfuric acid.
B They form a cream precipitate with silver nitrate solution.
C They form a cobalt(II) complex with a tetrahedral shape.
D They have the electron configuration 1s22s22p63s23p4
Mark scheme
Show the mark scheme
21 C 1 (AO1) They form a cobalt(II) complex with a tetrahedral shape.
How to answer it
Chloride ions: tests, redox & complexes (MCQ)
Core knowledge
- Recognising halide test outcomes with AgNO₃ (precipitate colours).
- Knowing when halide ions are oxidised by concentrated H₂SO₄ (Cl⁻ vs Br⁻/I⁻).
- Understanding ligand substitution/complex formation with Cl⁻ and metal ions.
- Electron configuration of Cl⁻ vs Cl.
Skill
- Eliminating options by checking one key fact per statement.
- Spotting “trap” facts (e.g., cream precipitate is Br⁻, not Cl⁻).
Part (multiple choice): Which statement about chloride ions is correct? 1 mark (AO1)
✅ Correct answer (from mark scheme)
C — They form a cobalt(II) complex with a tetrahedral shape.
Marking point: choose option C. No working required for MCQ; only one option is awarded.
💡 Key knowledge: why C is true
- Chloride ions can act as ligands and form complexes with transition metal ions.
- With cobalt(II), chloride can form the tetrachlorocobaltate(II) complex: [CoCl₄]²⁻ .
- [CoCl₄]²⁻ is typically tetrahedral (4 ligands around the metal centre).
❌ Why the other options are wrong (common traps)
- A (oxidised by concentrated H₂SO₄) — False for Cl⁻. Concentrated H₂SO₄ acts mainly as an acid with chloride salts giving HCl(g), not redox oxidation of Cl⁻. (Br⁻/I⁻ are the ones readily oxidised.)
- B (cream precipitate with AgNO₃) — False. For halides with AgNO₃:
- AgCl: white precipitate
- AgBr: cream precipitate
- AgI: yellow precipitate
- D (electron configuration 1s²2s²2p⁶3s²3p⁴) — False. That configuration is for S (sulfur) or a species with 16 electrons, not Cl⁻.
🧠 Exam technique: fastest route to 1 mark
- Scan for “high-certainty” facts:
- AgNO₃ precipitate colour table is a quick elimination tool → B is wrong immediately (cream = Br⁻).
- Electron configuration check: Cl has 17 e⁻, Cl⁻ has 18 e⁻ → D is wrong immediately.
- Between A and C:
- Remember: Cl⁻ is not oxidised by conc. H₂SO₄ (unlike Br⁻/I⁻) → A is wrong.
- So C must be correct.
- In MCQs, don’t overthink: one accurate keyword (e.g., “tetrahedral [CoCl₄]²⁻”) can secure the mark.
💡 Extra recall that supports this question (useful for similar items)
Halide + AgNO₃ test
- Add dilute HNO₃ first (removes CO₃²⁻/OH⁻ interference), then AgNO₃.
- Colours: AgCl white, AgBr cream, AgI yellow.
Conc. H₂SO₄ with halide salts
- Cl⁻: acid-base reaction gives HCl gas (no redox required).
- Br⁻/I⁻: also undergo redox producing Br₂/I₂ and SO₂/H₂S depending on halide.
Electron configurations
- Cl: 1s²2s²2p⁶3s²3p⁵ (17 e⁻)
- Cl⁻: 1s²2s²2p⁶3s²3p⁶ (18 e⁻, same as Ar)
❌ Examiner-style misconceptions to avoid
- Mixing up precipitate colours: cream is Br⁻, not Cl⁻.
- Assuming “concentrated H₂SO₄ oxidises all halides” — it’s a stronger oxidising agent relative to Br⁻/I⁻, but chloride is much harder to oxidise.
- Giving the electron configuration for the atom (Cl) when the question asks about the ion (Cl⁻), or confusing it with sulfur’s configuration.
Topics
Inorganic Chemistry · 3.2.5 Transition Metals · 3.2.3 Group 7(17), The Halogens · 3.2.6 Reactions of Ions in Aqueous Solution
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.