AQA A-Level Chemistry Paper 3, June 2022: Question 20

1 mark · Easy difficulty · Multiple Choice

Identify the type of reaction that occurs when excess concentrated hydrochloric acid is added to cobalt(II) chloride solution.

Practise this question

Question

Question 20 states: 'Cobalt(II) chloride solution changes colour when an excess of concentrated hydrochloric acid is added. What type of reaction takes place?' followed by four multiple choice options: A hydrolysis, B ligand substitution, C precipitation, and D redox, each with an oval selection box.
Question text

20 Cobalt(II) chloride solution changes colour when an excess of concentrated

hydrochloric acid is added.

What type of reaction takes place?

[1 mark]

A hydrolysis

B ligand substitution

C precipitation

D redox

Mark scheme

Show the mark scheme Mark scheme table row for question 20 showing the correct answer as B with assessment objective AO1, 1 mark, indicating 'ligand substitution'.

20 B (AO1) 1 ligand substitution

How to answer it

Cobalt(II) Complexes: Ligand Substitution

What this question tests

This question assesses your understanding of transition metal chemistry, specifically:

  • Recognising ligand substitution reactions involving monodentate ligands of different sizes.
  • Recalling the classic aqueous cobalt(II) system and its reaction with concentrated hydrochloric acid.
  • Distinguishing ligand exchange from redox, hydrolysis, and precipitation processes.

Question 20

Reaction Classification [1 mark]

✅ Correct Answer

B — ligand substitution

Mark Scheme Breakdown:
• 1 mark awarded for selecting B (AO1 recall).

💡 Key Knowledge

  • In aqueous solution, cobalt(II) chloride exists as the pink hexaaqua complex ion: [Co(H₂O)₆]²⁺ .
  • Adding concentrated HCl provides a high concentration of chloride ions ( Cl⁻ ), replacing the water ligands:

    [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l)
  • The colour shifts from pink to deep blue.
  • Geometry & Coordination Number Change: From 6-coordinate (octahedral) to 4-coordinate (tetrahedral), because Cl⁻ ligands are larger than uncharged H₂O molecules, creating greater steric repulsion.

🧠 Exam Technique: Elimination Strategy

  • Check oxidation states: Cobalt is +2 in [Co(H₂O)₆]²⁺ and +2 in [CoCl₄]²⁻ . There is no change in oxidation state, ruling out D (redox).
  • Check physical state: Both complexes are completely soluble in solution; no solid forms, ruling out C (precipitation).
  • Identify ligand role: Water is displaced rather than splitting a molecule/covalent bond, ruling out A (hydrolysis).

❌ Common Errors

  • Confusing colour change with redox: Many students associate vivid colour changes with oxidation/reduction. Always verify oxidation states of the central metal ion!
  • Assuming acid addition implies acid-base/hydrolysis: Concentrated HCl is used here primarily as a rich source of Cl⁻ ligands, not as a Bronsted-Lowry proton donor.
  • Misidentifying as precipitation: Students confuse the intense blue colour formation with a precipitate; it forms a clear blue solution.

Topics

Inorganic Chemistry · 3.2.5 Transition Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.