AQA A-Level Chemistry Paper 3, June 2025: Question 22

✅ Correct Answer: Option A

Tollens' reagent contains linear [Ag(NH₃)₂]⁺ ions.

Tollens' reagent is an ammoniacal silver nitrate solution. In the presence of aqueous ammonia, Ag⁺ ions form the diamminesilver(I) complex, [Ag(NH₃)₂]⁺ , which has a coordination number of 2 and a classic linear shape.

💡 Key Knowledge

• Tollens' Reagent: Active species is [Ag(NH₃)₂]⁺ .
  • Central ion: Ag⁺
  • Ligands: 2 monodentate NH₃ molecules
  • Coordination number: 2
  • Geometry: Linear (bond angle = 180°)
• Fehling's Solution: Contains Cu²⁺ ions complexed with tartrate ions in alkaline solution (deep blue solution). It contains no silver species.

🧠 Exam Technique: Elimination Strategy

1. Eliminate C and D immediately: Fehling's solution contains Cu²⁺ (tartrate complex), not silver complexes. Silver complexes belong exclusively to Tollens' reagent.
2. Compare A and B: Silver(I) ions in solution typically form 2-coordinate linear complexes with ammonia: [H₃N—Ag—NH₃]⁺ . A coordination number of 6 (octahedral) is common for 2+ and 3+ ions (such as Fe²⁺, Cu²⁺, Cr³⁺), but not Ag⁺ with NH₃.
3. Therefore, A must be correct.

❌ Common Errors & Pitfalls

• Defaulting to Octahedral: Many students associate all transition metal complexes with 6-coordinate octahedral geometry ( [M(ligand)₆] ). Remember: Ag⁺ complexes with NH₃ or CN⁻ are 2-coordinate and linear.
• Confusing Reagents: Mixing up Tollens' (Ag⁺ reduced to Ag mirror) with Fehling's (Cu²⁺ blue solution reduced to Cu₂O red precipitate).
• Formula Errors: Forgetting the overall +1 charge on the complex ion ( [Ag(NH₃)₂]⁺ ) because ammonia is a neutral ligand.