OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.2.6 Reactions of Ions in Aqueous Solution Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.2.6 Reactions of Ions in Aqueous Solution: what you need to know, common mistakes and exam-style questions with answers. Covers acidity of aqua ions and reactions of iron, copper and aluminium ions with hydroxide, ammonia and carbonate.
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About OxfordAQA International A Level Chemistry 3.2.6 Reactions of Ions in Aqueous Solution
Reactions of ions in aqueous solution is an A2 inorganic topic examined in Unit 3. You need to explain why aqua ions with a 3+ charge, such as [Al(H2O)6]3+ and [Fe(H2O)6]3+, are more acidic than 2+ aqua ions such as [Fe(H2O)6]2+ and [Cu(H2O)6]2+. You also need to describe and write equations for the test-tube reactions of Fe2+, Cu2+, Fe3+ and Al3+ with sodium hydroxide, ammonia and sodium carbonate, including the colours of solutions and precipitates and the amphoteric behaviour of aluminium hydroxide. Required practical 9, identifying transition metal ions by test-tube reactions, belongs here.
What you need to know
- Acidity of aqua ions The 3+ ions have a higher charge to size ratio, so they polarise co-ordinated water molecules more strongly and release H+ more readily. Solutions of 3+ ions are more acidic than solutions of 2+ ions.
- Reactions with NaOH Hydroxide ions remove protons to form precipitates: Fe(OH)2(H2O)4 is green, Cu(OH)2(H2O)4 is blue, Fe(OH)3(H2O)3 is brown and Al(OH)3(H2O)3 is white. Aluminium hydroxide dissolves in excess NaOH to form [Al(OH)4]-.
- Reactions with ammonia Ammonia acts as a base first, giving the same hydroxide precipitates. With excess ammonia, the copper precipitate dissolves to form deep blue [Cu(NH3)4(H2O)2]2+.
- Reactions with carbonate 2+ ions form carbonate precipitates: FeCO3 is green and CuCO3 is blue-green. 3+ ions are acidic enough to release CO2, giving hydroxide precipitates and effervescence.
- Required practical 9 Use test-tube reactions with NaOH, ammonia and sodium carbonate to identify unknown metal ions from their colours and behaviour in excess reagent.
Common mistakes in 3.2.6 Reactions of Ions in Aqueous Solution
- Saying Fe3+ forms iron(III) carbonate with sodium carbonate. It forms the brown hydroxide and bubbles of CO2.
- Giving the wrong colour for iron(II) hydroxide. It is green and slowly turns brown in air as it is oxidised.
- Forgetting that aluminium hydroxide dissolves in excess NaOH but not in excess ammonia.
- Explaining acidity of 3+ ions by charge alone. Use charge to size ratio and polarisation of the O-H bonds in water ligands.
Try three 3.2.6 Reactions of Ions in Aqueous Solution exam-style questions
1. Write an equation for the reaction of [Cu(H2O)6]2+ with excess ammonia. (1 mark)
Show the mark scheme answer
[Cu(H2O)6]2+ + 4NH3 → [Cu(NH3)4(H2O)2]2+ + 4H2O.
2. Explain why a solution of [Fe(H2O)6]3+ is more acidic than a solution of [Fe(H2O)6]2+. (3 marks)
Show the mark scheme answer
Fe3+ has a higher charge to size ratio; it polarises water ligands more, weakening the O-H bonds; so H+ ions are released more readily.
3. Describe what you would see when sodium carbonate solution is added to a solution containing Al3+ ions. (2 marks)
Show the mark scheme answer
White precipitate; effervescence / bubbles of gas (CO2).
3.2.6 Reactions of Ions in Aqueous Solution FAQs
What does OxfordAQA International A Level Chemistry 3.2.6 Reactions of Ions in Aqueous Solution cover?
It covers acidity of aqua ions and reactions of iron, copper and aluminium ions with hydroxide, ammonia and carbonate.
Which unit is 3.2.6 Reactions of Ions in Aqueous Solution examined in?
It is examined in Unit 3, an International A2 paper covering inorganic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
What colours do I need to learn?
Learn the solution colours of the aqua ions and the precipitate colours with hydroxide, ammonia and carbonate for Fe2+, Cu2+, Fe3+ and Al3+, plus the deep blue copper ammine complex in excess ammonia and the colourless aluminate in excess NaOH.
