AQA A Level Chemistry 3.2.6 Reactions of Ions in Aqueous Solution Exam Questions

About AQA A Level Chemistry 3.2.6 Reactions of Ions in Aqueous Solution

This A Level inorganic topic on the AQA course focuses on the metal aqua ions of iron(II), copper(II), iron(III) and aluminium(III). You need to explain why 3+ aqua ions are more acidic than 2+ aqua ions, write hydrolysis equations, and describe and write equations for their reactions with sodium hydroxide, ammonia and sodium carbonate, including the colours of precipitates and solutions. Required practical 11 involves test-tube reactions to identify transition metal ions in aqueous solution. The topic builds on 3.2.5 Transition Metals and on Brønsted-Lowry ideas from 3.1.12. Examiners expect full complex formulae, correct colours and balanced equations.

What you need to know

• Aqua ions In water, metal ions form [M(H2O)6]2+ and [M(H2O)6]3+ octahedral complexes. [Fe(H2O)6]2+ is pale green, [Cu(H2O)6]2+ is blue, [Fe(H2O)6]3+ is violet but usually looks yellow-brown in solution, and [Al(H2O)6]3+ is colourless.
• Acidity 3+ ions have a higher charge density than 2+ ions, so they polarise the O-H bonds in water ligands more and release H+ more easily: [Fe(H2O)6]3+ + H2O ⇌ [Fe(H2O)5(OH)]2+ + H3O+. 3+ solutions are more acidic.
• Hydroxide ions Each ion forms a hydroxide precipitate: [Fe(H2O)4(OH)2] green, [Cu(H2O)4(OH)2] blue, [Fe(H2O)3(OH)3] brown, [Al(H2O)3(OH)3] white. Only the aluminium precipitate dissolves in excess NaOH to form colourless [Al(OH)4(H2O)2]-, because it is amphoteric.
• Ammonia Ammonia acts first as a base to give the same hydroxide precipitates. In excess ammonia, the copper precipitate dissolves to form deep blue [Cu(NH3)4(H2O)2]2+ by ligand substitution.
• Carbonate ions 2+ ions form carbonate precipitates: FeCO3 (green) and CuCO3 (blue-green). 3+ ions are acidic enough to form hydroxide precipitates and release CO2 gas, so you see effervescence.
• Required practical 11 Add NaOH, NH3 and Na2CO3 solutions dropwise, then in excess, to unknown solutions and use the colours and solubility of precipitates to identify the metal ion. The green iron(II) precipitate darkens to brown on standing in air as it is oxidised to iron(III).

Common mistakes in 3.2.6 Reactions of Ions in Aqueous Solution

• Writing hydroxide precipitates without their water ligands, such as Fe(OH)3. Show the full neutral complex, for example [Fe(H2O)3(OH)3].
• Saying iron(III) and aluminium ions form carbonates with sodium carbonate. They form hydroxides and give off carbon dioxide.
• Writing [Cu(NH3)6]2+ for the deep blue complex. In excess ammonia, it is [Cu(NH3)4(H2O)2]2+.
• Explaining 3+ acidity simply by 'more charge'. Refer to charge density, polarisation of water ligands and weakening of O-H bonds.
• Forgetting that aluminium hydroxide also dissolves in acid. It is amphoteric and reacts with both acids and bases.

Try three 3.2.6 Reactions of Ions in Aqueous Solution exam-style questions