OxfordAQA A-Level Chemistry: Reactions of ions in aqueous solution (9620) – Revision Notes

These notes condense subsection 3.2.6.1, on the reactions of metal ions in water, from OxfordAQA’s International AS and A-level Chemistry specification (9620), Version 5.3, for exams from May/June 2020. It is International A2 content, so all of it is International A-level only. Unit 3 assesses 3.2.4 to 3.2.6, and Unit 5 can draw on any content. Required practical 9 sits in this section.

Every idea here is explained at length, with worked examples, in the study guide. Afterwards, check your recall on the practice questions. Ligand substitution in general is covered in the transition metals revision notes, and Ka work in acids and bases. The course hub, the printable checklist and the free diagnostics help you plan the rest.

The boundaries of 3.2.6.1

• 2+ aqua ions: iron and copper only. 3+ aqua ions: aluminium and iron only.
• Bases: hydroxide ions, ammonia and carbonate ions only.
• Acidity: explained through the charge/size ratio of the metal ion.
• Amphoteric hydroxide: the specification’s example is the hydroxide of Al³⁺.
• Required practical 9: identifying transition metal ions in solution by test-tube reactions.

Words you must be able to use

• Aqua ion: a metal ion surrounded by six water ligands, each joined through a co-ordinate bond from oxygen.
• Hydrolysis (of an aqua ion): transfer of a proton from a water ligand to a solvent water molecule, producing H₃O⁺.
• Charge/size ratio: the ion’s charge relative to its radius. A higher ratio means a more polarising ion.
• Amphoteric: dissolves in (reacts with) both acids and bases.
• Effervescence: bubbles of gas; here it means CO₂.

Ion profiles

Iron(II): [Fe(H₂O)₆]²⁺, pale green

• OH⁻ or NH₃: green precipitate, [Fe(H₂O)₄(OH)₂]. No change in excess of either.
• On standing in air, the precipitate browns at the surface as iron(II) is oxidised to iron(III).
• CO₃²⁻: green precipitate of FeCO₃, no gas.

Copper(II): [Cu(H₂O)₆]²⁺, blue

• OH⁻: blue precipitate, [Cu(H₂O)₄(OH)₂]. No change in excess.
• NH₃: blue precipitate first; in excess it dissolves to a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺.
• CO₃²⁻: blue-green precipitate of CuCO₃, no gas.

Iron(III): [Fe(H₂O)₆]³⁺, yellow to brown

• OH⁻ or NH₃: orange-brown precipitate, [Fe(H₂O)₃(OH)₃]. No change in excess of either.
• CO₃²⁻: orange-brown precipitate and effervescence; the precipitate is the hydroxide, not a carbonate.

Aluminium: [Al(H₂O)₆]³⁺, colourless

• OH⁻: white precipitate, [Al(H₂O)₃(OH)₃]; dissolves in excess to a colourless solution of [Al(H₂O)₂(OH)₄]⁻.
• NH₃: white precipitate; does not dissolve in excess.
• CO₃²⁻: white precipitate and effervescence.
• Acid: the hydroxide dissolves, re-forming [Al(H₂O)₆]³⁺.

Equation patterns

Situation	General equation
Hydrolysis (acidity)	[M(H₂O)₆]ⁿ⁺(aq) + H₂O(l) ⇌ [M(H₂O)₅(OH)]⁽ⁿ⁻¹⁾⁺(aq) + H₃O⁺(aq)
2+ ion with OH⁻	[M(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → M(H₂O)₄(OH)₂ + 2H₂O(l)
3+ ion with OH⁻	[M(H₂O)₆]³⁺(aq) + 3OH⁻(aq) → M(H₂O)₃(OH)₃ + 3H₂O(l)
2+ ion with NH₃ (base)	[M(H₂O)₆]²⁺(aq) + 2NH₃(aq) → M(H₂O)₄(OH)₂ + 2NH₄⁺(aq)
3+ ion with NH₃ (base)	[M(H₂O)₆]³⁺(aq) + 3NH₃(aq) → M(H₂O)₃(OH)₃ + 3NH₄⁺(aq)
2+ ion with CO₃²⁻	[M(H₂O)₆]²⁺(aq) + CO₃²⁻(aq) → MCO₃(s) + 6H₂O(l)
3+ ion with CO₃²⁻	2[M(H₂O)₆]³⁺(aq) + 3CO₃²⁻(aq) → 2M(H₂O)₃(OH)₃ + 3CO₂(g) + 3H₂O(l)

Three specific equations sit outside the patterns:

Al hydroxide + excess OH⁻:  [Al(H₂O)₃(OH)₃](s) + OH⁻(aq) → [Al(H₂O)₂(OH)₄]⁻(aq) + H₂O(l)
Al hydroxide + acid:        [Al(H₂O)₃(OH)₃](s) + 3H⁺(aq) → [Al(H₂O)₆]³⁺(aq)
Cu hydroxide + excess NH₃:  [Cu(H₂O)₄(OH)₂](s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)

Mole ratios you will use in calculations:

Reaction	Ratio
M²⁺ : OH⁻ (or NH₃) to precipitate	1 : 2
M³⁺ : OH⁻ (or NH₃) to precipitate	1 : 3
Al³⁺ : OH⁻ to reach the soluble aluminate ion	1 : 4
M³⁺ : CO₂ with carbonate	2 : 3

Method in steps: the acidity explanation

1. State the equation for hydrolysis of the aqua ion, giving H₃O⁺.
2. Compare the ions: the 3+ ion has a higher charge and a smaller radius.
3. Conclude: a higher charge/size ratio, so a more polarising ion.
4. Mechanism: more electron density is pulled from the O–H bonds of the water ligands; the O–H bonds weaken.
5. Result: H⁺ is released more easily, the equilibrium lies further right, and the pH is lower at equal concentration.

Method in steps: naming an unknown

1. Colourless solution? Think Al³⁺. Blue? Think Cu²⁺. Pale green or yellow-brown? Think iron.
2. Add NaOH(aq) dropwise, then to excess. A white precipitate that dissolves means Al³⁺.
3. Use a fresh sample and add NH₃(aq) to excess. A blue precipitate that turns into a deep blue solution means Cu²⁺.
4. Use a fresh sample and add Na₂CO₃(aq). Bubbles mean a 3+ ion; no bubbles mean a 2+ ion.
5. For iron, use the precipitate colour (green or orange-brown) and the carbonate result together.

Must-know distinctions

• Precipitate forms vs precipitate dissolves. Formation is always the neutral hydroxide (or MCO₃ for 2+ with carbonate). Dissolving needs either a further proton loss (aluminium) or new ligands (copper with ammonia).
• Base vs ligand. NH₃ removes H⁺ in small amounts; with Cu²⁺ in excess it binds to the metal.
• Strong vs weak base. Excess OH⁻ can take a fourth proton from aluminium hydroxide; ammonia cannot.
• Carbonate vs hydroxide product. 2+ ions give a metal carbonate. 3+ ions give a hydroxide plus CO₂.
• Colourless vs clear. Any transparent solution is clear, including a blue one. Use “colourless” for no colour.
• Fe²⁺ vs Fe³⁺ by test, not by colour alone. Green vs orange-brown precipitate; no gas vs gas with carbonate.

Required practical 9 in brief

• Separate sample of the unknown for each reagent.
• Add the reagent drop by drop, shaking, so you see any precipitate before an excess is present.
• Continue to excess and record whether the precipitate dissolves.
• Look for gas with sodium carbonate.
• Let an iron(II) precipitate stand; record any browning at the surface.
• Eye protection throughout: NaOH(aq) is corrosive, and NH₃(aq) irritates eyes and lungs.

Small worked reminders

Base needed. 0.0350 mol of [Cu(H₂O)₆]²⁺ needs 2 × 0.0350 = 0.0700 mol of NH₃ to reach [Cu(H₂O)₄(OH)₂], before any excess.

Gas released. 0.0240 mol of [Fe(H₂O)₆]³⁺ with excess carbonate gives 0.0240 × 3/2 = 0.0360 mol of CO₂.

Quick self-test

1. Name the two metals in this section that form 2+ aqua ions.
2. Give the formula of the species formed when an aqua ion of iron(III) loses one proton.
3. State the two properties of Fe³⁺ that give it a greater charge/size ratio than Fe²⁺.
4. What do you see when NH₃(aq) is added dropwise and then in excess to aluminium nitrate solution?
5. Give the formula, with charge, of the soluble aluminium species in excess NaOH(aq).
6. Which ion in this section gives a precipitate that dissolves in excess NH₃(aq)?
7. Write the equation for copper(II) aqua ions reacting with carbonate ions.
8. Why is no aluminium carbonate formed when sodium carbonate is added to Al³⁺(aq)?
9. What species is formed from ammonia when it acts as a base in these reactions?
10. Precipitating all the Al³⁺ in a solution uses 0.0270 mol of OH⁻. How many more moles of OH⁻ are needed to redissolve the precipitate?
11. 0.0130 mol of [Fe(H₂O)₆]³⁺ reacts fully with NH₃(aq). How many moles of NH₄⁺ form?
12. A solution gives a white precipitate with NaOH(aq) that does not dissolve in excess. Is Al³⁺ present?

Answers

1. Iron and copper.
2. [Fe(H₂O)₅(OH)]²⁺.
3. A higher charge (3+) and a smaller ionic radius.
4. A white precipitate forms and remains in excess ammonia.
5. [Al(H₂O)₂(OH)₄]⁻, also written [Al(OH)₄]⁻.
6. Cu²⁺.
7. [Cu(H₂O)₆]²⁺ + CO₃²⁻ → CuCO₃ + 6H₂O.
8. [Al(H₂O)₆]³⁺ is acidic enough to protonate carbonate ions, which form CO₂ and water, leaving [Al(H₂O)₃(OH)₃] as the precipitate.
9. The ammonium ion, NH₄⁺.
10. n(Al³⁺) = 0.0270/3 = 0.00900 mol, so a further 0.00900 mol (1 : 1).
11. 3 × 0.0130 = 0.0390 mol.
12. No. Aluminium hydroxide dissolves in excess NaOH(aq); a white precipitate that stays points to another ion, such as Mg²⁺.

Where marks are usually lost

• Explaining acidity with charge alone, or with “charge density” and no link to the O–H bond.
• Writing the hydrolysis equation with H⁺ on the right but forgetting that the complex’s charge must fall by one.
• Describing “a blue solution” for copper with excess NH₃; the answer needs “deep blue” and the earlier precipitate.
• Writing [Cu(NH₃)₆]²⁺ instead of [Cu(NH₃)₄(H₂O)₂]²⁺.
• Forgetting that the iron(II) precipitate changes on standing, when the question asks for all observations.
• Giving effervescence for Fe²⁺ or Cu²⁺ with carbonate.
• Using a 1 : 1 or 1 : 3 ratio between M³⁺ and CO₂ instead of 2 : 3.
• Naming the reagent without saying “dropwise” and “in excess” when a question asks how to tell two ions apart.

Official syllabus

Source: OxfordAQA’s International AS and A-level Chemistry specification (9620), Version 5.3, for International AS and A-level exams from May/June 2020. Section 3.2.6 (International A2), the topic on reactions of ions in aqueous solution, subsection 3.2.6.1, including required practical 9.