1. Acidity of Aqueous Metal Ions (2+ vs 3+)
Transition metal ions and aluminum ions dissolve in water to form octahedral hexaaqua complex ions: [M(H2O)6]2+ or [M(H2O)6]3+.
- 2+ ions ([Fe(H2O)6]2+, [Cu(H2O)6]2+): Form weakly acidic solutions with pH ~ 5 to 6.
- 3+ ions ([Fe(H2O)6]3+, [Al(H2O)6]3+): Form distinctly acidic solutions with pH ~ 2 to 3.
Polarisation & Charge Density Mechanism
A 3+ metal ion has a higher ionic charge and a smaller ionic radius than a 2+ ion, resulting in a substantially higher charge density.
- The small, highly charged 3+ cation strongly attracts electron density from the coordinate bonds toward itself.
- This polarises the O-H bonds within the coordinated water ligands, pulling electron density away from hydrogen.
- The O-H bond is significantly weakened, enabling water molecules from the surrounding solvent to act as Bronsted-Lowry bases and remove a proton:
[M(H2O)6]3+(aq) + H2O(l) <=> [M(H2O)5(OH)]2+(aq) + H3O+(aq) - Because 2+ ions have lower charge density, they polarise O-H bonds much less effectively, releasing far fewer protons into solution.
2. Precipitation Reactions with Dilute Sodium Hydroxide (NaOH)
Adding aqueous hydroxide ions (OH-) progressively removes protons from coordinated water ligands until an uncharged, insoluble neutral metal hydroxide precipitates:
| Metal Ion | Initial Solution | Precipitate with Dilute NaOH | Ionic Equation for Precipitation | Effect of Excess NaOH |
|---|---|---|---|---|
| Fe2+ | Pale green | Green precipitate of Fe(H2O)4(OH)2(s) | [Fe(H2O)6]2+ + 2OH- -> Fe(H2O)4(OH)2(s) + 2H2O | Insoluble in excess. (Darkens to brown on standing as air oxidises Fe2+ to Fe3+). |
| Cu2+ | Pale blue | Blue precipitate of Cu(H2O)4(OH)2(s) | [Cu(H2O)6]2+ + 2OH- -> Cu(H2O)4(OH)2(s) + 2H2O | Insoluble in excess. |
| Fe3+ | Yellow / brown | Brown precipitate of Fe(H2O)3(OH)3(s) | [Fe(H2O)6]3+ + 3OH- -> Fe(H2O)3(OH)3(s) + 3H2O | Insoluble in excess. |
| Al3+ | Colourless | White precipitate of Al(H2O)3(OH)3(s) | [Al(H2O)6]3+ + 3OH- -> Al(H2O)3(OH)3(s) + 3H2O | DISSOLVES in excess to form a colourless solution of [Al(OH)4]-(aq). |
3. Amphoteric Dissolution of Aluminum Hydroxide
Aluminum hydroxide, Al(H2O)3(OH)3, is amphoteric. It behaves as a base by accepting protons in acid, and as an acid by donating a proton to excess hydroxide:
Al(H2O)3(OH)3(s) + OH-(aq) -> [Al(OH)4]-(aq) + 3H2O(l)
The white precipitate re-dissolves in excess NaOH to yield a completely colourless solution of sodium tetrahydroxoaluminate.
4. Reactions with Dilute & Excess Aqueous Ammonia (NH3)
Aqueous ammonia establishes an equilibrium: NH3 + H2O <=> NH4+ + OH-. In dilute ammonia, OH- acts as a Bronsted-Lowry base, producing the exact same hydroxide precipitates as dilute NaOH.
Behavior with Excess Ammonia: Ligand Substitution in Cu2+
When excess concentrated ammonia is added to copper(II) hydroxide, ammonia molecules act as Lewis base ligands and displace water molecules in a ligand substitution reaction:
Cu(H2O)4(OH)2(s) + 4NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq)
The pale blue precipitate dissolves completely to yield an intense, deep royal blue solution of tetraamminediaquacopper(II).
In contrast, the precipitates of Fe2+, Fe3+, and Al3+ do NOT dissolve in excess ammonia.
5. Precipitate Color Rack Diagram
6. Reactions with Sodium Carbonate (Na2CO3)
The reaction of aqueous metal ions with sodium carbonate illustrates the profound chemical difference between 2+ and 3+ ions:
| Ion Type | Reaction Behavior with Na2CO3 | Observations | Balanced Ionic Equation |
|---|---|---|---|
| 2+ Ions (Fe2+, Cu2+) | Simple precipitation of insoluble metal carbonate. No gas formed. |
Fe2+: Green precipitate of FeCO3(s).
Cu2+: Blue-green precipitate of CuCO3(s). |
Fe2+(aq) + CO3 2-(aq) -> FeCO3(s)
Cu2+(aq) + CO3 2-(aq) -> CuCO3(s) |
| 3+ Ions (Fe3+, Al3+) | Acid-base reaction! 3+ ions are too acidic to form carbonates. Protons released react with CO3 2- to produce CO2 gas and precipitate the metal hydroxide. |
Fe3+: Brown precipitate of Fe(OH)3 and effervescence (bubbles of CO2).
Al3+: White precipitate of Al(OH)3 and effervescence (bubbles of CO2). |
2[Fe(H2O)6]3+ + 3CO3 2- -> 2Fe(H2O)3(OH)3(s) + 3CO2(g) + 3H2O(l)
2[Al(H2O)6]3+ + 3CO3 2- -> 2Al(H2O)3(OH)3(s) + 3CO2(g) + 3H2O(l) |
Never write Fe2(CO3)3 or Al2(CO3)3 as products! Because hexaaqua 3+ ions are strongly acidic (pH 2 to 3), carbonate ions are protonated to hydrogencarbonate and then decompose into carbon dioxide and water. The products are ALWAYS the metal hydroxide precipitate and carbon dioxide gas.
7. Worked Calculations & Identification Schemes
Test 1: Addition of aqueous sodium hydroxide (NaOH):
- If Fe2+: A green precipitate of Fe(H2O)4(OH)2 forms, which darkens to brown at the surface upon standing in air.
- If Fe3+: A red-brown precipitate of Fe(H2O)3(OH)3 forms immediately.
Test 2: Addition of aqueous sodium carbonate (Na2CO3):
- If Fe2+: A green precipitate of FeCO3 forms with no bubbles of gas.
- If Fe3+: A brown precipitate of Fe(H2O)3(OH)3 forms alongside vigorous effervescence (bubbles of CO2 gas) that turns limewater cloudy.
Stage 1: Precipitation with dilute ammonia:
[Cu(H2O)6]2+(aq) + 2NH3(aq) -> Cu(H2O)4(OH)2(s) + 2NH4+(aq)
Stage 2: Dissolution in excess concentrated ammonia:
Cu(H2O)4(OH)2(s) + 4NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq)
Exam-Style Practice Questions
Test your understanding of these core syllabus concepts with targeted questions and detailed explanations.
Question 1: Why is an aqueous solution of iron(III) chloride significantly more acidic (pH ~2) than an aqueous solution of iron(II) chloride (pH ~5)?
Show Answer & Explanation
Correct Answer: B
Fe3+ has a higher charge and smaller ionic radius than Fe2+, giving it higher charge density. This polarises the O-H bonds of water ligands, weakening them and releasing H+ ions into solution.
Question 2: Which metal hydroxide precipitate dissolves in excess sodium hydroxide to form a colourless solution?
Show Answer & Explanation
Correct Answer: D
Al(OH)3 is amphoteric. It reacts with excess OH- to form the soluble tetrahydroxoaluminate ion, [Al(OH)4]-, producing a completely colourless solution.
Question 3: What is observed when aqueous sodium carbonate is added to an aqueous solution of aluminum chloride?
Show Answer & Explanation
Correct Answer: B
[Al(H2O)6]3+ is strongly acidic. It undergoes an acid-base reaction with carbonate ions (CO3 2-) to form carbon dioxide gas (bubbles) and precipitates aluminum hydroxide: 2[Al(H2O)6]3+ + 3CO3 2- -> 2Al(H2O)3(OH)3 + 3CO2 + 3H2O.
Question 4: What color change is observed when excess concentrated ammonia is added to a blue precipitate of copper(II) hydroxide?
Show Answer & Explanation
Correct Answer: B
Copper(II) hydroxide undergoes ligand exchange with excess ammonia to form the soluble [Cu(NH3)4(H2O)2]2+ complex ion, which has a characteristic intense deep royal blue color.