5.3.3Unit 5. Transition Metals and Organic Nitrogen Chemistry
Ions in Aqueous Solution — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 43 Unit quiz
Section 1 of 5Acidity of Hexaaqua Ions
Structure
- Metal ions in water exist as [M(H₂O)₆]ⁿ⁺ complexes held by dative bonds from oxygen atoms.
Polarisation
- The positive metal ion attracts electrons away from the O-H bonds, making it easier for a water ligand to lose H⁺.
Equilibrium
- [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺, so the solution is acidic.
Charge density
- 3+ ions have a higher charge density than 2+ ions, so [M(H₂O)₆]³⁺ solutions are more acidic.
Section 2 of 5Reactions With Hydroxide and Ammonia
Reactions With Hydroxide and Ammonia
Cu²⁺
- Pale blue precipitate of Cu(OH)₂ with NaOH, insoluble in excess; with excess NH₃ gives a deep blue solution.
Fe²⁺
- Green precipitate of Fe(OH)₂ that darkens as it is oxidised in air.
Fe³⁺
- Orange-brown precipitate of Fe(OH)₃ that does not dissolve in excess NaOH or NH₃.
Al³⁺
- White precipitate of Al(OH)₃ that dissolves in excess NaOH, forming [Al(OH)₄]⁻, but not in ammonia.
| Ion | Colour of solution | With NaOH |
|---|---|---|
| [Cu(H₂O)₆]²⁺ | Pale blue | Pale blue ppt |
| [Fe(H₂O)₆]²⁺ | Pale green | Green ppt |
| [Fe(H₂O)₆]³⁺ | Yellow-brown | Orange-brown ppt |
| [Al(H₂O)₆]³⁺ | Colourless | White ppt, soluble in excess |
Section 3 of 5Reaction With Carbonate
1
With a 2+ ion such as Cu²⁺, sodium carbonate gives a precipitate of the metal carbonate, for example CuCO₃.
2
With a 3+ ion such as Al³⁺ or Fe³⁺, the solution is acidic enough to react with carbonate and release carbon dioxide.
3
A hydroxide precipitate forms at the same time, because the base removes H⁺ from the hexaaqua ion.
4
2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Al(OH)₃(H₂O)₃ + 3CO₂ + 3H₂O
5
Effervescence together with a coloured or white precipitate identifies a 3+ aqua ion.
Section 4 of 5Worked Example
Question Write the equation for [Cu(H₂O)₆]²⁺ reacting with two moles of hydroxide ions.
- Two hydroxide ions remove two H⁺ from two water ligands, leaving two OH⁻ ligands.
- The complex becomes neutral[Cu(H₂O)₄(OH)₂].
- Two molecules of water are formed from H⁺ and OH⁻.
Answer [Cu(H₂O)₆]²⁺ + 2OH⁻ → [Cu(OH)₂(H₂O)₄] + 2H₂O
Section 5 of 5Exam Checklist & Tips
5.3.3 – Key Points for the Exam
- Hexaaqua ion A metal ion surrounded by six water ligands.
- Amphoteric hydroxide A hydroxide that dissolves in both acid and excess alkali.
- Charge density Charge divided by size of an ion.
- Precipitate An insoluble solid formed from solution.
Common Mistakes
- Writing Fe(OH)₃ as green.
- Saying that Al(OH)₃ dissolves in excess ammonia.
- Forgetting that 3+ ions release CO₂ with carbonate.
- Describing the acid behaviour as the metal losing electrons.
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Chemistry 0620 & Edexcel OL
Product Prediction Rules
Aqueous Electrolysis Rules (Inert Electrodes)
- At Cathode (-): Least reactive cation discharged. If metal is more reactive than hydrogen (Na+, K+, Mg2+), H2 gas is produced:
2H+ + 2e- → H2. If Cu2+ or Ag+, the metal coats the cathode. - At Anode (+): If concentrated halide present (Cl-, Br-, I-), halogen is formed (e.g.
2Cl- → Cl2 + 2e-). If sulfate (SO42-) or nitrate (NO3-), O2 gas is discharged:4OH- → O2 + 2H2O + 4e-.