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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

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Part 1 of 5Acidity of Hexaaqua Ions

5.3.3 Ions in Aqueous Solution
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.

Part 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.
IonColour of solutionWith NaOH
[Cu(H₂O)₆]²⁺Pale bluePale blue ppt
[Fe(H₂O)₆]²⁺Pale greenGreen ppt
[Fe(H₂O)₆]³⁺Yellow-brownOrange-brown ppt
[Al(H₂O)₆]³⁺ColourlessWhite ppt, soluble in excess

Part 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.

Part 4 of 5Worked Example

Question Write the equation for [Cu(H₂O)₆]²⁺ reacting with two moles of hydroxide ions.

  1. Two hydroxide ions remove two H⁺ from two water ligands, leaving two OH⁻ ligands.
  2. The complex becomes neutral[Cu(H₂O)₄(OH)₂].
  3. Two molecules of water are formed from H⁺ and OH⁻.

Answer [Cu(H₂O)₆]²⁺ + 2OH⁻ → [Cu(OH)₂(H₂O)₄] + 2H₂O

Part 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-.
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