Redox Reactions

Full Notes

Redox Reactions and Colour Changes of Transition Metal Ions you need to know for OCR (A) A-level Chemistry (download sheet here).

Iron: Fe²⁺ ⇌ Fe³⁺

Fe²⁺: Pale green solution
Fe³⁺: Yellow/orange solution

This is a classic one-electron redox process.

• Oxidation: Fe²⁺ → Fe³⁺ + e⁻
• Reduction: Fe³⁺ + e⁻ → Fe²⁺

Seen in reactions with oxidising agents (e.g. acidified MnO₄⁻) or reducing agents (e.g. iodide ions).

Chromium Redox Interconversions: Cr³⁺ ⇌ CrO₄²⁻ ⇌ Cr₂O₇²⁻

Cr³⁺ to CrO₄²⁻ to Cr₂O₇²⁻: Stepwise Oxidation Pathway

Chromium can transition between multiple oxidation states. In redox reactions, especially under alkaline and acidic conditions, it undergoes a stepwise transformation:

Starting Ion: Cr³⁺ – green ion in aqueous solution.

• Cr³⁺ → CrO₄²⁻ (Chromate(VI))
  • Add H₂O₂ and OH⁻ in alkaline conditions.
  • Cr³⁺ oxidised to CrO₄²⁻, a yellow solution.
  • Balanced: 2Cr³⁺ + 3H₂O₂ + 10OH⁻ → 2CrO₄²⁻ + 8H₂O
• CrO₄²⁻ ⇌ Cr₂O₇²⁻
  • Add H⁺ to acidify yellow CrO₄²⁻.
  • Equilibrium shifts to Cr₂O₇²⁻, orange solution.
  • Balanced: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O
• Cr₂O₇²⁻ → Cr³⁺ (Reduction)
  • Reduced back to Cr³⁺ using zinc in acidic conditions.
  • Colour: orange → green.
  • Balanced: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Summary of Colour Changes:
Cr³⁺ (green) → CrO₄²⁻ (yellow) → Cr₂O₇²⁻ (orange) → Cr³⁺ (green)

Copper: Cu²⁺ ⇌ Cu⁺ ⇌ Cu

Copper (II) ions can be reduced by iodide (I⁻) ions in aqueous solution.

Forms a white CuI precipitate and iodine (I₂), which makes the solution brown.

Disproportionation: Copper (I) readily undergoes disproportionation. For example, when Cu₂O is heated with H₂SO₄, this forms a blue Cu(II) solution and a brown precipitate of solid copper (Cu).

Predicting Reactions Using Ligand, Precipitation and Redox Knowledge

OCR expects you to apply knowledge to unfamiliar situations using these core principles:

• Oxidation States – Look for changes in oxidation number to spot redox behaviour.
  • For example, if a transition metal changes from +3 to +2, it is being reduced.
• Ligand Substitution – Understand which ligands are monodentate, bidentate or multidentate:
  • H₂O, NH₃, Cl⁻ → Monodentate
  • NH₂CH₂CH₂NH₂ (en) → Bidentate
  • EDTA⁴⁻ → Multidentate

  Ligand substitution can be partial or complete and often produce colour changes. [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
  Blue solution → deep blue solution

• Precipitation – Transition metal hydroxides often form coloured precipitates with OH⁻:
  • Fe²⁺ + 2OH⁻ → Fe(OH)₂ (green ppt)
  • Fe³⁺ + 3OH⁻ → Fe(OH)₃ (orange ppt)
  • Cu²⁺ + 2OH⁻ → Cu(OH)₂ (blue ppt)
• Combining Principles – In complex scenarios, combine:
  • Redox changes → predict oxidation state and electron transfer
  • Ligands → predict coordination complexes
  • Precipitate solubility → predict visual changes

Summary

• Fe²⁺ ⇌ Fe³⁺ involves green to yellow colour change in one-electron redox.
• Cr³⁺, CrO₄²⁻, and Cr₂O₇²⁻ interconvert with pH-dependent colour changes.
• Cu²⁺ ⇌ Cu⁺ ⇌ Cu shows reduction and disproportionation, forming CuI and iodine.
• OCR requires linking oxidation states, ligand substitution, and precipitation to explain redox observations.