1. Potassium permanganate: structure and preparation
Dark purple crystals; Mn +7 (3d⁰), diamagnetic; colour by charge transfer. is tetrahedral; manganate (Mn +6, d¹, green, paramagnetic) is also tetrahedral. Decomposes at 513 K: .
Preparation from pyrolusite: (1) fuse with KOH and air (or KNO₃) → green ; (2) oxidise manganate electrolytically (industry), or with Cl₂/O₃, or let it disproportionate in neutral/acid solution: .
Formulas
- Fusion of pyrolusite
Key terms
- Charge transfer
Movement of an electron from ligand (oxygen) to metal on absorbing light; causes the colour of d⁰ ions such as MnO₄⁻.
- Disproportionation
A reaction in which the same species is both oxidised and reduced.
2. Oxidising action of KMnO₄ and titrations
Acid (dil. H₂SO₄): — oxidises Fe²⁺ (1 : 5), oxalic acid (2 : 5, warm), I⁻ → I₂. Basic, with KI: . Neutral, with H₂S: .
Titrations: self-indicator (end point = first permanent pale pink); dilute H₂SO₄ only (HCl is oxidised to Cl₂; HNO₃ is an oxidant); oxalic acid titrations warm, Mn²⁺ autocatalyses; KMnO₄ is not a primary standard. Equivalent mass in acid = M/5 = 31.6.
| Medium | e⁻ gained | Product |
|---|---|---|
| Acidic | 5 | Mn²⁺ |
| Neutral/faintly alkaline | 3 | MnO₂ |
| Strongly alkaline | 1 | MnO₄²⁻ |
Formulas
- Acidic medium
- Neutral/alkaline medium
Key terms
- Self-indicator
A titrant whose own colour change shows the end point, so no separate indicator is needed.
- Autocatalysis
Catalysis of a reaction by one of its own products (Mn²⁺ in the permanganate–oxalate reaction).
3. Potassium dichromate: structure and preparation
Orange crystals; Cr +6. tetrahedral; = two tetrahedra sharing one O (Cr–O–Cr ≈ 126°).
From chromite (, Cr +3):
- Roast with Na₂CO₃ in air: (Cr oxidised to +6).
- Acidify: .
- Add KCl: (K salt less soluble).
Key terms
- Chromite
The ore FeCr₂O₄, in which chromium is +3.
4. Oxidising action of K₂Cr₂O₇; chromate ⇌ dichromate
Acid: (orange → green; V). Oxidises Fe²⁺ (1 : 6), I⁻ → I₂, SO₂ → SO₄²⁻, H₂S → S, ethanol → ethanal.
pH: — acid gives orange dichromate, alkali gives yellow chromate; Cr stays +6 (not redox).
Titrations: primary standard (pure, non-hygroscopic, stable solutions); internal indicator diphenylamine; HCl may be used. Equivalent mass = M/6 = 49.
| Feature | KMnO₄ | K₂Cr₂O₇ |
|---|---|---|
| Primary standard | No | Yes |
| Indicator | Self | Diphenylamine |
| e⁻ per ion (acid) | 5 | 6 |
Formulas
- Dichromate in acid
- Chromate–dichromate equilibrium
Key terms
- Primary standard
A substance pure and stable enough for a standard solution to be made by direct weighing.
Exam tips
Common mistakes
- Using HCl to acidify permanganate titrations.
- Giving Mn²⁺ as the product in neutral medium (it is MnO₂).
- Treating chromate ⇌ dichromate as a redox change.
- Wrong mole ratios: use 1 : 5 (MnO₄⁻ : Fe²⁺), 2 : 5 (MnO₄⁻ : H₂C₂O₄), 1 : 6 (Cr₂O₇²⁻ : Fe²⁺).
How marks are usually awarded
- Preparation (3 marks): one balanced equation per step with conditions.
- Ionic equations (1 mark each): atoms and charge balanced.
- Titration numericals (2–3 marks): moles, ratio, answer with unit.