Identify redox reactions by observing the colour changes that occur when using acidified aqueous potassium manganate(VII) or aqueous potassium iodide during a test. 🔬
Potassium manganate(VII) in water is green because the ion MnO₄²⁻ gives a green colour. When you add a little acid, the green ion is protonated and rearranges to the purple MnO₄⁻ (permanganate). This is a classic redox change: the manganese is oxidised from +6 to +7. Think of it like a green traffic light that suddenly turns purple when the driver (acid) steps on the accelerator. ⚙️
The reaction can be written as:
\$\text{MnO}4^{2-} + 2\text{H}^+ \rightarrow \text{MnO}4^- + \text{H}_2\text{O}\$
Notice how the colour shifts from green to purple – that’s your redox indicator. 💡
Potassium iodide (KI) is colourless in solution. When an oxidising agent (like hydrogen peroxide or a mild acid) is added, iodide ions are oxidised to iodine (I₂), giving a brownish‑violet colour. The reaction is:
\$\text{2KI} + \text{H}2\text{O}2 + \text{H}^+ \rightarrow \text{I}2 + 2\text{KOH} + \text{H}2\text{O}\$
Imagine a clear glass turning into a dark lake – that sudden colour change tells you that a redox reaction has happened. 🌊
Look for a colour change when an acid or oxidiser is added.
Identify the oxidising agent (e.g. acidified KMnO₄ or H₂O₂).
Check the oxidation states of the key element (Mn +6 → +7, I –1 → 0).
Remember that colour changes are often the quickest evidence of a redox reaction.
In exam questions, draw the balanced equation and annotate the colour change.
| Reagent | Initial Colour | After Acid/Oxidiser | Redox Change |
|---|---|---|---|
| KMnO₄ (acidified) | Green (MnO₄²⁻) | Purple (MnO₄⁻) | \$+6 \rightarrow +7\$ |
| KI + H₂O₂ + H⁺ | Colourless | Brownish‑violet (I₂) | \$-1 \rightarrow 0\$ |