Topic 3 of 6
Three redox systems
Balance iron, manganate(VII) and dichromate(VI) reactions.
A-Level 9476 (2026-2027)
Use three redox systems confidently
Write the correct acidic half-equation before combining electrons or comparing potentials.
| System | Half-equation | Useful observation |
|---|---|---|
| Fe(III)/Fe(II) | Fe3+(aq) + e- → Fe2+(aq) | Common Fe(III) solutions are yellow and Fe(II) solutions pale green; intensity and ligands affect the appearance. |
| Manganate(VII)/Mn(II) | MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l) | Purple disappears to a very pale pink, often effectively colourless, dilute Mn(II) solution. |
| Dichromate(VI)/Cr(III) | Cr2O72-(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 7H2O(l) | Orange changes to green under the usual reaction conditions. |
The oxidising agent is reduced. Acidified manganate(VII) and dichromate(VI) accept electrons from reducing agents such as Fe2+, which becomes Fe3+. Acid is a reactant in both oxyanion half-equations, not just a label above the arrow. Changing pH can change both the potential and the manganese or chromium product.
Worked example
Will dichromate oxidise iron(II)?
Use E°(Cr2O72-/Cr3+) = +1.33 V and E°(Fe3+/Fe2+) = +0.77 V. Predict the reaction under standard conditions and balance it.
- Dichromate has the more positive reduction potential: reduce it and reverse the iron reduction half-equation.
- Multiply Fe2+ → Fe3+ + e- by six to cancel six electrons. Do not multiply its electrode potential.
- E°cell = 1.33 - 0.77 = +0.56 V.
- Add the half-equations: Cr2O72- + 14H+ + 6Fe2+ → 2Cr3+ + 7H2O + 6Fe3+.
The positive standard cell potential predicts thermodynamic feasibility for the written reaction. It does not predict its speed. Atoms and total charge (+24 on each side) balance.