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9476 / 2027
Transition Elements

Lesson 3 of 6

Three redox systems

Balance iron, manganate(VII) and dichromate(VI) reactions.

A-Level 9476 (2026-2027)

  • I can explain the three specified redox systems.Syllabus

    Syllabus 9476, 13(g). Explain the three specified redox systems.

  • I can predict redox likelihood from standard potentials.Syllabus

    Syllabus 9476, 13(h). Predict redox likelihood from standard potentials.

Use three redox systems confidently

Write the correct acidic half-equation before combining electrons or comparing potentials.

Reduction half-equations in acidic aqueous solution
SystemHalf-equationUseful 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 exampleWill 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.

Try it first. Then show the worked stepsHide the worked steps
  1. Dichromate has the more positive reduction potential: reduce it and reverse the iron reduction half-equation.
  2. Multiply Fe2+ → Fe3+ + e- by six to cancel six electrons. Do not multiply its electrode potential.
  3. E°cell = 1.33 - 0.77 = +0.56 V.
  4. Add the half-equations: Cr2O72- + 14H+ + 6Fe2+ → 2Cr3+ + 7H2O + 6Fe3+.
Answer

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.

Check your understanding

How many moles of Fe2+ react with 0.00400 mol of acidified MnO4-?

  1. 0.0200 mol, since MnO4- : Fe2+ = 1 : 5
  2. 0.00400 mol, since the ions react 1 : 1
  3. 0.000800 mol, since Fe2+ : MnO4- = 1 : 5
Show the answer

0.0200 mol, since MnO4- : Fe2+ = 1 : 5

MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+. Each MnO4- gains five electrons and each Fe2+ gives one, so 5 x 0.00400 = 0.0200 mol.