Comparing Dichromate and Permanganate

Introduction

Dichromate and permanganate are both textbook oxidants, yet their acidic half-reactions differ in atoms, proton demand and electron count. Dichromate has two Cr(VI) atoms accepting six electrons total to become two Cr³⁺ ions; permanganate has one Mn(VII) accepting five to become Mn²⁺. Mixing these numbers is a common balancing error.

Core explanation

Write both reductions under their stated acidic conditions. Dichromate: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. Permanganate: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. The coefficients are not arbitrary memorised decorations. Seven dichromate oxygens become seven waters, requiring fourteen protons; four permanganate oxygens become four waters, requiring eight protons. Charge balance then determines six or five electrons, matching formal oxidation-state decreases.

Suppose Fe²⁺ is oxidised to Fe³⁺, a one-electron oxidation. One dichromate ion requires six Fe²⁺; one acidic permanganate requires five. The dichromate net ionic equation is Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺. The permanganate version is MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Both conserve atoms and charge. Their stoichiometric ratio to Fe²⁺ is different, so swapping oxidant formulas without rebalancing gives a false titration calculation.

To compare oxidising strength thermodynamically, use standard reduction potentials for the exact half-reactions in a specified medium. OpenStax lists illustrative standard values around +1.33 V for the acid dichromate/Cr³⁺ couple and +1.51 V for the acid permanganate/Mn²⁺ couple under its stated conditions. The more positive reduction potential indicates the stronger oxidising tendency for that defined comparison, but neither number is a universal reagent ranking at every pH, ligand concentration or reaction rate. The Nernst relation and changing products modify actual conditions.

Permanganate's reduction product changes markedly with medium: MnO₂ or MnO₄²⁻ may be relevant outside strong acid. Chromium(VI) also has pH-dependent chromate, hydrogen chromate and dichromate speciation. Therefore, stating a single “electron equivalent” per formula without the product and medium is incomplete. In acid the familiar counts are six for dichromate and five for permanganate; in other media the count may differ.

Colour is a useful observation but not a balancing method. Dichromate is often orange and acidic Cr³⁺ solutions can be green or violet depending on ligands; permanganate is purple and Mn²⁺ pale, while MnO₂ is brown. A colour change can suggest progress or reveal an unexpected species, but concentration, ligand exchange and side reactions affect appearance. Calculate electron balance independently.

Safety also differs from a purely mathematical comparison. Cr(VI) compounds present serious occupational health hazards, while permanganate is a strong oxidiser with its own handling concerns. Choose educational examples responsibly; avoid substituting one reagent for another in a practical procedure without a reviewed method and waste plan.

Step-by-step reasoning

1. Identify the oxidant ion and solution medium. 2. Write its matching reduction product and half-reaction. 3. Determine electrons accepted per ion from oxidation-state change. 4. Multiply the partner oxidation to cancel electrons. 5. Check atom and charge balance, then assess potential only for the specified conditions.

Visual explanation

Draw a two-row ledger. Dichromate row: 2 Cr, +6→+3 each, 6e⁻, 14H⁺. Permanganate row: 1 Mn, +7→+2, 5e⁻, 8H⁺. Place a note that all counts refer to the acidic products shown.

Real-world analogy

Two trucks can carry different numbers of boxes even if both are delivery vehicles. Dichromate and permanganate both accept electrons, but their specified formula units and destinations determine capacities of six and five in acid.

Real-world example

In titration calculations for Fe²⁺, a mole of acidic dichromate corresponds stoichiometrically to six moles of Fe²⁺, while a mole of acidic permanganate corresponds to five. Using the wrong ratio directly biases the inferred iron concentration.

Why?

Why do dichromate and permanganate need different Fe²⁺ coefficients? Fe²⁺ loses one electron, but dichromate accepts six per ion in acid whereas permanganate accepts five per ion in acid.

Common misconception

“Both are strong oxidants, so they use the same electron coefficient.” Their oxidation-state changes and numbers of metal atoms differ; half-reactions must be balanced separately.

Worked example

If 0.0100 mol of Cr₂O₇²⁻ reacts completely with Fe²⁺ under the stated acidic equation, it oxidises 6(0.0100) = 0.0600 mol Fe²⁺. The same amount of MnO₄⁻ under its acidic Mn²⁺ half-reaction oxidises 5(0.0100) = 0.0500 mol Fe²⁺. This comparison assumes complete reactions and the stated products; it does not compare practical safety or rates.

Quick check

1. How many Fe²⁺ ions are oxidised per dichromate ion in the standard acidic Fe²⁺ reaction? Answer: Six Fe²⁺ ions, each donating one electron.

Exam focus

Write medium and products before calculating an electron equivalent. Keep potential separate from stoichiometry and never multiply E° by electron count. Verify every net equation's charge balance.

Advanced insight

OpenStax's transition-metal chapter gives the defined acidic potentials and reactions at https://openstax.org/books/chemistry-2e/pages/19-1-occurrence-preparation-and-properties-of-transition-metals-and-their-compounds. Conditional potentials at other pH values can change relative oxidising tendencies.

Summary

Acidic dichromate accepts six electrons per Cr₂O₇²⁻ and acidic permanganate accepts five per MnO₄⁻. Their proton and product coefficients differ. Redox comparison requires specified medium, balanced half-reactions and conditional potential awareness.

Practice questions

1. What is the acidic electron count for Cr₂O₇²⁻ → 2Cr³⁺? Answer: Six electrons. 2. What is the acidic electron count for MnO₄⁻ → Mn²⁺? Answer: Five electrons. 3. Does E° multiply when a half-reaction is scaled to cancel electrons? Answer: No. Potential is intensive. 4. Why can neutral permanganate chemistry require a different electron count? Answer: MnO₂ rather than Mn²⁺ may be the reduction product, changing Mn's oxidation-state drop.