Explain oxidation-reduction and assign oxidation numbers in a redox reaction: MnO4- to Mn2+ in acidic solution.

• A. In MnO4- to Mn2+ in acidic solution, Mn is +7; Mn2+ is +2.
• B. In MnO4- to Mn2+ in acidic solution, Mn is +2; Mn2+ is +7.
• C. MnO4- is oxidized to Mn2+ in acidic solution.
• D. Oxidation numbers cannot be assigned.

Answer: (A)

Oxidation-reduction hinges on electron transfer, tracked by oxidation numbers. In a redox couple, the species that increases its oxidation state is oxidized, while the one that decreases its oxidation state is reduced. Here, manganese goes from MnO4− to Mn2+ in acidic solution. To assign oxidation numbers, treat oxygen as −2 in MnO4−. Let manganese have oxidation state x: x + 4(−2) = −1, so x = +7. In Mn2+, manganese is +2. So the manganese center is reduced from +7 to +2, a gain of 5 electrons. This shows MnO4− acts as the oxidizing agent because it accepts electrons and is reduced itself.

A balanced acidic half-reaction is MnO4− + 8 H+ + 5 e− → Mn2+ + 4 H2O, illustrating the 5-electron gain by Mn. This concrete bookkeeping confirms the oxidation numbers and the redox flow.