In the MnO4- to MnO2 half-reaction balanced in basic solution, how many electrons are transferred?

• A. 1 electron
• B. 2 electrons
• C. 3 electrons
• D. 4 electrons

Answer: (C)

Balancing redox half-reactions in basic solution hinges on tracking how much the oxidation state of the element being reduced changes, since that change equals the number of electrons exchanged. Here, manganese goes from MnO4- where manganese is in the +7 state, to MnO2 where manganese is in the +4 state. That drop of three oxidation units means three electrons are gained.

In basic solution, a properly balanced half-reaction for this reduction is:

MnO4- + 2 H2O + 3 e- -> MnO2 + 4 OH-

This shows three electrons are transferred. If only one, two, or four electrons were transferred, the manganese oxidation state would change by 1, 2, or 4, which would not yield MnO2 with manganese in the +4 state.