How many electrons are transferred per formula unit in the redox couple Al + Ni2+ → Ni + Al3+?

• A. 2
• B. 4
• C. 8
• D. 6

Answer: (D)

The concept here is how electrons are transferred in a redox reaction and how to balance the electrons between the oxidation and reduction halves. Aluminum metal starts at oxidation state 0 and is oxidized to Al3+, so each Al atom loses 3 electrons. Nickel(II) ion a Ni2+ is reduced to Ni metal, so each Ni2+ gains 2 electrons.

To conserve electrons, the electrons lost must equal the electrons gained. Since 3 and 2 have no common factor, multiply the aluminum oxidation by 2 (giving 2 Al -> 2 Al3+ + 6 e−) and the nickel reduction by 3 (giving 3 Ni2+ + 6 e− -> 3 Ni). Adding these gives the balanced overall equation: 2 Al + 3 Ni2+ -> 2 Al3+ + 3 Ni, with a total of 6 electrons transferred.

Thus, six electrons are transferred in the balanced reaction.