In the reaction 2Al(s) + 3Fe2+(aq) → 2Al3+(aq) + 3Fe(s), where do the electrons have the lowest free energy?

• A. In solid aluminum
• B. In solution
• C. In the wire
• D. In solid iron

Answer: (D)

The main idea is that electrons flow from the species that is oxidized to the species that is reduced, ending up in the reduced form which is the most stable (lowest free energy) state for those electrons. In this reaction, aluminum loses electrons and iron(II) in solution gains electrons to become solid iron. The electrons are thus gathered into the solid iron lattice, which is the reduced form and provides a very stable environment thanks to metallic bonding. So the electrons reside in solid iron because that is where the reduction product is formed and where their energy is minimized.