During electrolysis of molten sodium iodide, what is the half-reaction occurring at the anode?

• A. I- -> I+ + e-
• B. Na -> Na+ + e-
• C. 2 I- -> I2 + 2 e-
• D. I2 + 2 e- -> 2 I-

Answer: (C)

In molten salt electrolysis, the anode is the site where oxidation occurs, so the species present as anions are the ones that get oxidized. Here, iodide (I−) is oxidized to diatomic iodine (I2), releasing electrons. The balanced half-reaction is 2 I− → I2 + 2 e−. This is why iodine gas is formed at the anode. The electrons travel to the cathode, where Na+ would be reduced to Na metal: Na+ + e− → Na(s). The other possibilities either propose oxidation of a species that isn’t present as a metal at the anode or depict the reverse (a reduction) of iodine, which would occur at the cathode, not the anode.