Question

In these reactions involving iodine (I), determine whether iodine is oxidized or reduced.

Iodine (I) and hydrogen sulfide (H2S) combine to form hydrogen iodide and sulfur (S8).
8I2 + 8H2S es001-1.jpg 16HI + S8 Iodine is
Iodine (I2) and aluminum (Al) combine to form aluminum iodide (AlI3).
3I2 + 2Al es002-1.jpg 2AlI3 Iodine is
Hydrogen iodide (2HI) and chlorine gas (Cl2) combine to form hydrogen chloride (2HCl) and iodine (I2).
2HI + Cl2 ® 2HCl + I2 Iodine is

Answer 1

1) 8I2 + 8H2S → 16HI + S8
I in I2: 0 → I in HI: −1, so iodine is reduced (I2 gains electrons).

2) 3I2 + 2Al → 2AlI3
I in I2: 0 → I in AlI3: −1, so iodine is reduced.

3) 2HI + Cl2 → 2HCl + I2
I in HI: −1 → I in I2: 0, so iodine is oxidized (I− loses electrons).