During the decomposition of cobalt(II) chloride, which element is oxidized?

• A. Cobalt(II) chloride
• B. Chlorine
• C. Cobalt
• D. Chloride ion

Answer: (B)

Oxidation is an increase in oxidation state. When cobalt(II) chloride thermally decomposes, it breaks into cobalt metal and chlorine gas: CoCl2 → Co + Cl2. In this change, cobalt goes from +2 to 0, so it is reduced, not oxidized. The chlorine atoms go from −1 in the chloride form to 0 in Cl2, which is an increase in oxidation state, so chlorine is oxidized. Since the question asks which element is oxidized, chlorine is the correct answer. The chloride ion is simply the source of chlorine in the compound, but the element undergoing oxidation is chlorine itself.