In the balanced reaction Ca2+ (aq) + C(s) + ClO2(g) → CaCO3(s) + ClO2− (aq) balanced in basic conditions, what is the oxidation state of carbon in CaCO3(s)?

• A. +2
• B. +4
• C. +6
• D. 0

Answer: (B)

Knowing how to assign oxidation states lets you see which element is oxidized in a redox process. In calcium carbonate, the carbonate ion has three oxygens each at -2, giving a total of -6 from oxygen. The carbonate ion carries an overall -2 charge, so the carbon must be +4 to balance: x + (-6) = -2, hence x = +4. Elemental carbon starts with an oxidation state of 0, so it loses four electrons to become +4 in CaCO3. That makes carbon the species being oxidized in the reaction. Therefore, the carbon in CaCO3(s) has an oxidation state of +4.