In which of the following reactions does the oxidation number of oxygen increase?

• A. C + O2 → CO2
• B. 2 KClO3 → 2 KCl + 3 O2
• C. SO2 + O2 → 2 SO3
• D. 2 H2O2 → 2 H2O + O2

Answer: (D)

Verified step by step guidance

1. Recall that the oxidation number of oxygen in most compounds is usually -2, except in peroxides where it is -1, and in elemental oxygen (O2) where it is 0.
2. Analyze each reaction by assigning oxidation numbers to oxygen before and after the reaction to see if the oxidation number increases (which means oxygen is oxidized).
3. For the reaction C + O2 → CO2, oxygen goes from 0 in O2 to -2 in CO2, so the oxidation number decreases (oxygen is reduced).
4. For the reaction 2 KClO3 → 2 KCl + 3 O2, oxygen in KClO3 is typically -2, and in O2 it is 0, so oxygen's oxidation number increases (oxygen is oxidized).
5. For the reaction 2 H2O2 → 2 H2O + O2, oxygen in H2O2 (a peroxide) is -1, and in O2 it is 0, so oxygen's oxidation number increases, indicating oxidation of oxygen.