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

• A. 2 H_2O_2 ightarrow 2 H_2O + O_2
• B. C + O_2 ightarrow CO_2
• C. 2 H_2 + O_2 ightarrow 2 H_2O
• D. 2 KClO_3 ightarrow 2 KCl + 3 O_2

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that the oxidation number of oxygen in most compounds is usually -2, except in peroxides where it is -1, and in elemental oxygen (O_2) where it is 0.
2. Step 2: Analyze each reaction by assigning oxidation numbers to oxygen in the reactants and products. For example, in H_2O_2 (hydrogen peroxide), oxygen has an oxidation number of -1, while in H_2O and O_2, oxygen has oxidation numbers of -2 and 0 respectively.
3. Step 3: Compare the oxidation numbers of oxygen on both sides of each reaction to determine if the oxidation number increases (which means oxygen is oxidized). An increase means the oxidation number becomes less negative or more positive.
4. Step 4: For the reaction 2 H_2O_2 \rightarrow 2 H_2O + O_2, observe that oxygen goes from -1 in H_2O_2 to -2 in H_2O and 0 in O_2. Since 0 is greater than -1, oxygen is oxidized in this reaction.
5. Step 5: Confirm that in the other reactions, oxygen either maintains the same oxidation number or is reduced (oxidation number decreases), so the only reaction where oxygen's oxidation number increases is the decomposition of hydrogen peroxide.