Chemistry 9701/13 — May/June 2013

Background Concept

Oxidation numbers (oxidation states) are a bookkeeping tool to track electron transfer in redox reactions. For a polyatomic ion, the sum of the oxidation numbers of all atoms equals the charge on the ion. Oxygen is usually assigned -2 in compounds (except in peroxides and with fluorine).

Understanding the Question

The reaction is:

$\text{VO}_2^+ + \text{SO}_2 \rightarrow \text{V}^{3+} + \text{SO}_4^{2-}$

We need to find the oxidation numbers of vanadium and sulfur on both sides and see how they change.

Approach

1. Determine the oxidation number of vanadium in $\text{VO}_2^+$ and in $\text{V}^{3+}$.
2. Determine the oxidation number of sulfur in $\text{SO}_2$ and in $\text{SO}_4^{2-}$.
3. Compare the changes and select the correct option.

Step-by-Step Reasoning

Vanadium in $\text{VO}_2^+$:

• Each oxygen is -2, so two oxygens contribute -4.
• The ion has a total charge of +1.
• Let V = x. Then x + (-4) = +1 → x = +5.

Vanadium in $\text{V}^{3+}$:

• The ion has a charge of +3, so V = +3.

Sulfur in $\text{SO}_2$:

• Each oxygen is -2, so two oxygens contribute -4.
• The molecule is neutral, so S = +4.

Sulfur in $\text{SO}_4^{2-}$:

• Four oxygens contribute -8.
• The ion has a charge of -2.
• Let S = y. Then y + (-8) = -2 → y = +6.

Thus vanadium goes from +5 to +3, and sulfur goes from +4 to +6. This matches option D.

Key Takeaways

• Always assign oxidation numbers using the known rules: oxygen is -2 (except in peroxides), hydrogen is +1, and the sum equals the charge on the species.
• In redox reactions, the oxidation number of one species increases (oxidation) and another decreases (reduction). Here sulfur is oxidised and vanadium is reduced.

Common Mistakes

• Forgetting that the sum of oxidation numbers in a polyatomic ion equals the ion's charge, not zero.
• Assigning oxygen as -1 instead of -2.
• Confusing the oxidation number of sulfur in $\text{SO}_2$ (which is +4) with that in $\text{SO}_3$ or sulfate.

Things to Be Careful About

• Read the table carefully: the columns are vanadium (from), vanadium (to), sulfur (from), sulfur (to).
• Double-check the charge on each ion before assigning oxidation numbers.
• The reaction is not balanced, but that does not affect oxidation number assignment.