Chemistry 9701/11 — May/June 2013

Background Concept

An oxidation number (oxidation state) is a bookkeeping number assigned to each atom in a species to track the transfer of electrons in redox reactions. The key rules are:

• Oxygen is usually $-2$ (except in peroxides, where it is $-1$, and in $\text{F}_2\text{O}$, where it is $+2$).
• Hydrogen is usually $+1$.
• In a neutral molecule, the sum of all oxidation numbers is zero.
• In an ion, the sum of all oxidation numbers equals the charge on the ion.
• For a monatomic ion, the oxidation number is simply equal to the charge on the ion.

In oxyanions such as $\text{ClO}^-$ and $\text{ClO}_3^-$, oxygen takes $-2$ and the chlorine is found by solving for the unknown so that the total matches the ion's charge.

Understanding the Question

The question gives a decomposition reaction of chlorate(I) ions:

and asks for the oxidation state of chlorine in each of the three species: $\text{ClO}^-$, $\text{ClO}_3^-$, and $\text{Cl}^-$. This is a recall-and-apply MCQ: you simply use the oxidation-number rules on each species. Notice that this reaction is a disproportionation — chlorine in the $+1$ state is simultaneously oxidised to $+5$ (in $\text{ClO}_3^-$) and reduced to $-1$ (in $\text{Cl}^-$).

Approach

Set the oxidation number of oxygen to $-2$ in each oxyanion, then set the sum of all oxidation numbers equal to the charge on the ion and solve for chlorine. For the monatomic ion $\text{Cl}^-$, the oxidation number is just the charge, $-1$.

Step-by-Step Reasoning

1. $\text{ClO}^-$: Let the oxidation number of chlorine be $x$. Oxygen is $-2$, and the ion has charge $-1$: $$x + (-2) = -1 \rightarrow x = +1$$
2. $\text{ClO}_3^-$: Three oxygens give $3 \times (-2) = -6$, and the ion has charge $-1$: $$x + 3(-2) = -1 \rightarrow x - 6 = -1 \rightarrow x = +5$$
3. $\text{Cl}^-$: This is a monatomic ion, so the oxidation number equals the charge: $$x = -1$$

So the correct set is $+1$, $+5$, $-1$, which is option C.

Why the distractors are wrong:

• Option A gives $\text{ClO}_3^-$ as $+3$. That would make the sum $+3 + (-6) = -3$, which does not match the $-1$ charge.
• Option B gives $\text{ClO}^-$ as $-1$ and $\text{Cl}^-$ as $+1$. In $\text{ClO}^-$, oxygen is more electronegative than chlorine, so oxygen takes $-2$ and chlorine must be $+1$; and $\text{Cl}^-$ is clearly $-1$, not $+1$.
• Option D repeats the same error for $\text{ClO}^-$ ($-1$ instead of $+1$).

Key Takeaways

• In an oxyanion, oxygen is $-2$; solve for the central atom using the ion's charge.
• A monatomic ion's oxidation number equals its charge.
• The sum of oxidation numbers equals the ion's charge (not zero, which applies only to neutral molecules).
• This reaction is a classic disproportionation: the same element in one oxidation state is both oxidised and reduced.

Common Mistakes

• Forgetting that the sum of oxidation numbers must equal the ion's charge, not zero — this is the most common error here.
• Assuming chlorine is more electronegative than oxygen. In fact, oxygen is more electronegative than chlorine, so in $\text{ClO}^-$ oxygen takes $-2$ and chlorine is $+1$.
• Confusing the oxidation number of an atom with the overall charge of the ion.

Things to Be Careful About

• Always include the sign of the charge when setting the sum of oxidation numbers.
• Oxygen is $-2$ except in peroxides and $\text{F}_2\text{O}$ — neither applies here.
• Check the reaction is balanced in charge: left side $3 \times (-1) = -3$; right side $-1 + 2(-1) = -3$. It is balanced, consistent with the oxidation states found.