Redox Reactions (Class 11): Oxidation Numbers and Balancing That Works

Assigning oxidation numbers without guessing

Most errors in this chapter happen in the first thirty seconds, when a student assigns an oxidation number by feel. The rules are short and they resolve almost every case:

The ion-electron method, in fixed order

1. Write the unbalanced ionic equation and assign oxidation numbers.
2. Identify the species oxidised and the species reduced, and split into two half reactions.
3. Balance all atoms except oxygen and hydrogen.
4. Balance oxygen by adding H₂O, then hydrogen by adding H⁺.
5. Balance the charge on each side by adding electrons.
6. Multiply the half reactions so the electrons cancel, then add them.

For basic medium, complete all six steps as though the solution were acidic, then add as many OH⁻ to both sides as there are H⁺, and combine H⁺ with OH⁻ into water. Doing this conversion at the end rather than partway through avoids nearly every mistake students make in basic-medium questions.

Disproportionation, and why it needs an intermediate state

A species disproportionates when the same element is simultaneously oxidised and reduced. This is only possible if the element starts in an intermediate oxidation state, with room to move in both directions. That is why H₂O₂ disproportionates (oxygen at −1) while water does not (oxygen already at its lowest common state). Questions asking whether a given species can disproportionate are really asking whether the oxidation state is intermediate.

What to practise

• Ten balancings, at least four in basic medium, always using the same six steps.
• Oxidation number assignment in awkward species: S₂O₃²⁻, Cr₂O₇²⁻, MnO₄⁻, CaOCl₂.
• Three disproportionation examples, with the reasoning stated.

FAQs

Can an oxidation number be fractional?

Yes, and it is not an error. In species such as Fe₃O₄ or the superoxide ion, the value obtained is an average across atoms in different environments. Fractional answers are acceptable and often expected.

Why balance in acidic medium first even when the question says basic?

Because the procedure is identical up to the last step, and converting at the end is a single mechanical operation. Trying to work in basic medium throughout means juggling OH⁻ and water simultaneously, which is where most errors appear.

What is the difference between an oxidising agent and the species oxidised?

The oxidising agent is itself reduced — it takes electrons. The species oxidised loses them. Reversing these two in a written answer is one of the most common ways to lose a mark that the working otherwise earned.

How much of the Class 11 paper does redox carry?

It is a compact chapter but it underpins electrochemistry in Class 12 and appears throughout inorganic chemistry, so the practical return on getting it secure is much larger than its own mark weight.