Question
When chlorine gas reacts with cold dilute NaOH, the products are NaCl, NaOCl, and water. Show that this is a disproportionation reaction by tracking the oxidation state of chlorine throughout.
Solution — Step by Step
In elemental chlorine (Cl₂), the oxidation state is 0. This is always true for any element in its pure form — no electronegativity difference, no charge transfer.
NaCl is ionic: Na is +1, so Cl must be −1 to keep the compound neutral. Chlorine has been reduced (gained electrons: 0 → −1).
NaOCl: Na = +1, O = −2. So: (+1) + x + (−2) = 0, which gives x = +1. Chlorine has been oxidised (lost electrons: 0 → +1).
Both Cl atoms started at oxidation state 0 (in Cl₂). One went to −1, the other went to +1. Same element, same starting state, two opposite fates. That is the definition of disproportionation.
Oxidation half-reaction:
Reduction half-reaction:
One Cl from each Cl₂ molecule goes each way. The electrons are transferred internally between the two chlorine atoms. This is disproportionation.
Why This Works
In a normal redox reaction, one species is the reducing agent and a completely different species is the oxidising agent. Disproportionation is special — the same element plays both roles simultaneously. The element’s oxidation state splits into a higher value and a lower value.
This is only possible when the element starts at an intermediate oxidation state. Chlorine in Cl₂ is at 0, which sits between its common states of −1 and +1. It has room to go both ways. If it were already at its maximum or minimum oxidation state, one of the half-reactions would be impossible.
This reaction is also why chlorine water is used as a disinfectant. The NaOCl (hypochlorite) produced is the actual bleaching and disinfecting agent — it forms in situ through this very disproportionation.
Alternative Method — The Electron Transfer Count
Instead of just identifying oxidation states, we can verify the reaction is balanced by counting electrons transferred. This is what NCERT asks us to do in Chapter 8 exercises.
For every one Cl₂ molecule:
- One Cl atom loses 1 electron (0 → +1): 1 electron lost
- One Cl atom gains 1 electron (0 → −1): 1 electron gained
Electrons lost = electrons gained. ✓ The reaction is internally balanced with respect to charge. No external oxidising or reducing agent is needed — Cl₂ disproportionates on its own in alkaline medium.
A quick check for disproportionation: if you see the same element appear in two different products with different oxidation states, and it started at an intermediate state in the reactant — it is disproportionation. Cl₂ → NaCl (Cl = −1) + NaOCl (Cl = +1) is the textbook example. H₂O₂ → H₂O + O₂ is another one worth memorising (O goes from −1 to −2 and 0).
Common Mistake
Students often write the oxidation state of Cl in NaOCl as −1 by confusing it with NaCl. The key difference: in NaOCl, oxygen is also present and oxygen takes −2. So the chlorine cannot be −1 (that would make the total charge −2, not 0). Always account for oxygen first when oxygen and chlorine are both in the same formula. Cl in hypochlorite (OCl⁻) is always +1.
This mistake costs marks in NCERT-based CBSE board questions where you are asked to “justify that the reaction is a disproportionation reaction.” If you get the oxidation state of Cl in NaOCl wrong, the entire argument collapses — you’d show Cl going from 0 to −1 in both products, which looks like a simple reduction, not disproportionation.
Final answer: Cl goes from 0 in Cl₂ to −1 in NaCl (reduction) and +1 in NaOCl (oxidation). Since both changes involve the same element, this is a disproportionation reaction.