11.4 The Reactions of Chlorine

Cambridge International AS Level Chemistry 9701 | Group 17

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and interpret, in terms of changes in oxidation number, the reaction of chlorine with cold aqueous sodium hydroxide and with hot aqueous sodium hydroxide, and recognise both reactions as disproportionation reactions.
  2. Explain, using equations, how chlorine is used in water purification, including how the active species HOCl and ClO⁻ are produced and how they kill bacteria.

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Section 1 — Background: What Is an Oxidation Number?

Before diving into the reactions, you need to understand two key ideas.

Oxidation number (also called oxidation state) is a number we assign to an atom in a molecule or ion to keep track of how electrons are being shared or transferred. Think of it like a "charge score" for each atom.

The rules you need here are:

  • An element in its pure form (not bonded to anything different) always has an oxidation number of 0. So in Cl₂, each chlorine atom has an oxidation number of 0.
  • In a simple negative ion, the oxidation number equals the charge. So in Cl⁻, the oxidation number is −1.
  • In a compound or complex ion, you work out the oxidation number from the overall charge and the other atoms present.

Oxidation means the oxidation number increases (goes up). The atom loses electron control. Reduction means the oxidation number decreases (goes down). The atom gains electron control.

A simple memory trick: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Section 2 — What Is a Disproportionation Reaction?

Disproportionation is a special type of reaction where the same element is simultaneously oxidised AND reduced in the same reaction.

In other words, one part of the element goes "up" in oxidation number (gets oxidised), and another part of the same element goes "down" in oxidation number (gets reduced) — all in the same reaction.

Chlorine (Cl₂) is particularly good at disproportionation because it starts at oxidation number 0 and can go either up or down from there.

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