Chlorine and chlorate reactions for A-Level Chemistry
Chlorine reacts with cold dilute sodium hydroxide to form sodium chloride, sodium chlorate(I), and water. This is a disproportionation reaction because chlorine is simultaneously oxidised and reduced: It goes from oxidation state 0 in Cl2 to -1 in NaCl and +1 in NaClO. The same family of reactions explains why chlorine is used in bleach, in swimming pools, and in drinking water treatment.
This guide covers the two key chlorine and chlorate equations, the oxidation states you need to assign, the uses in water treatment, and the safety arguments examiners want you to weigh. By the end you will be ready for any 4-mark or 6-mark question on chlorine chemistry from AQA Topic 3.2.3.
Two disproportionation reactions
Cold dilute NaOH gives ClO- (bleach). Hot concentrated NaOH gives ClO3-. Both are disproportionation. Both come up in AQA papers.
Oxidation states matter
Chlorine moves from 0 in Cl2 to -1 in chloride and +1 or +5 in the chlorate. Showing the change earns the disproportionation mark.
Risk vs benefit in water
Chlorine kills bacteria in drinking water but produces toxic byproducts. AQA wants both sides in the evaluation question.
Chlorine with cold dilute sodium hydroxide
When chlorine is bubbled into cold dilute sodium hydroxide, it disproportionates to form sodium chloride and sodium chlorate(I), with water as a byproduct. The equation is: Cl2 + 2NaOH → NaCl + NaClO + H2O.
Sodium chlorate(I), NaClO, is the active ingredient in household bleach. The chlorine atom in NaClO has an oxidation state of +1, while the chlorine in NaCl has an oxidation state of -1. Because one chlorine atom is oxidised and another is reduced in the same reaction, this is the textbook example of disproportionation.
Assigning the oxidation states In Cl2, each chlorine is 0. In NaCl, chlorine is -1 (sodium is +1, the compound is neutral). In NaClO, chlorine is +1 (sodium is +1, oxygen is -2, so chlorine must be +1 to make the compound neutral). Show this working in any disproportionation answer.
Chlorine with hot concentrated sodium hydroxide
When chlorine is bubbled into hot concentrated sodium hydroxide, it disproportionates further to form sodium chloride and sodium chlorate(V), again with water as a byproduct. The equation is: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O.
Sodium chlorate(V), NaClO3, is used as a weedkiller and oxidising agent. The chlorine in NaClO3 has an oxidation state of +5 (sodium +1, three oxygens at -2 each totalling -6, so chlorine must be +5). Five chlorine atoms are reduced from 0 to -1, and one chlorine atom is oxidised from 0 to +5.
| Reaction conditions | Equation | Chlorine oxidation states |
|---|---|---|
| Cl2 with cold dilute NaOH | Cl2 + 2NaOH → NaCl + NaClO + H2O | 0 in Cl2, -1 in NaCl, +1 in NaClO |
| Cl2 with hot concentrated NaOH | 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O | 0 in Cl2, -1 in NaCl, +5 in NaClO3 |
| Cl2 with water (for treatment) | Cl2 + H2O ⇌ HClO + HCl | 0 in Cl2, -1 in HCl, +1 in HClO |
Why these reactions are disproportionation
Disproportionation is a reaction in which the same element is both oxidised and reduced. In the chlorine reactions, chlorine starts in oxidation state 0 in Cl2 and ends up in both a lower state (-1, reduced) and a higher state (+1 or +5, oxidised) in the products.
AQA mark schemes for disproportionation questions almost always want three things: The word "disproportionation", the oxidation states clearly shown, and an explanation that the same element is oxidised and reduced. Hit all three and you will pick up the application mark.
How to spot disproportionation in any equation Find the element whose oxidation state changes. If the same element ends up in two different oxidation states in the products, one higher and one lower than the starting state, the reaction is disproportionation. Practise this on hydrogen peroxide decomposition and copper(I) reactions to build confidence.
Uses of chlorine in water treatment
Chlorine is added to drinking water and swimming pools because it kills bacteria, viruses, and other pathogens. When chlorine dissolves in water it sets up an equilibrium: Cl2 + H2O ⇌ HClO + HCl. The HClO formed is the active germ-killer.
In drinking water treatment, the chlorine concentration is kept low (typically less than 1 ppm) so that pathogens are killed without leaving a noticeable taste. In swimming pools, higher concentrations are used because the water is continuously contaminated by swimmers.
The risk versus benefit argument
AQA loves the evaluation question: Should chlorine be added to drinking water? The benefit is clear: Chlorination has saved millions of lives by reducing cholera, typhoid, and other waterborne diseases. The risk is that chlorine can react with organic matter in water to form trihalomethanes (THMs), which are suspected carcinogens.
A strong evaluation answer weighs both sides and concludes that the public health benefit far outweighs the very small risk from byproducts, while noting that water companies monitor THM levels and use additional treatment steps to minimise them.
Exam-style evaluation structure In 4- to 6-mark evaluation questions, set out one benefit, set out one risk, then state and justify a conclusion. "Chlorination prevents waterborne disease. It can also form trihalomethanes which are potentially harmful. On balance the public health benefits outweigh the risks at the concentrations used." That earns full marks if the science is accurate.
Chlorine in bleach and the silver nitrate test
Household bleach is mainly an aqueous solution of sodium chlorate(I), NaClO. It works because ClO- is a strong oxidising agent: It oxidises coloured organic molecules (the stains) into colourless products and oxidises the cell components of bacteria, killing them.
To test for chloride ions in solution, add dilute nitric acid followed by silver nitrate solution. A white precipitate of silver chloride (AgCl) confirms chloride. Silver bromide is cream and silver iodide is yellow, so the test distinguishes all three halides. The precipitate dissolves in dilute ammonia for chloride, concentrated ammonia for bromide, and not at all for iodide.
| Halide ion | Colour of silver halide precipitate | Solubility in ammonia |
|---|---|---|
| Chloride (Cl-) | White | Dissolves in dilute NH3 |
| Bromide (Br-) | Cream | Dissolves only in concentrated NH3 |
| Iodide (I-) | Yellow | Insoluble in NH3 |
Worked example: Identifying disproportionation
Question: "Explain why the reaction of chlorine with cold dilute sodium hydroxide is a disproportionation reaction." (3 marks)
Step 1: Write the equation. Cl2 + 2NaOH → NaCl + NaClO + H2O.
Step 2: Assign oxidation states. Cl in Cl2 is 0. Cl in NaCl is -1. Cl in NaClO is +1.
Step 3: State the rule. One chlorine has been reduced from 0 to -1, another has been oxidised from 0 to +1. The same element is both oxidised and reduced, which is the definition of disproportionation. Three clear sentences, three marks.
Common mistakes that lose marks Writing "redox" instead of "disproportionation". Getting the oxidation state of chlorine in NaClO wrong (it is +1, not -1). Forgetting to balance the hot concentrated NaOH equation (it needs 3Cl2, 6NaOH, 5NaCl). Calling NaClO "sodium chlorate" without specifying the oxidation state in brackets when the question asks for full IUPAC naming.
Key facts to memorise for the exam
- Cl2 + 2NaOH → NaCl + NaClO + H2O is the cold dilute NaOH reaction (makes bleach)
- 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O is the hot concentrated NaOH reaction (makes weedkiller)
- Cl2 + H2O ⇌ HClO + HCl is the reaction in water treatment
- Chlorine in Cl2 is 0, in NaCl is -1, in NaClO is +1, in NaClO3 is +5
- Disproportionation: The same element is simultaneously oxidised and reduced
- Chlorinated water kills bacteria but can form trihalomethanes; benefit outweighs risk at normal concentrations
- Silver nitrate test: White AgCl, cream AgBr, yellow AgI
- AQA exam reference: This topic sits in 3.2.3 of the A-Level Chemistry specification
