Uses of chlorine and chlorate(I)AQA A-Level Chemistry: Revision notes
Section 1
Chlorine and water
Chlorine reacts reversibly with water in a disproportionation reaction:
Cl₂(aq) + H₂O(l) ⇌ 2H⁺(aq) + Cl⁻(aq) + ClO⁻(aq)
The oxidation state of chlorine is 0 in Cl₂, −1 in Cl⁻ (reduced) and +1 in ClO⁻ (oxidised), so the same element is both oxidised and reduced. ClO⁻ is the chlorate(I) ion.
The H⁺ ions make the solution acidic. The chlorate(I) ion is the species that kills microorganisms and has bleaching action.
In sunlight chlorine reacts differently, forming chloride ions and oxygen: 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂. Here chlorine is reduced and oxygen in water is oxidised.
Chlorate(I) is ClO⁻ (oxidation state +1). Chlorate(V) is ClO₃⁻, which is not formed here.
Section 2
Chlorine in water treatment
Chlorine is added to drinking water and swimming pools to kill bacteria and other microorganisms. The active steriliser is chlorate(I) formed when chlorine reacts with water.
Benefits: it prevents the spread of waterborne diseases such as cholera and typhoid, and the effect lasts through the supply system.
Risks: chlorine is a toxic, irritant gas that is hazardous to store and transport. It can react with organic compounds in water to form chlorinated hydrocarbons, some of which are carcinogenic.
Only very low concentrations are used. Society judges that the benefits to health of water treatment by chlorine outweigh its toxic effects.
Evaluation answers need both an advantage and a disadvantage, then a conclusion about which outweighs the other.
Section 3
Chlorine with cold, dilute sodium hydroxide
Chlorine reacts with cold, dilute aqueous sodium hydroxide to form sodium chlorate(I), used as bleach:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
Ionic form: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O.
This is also a disproportionation: chlorine goes from 0 to −1 in NaCl and from 0 to +1 in NaClO.
The solution is used as a household bleach and as a disinfectant, for example for sterilising water and surfaces.
Must Know
- Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻ is a disproportionation (0 to −1 and +1)
- In sunlight: 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂
- Chlorate(I) kills microorganisms in water treatment
- Benefits (preventing disease) outweigh risks (toxicity, chlorinated hydrocarbons)
- Cl₂ + 2NaOH → NaCl + NaClO + H₂O gives bleach
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Uses of chlorine and chlorate(I)
- Chlorine gas is bubbled into water in a laboratory. The pale green solution formed is acidic and has bleaching properties.Write an equation for the reaction of chlorine with water that forms chloride ions and chlorate(I) ions, and explain why the reaction is a disproportionation.2 marks
- A local council treats its drinking water by adding chlorine. A resident complains that chlorine is a poisonous gas and asks why it is added to the water supply.Explain why the benefit to health of treating drinking water with chlorine is considered to outweigh its toxic effects.2 marks
- A swimming pool manager makes a bleach solution by reacting chlorine gas with cold, dilute aqueous sodium hydroxide. She also considers adding chlorine gas directly to the pool water.Write an equation for the reaction of chlorine with cold, dilute aqueous sodium hydroxide, name the chlorine-containing compound in the solution that acts as the bleach, and state one other use of the solution.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).