Uses of Chlorine & Chlorate(I) (Oxford AQA International A Level Chemistry)

Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Reactions of Chlorine

  • A disproportionation reaction is a reaction in which the same species is both oxidised and reduced

  • Examples of these reactions are:

    • The reaction of chlorine with water

    • The reaction of chlorine with cold, dilute alkali

  • In both cases, the chlorine gets oxidised and reduced at the same time

Reaction with water

  • When chlorine reacts with water, chloric(I) acid, and hydrochloric acid are formed

Group 17 - Chlorine and Water Reaction
  • This is a redox reaction because:

    • The oxidation state of one chlorine atom increases from 0 to +1

    • The oxidation state of the other decreases from 0 to -1

  • Due to the same element being oxidised and reduced, it is a disproportionation reaction

Reaction with cold alkali (15 oC)

  • When chlorine reacts with cold, dilute sodium hydroxide, sodium chlorate(I) is formed

Cl2 (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H2O (l)

  • The ionic equation is:

Cl2 (aq) + 2OH (aq) → Cl (aq) + ClO (aq) + H2O (l)

  • Chlorine gets oxidised

    • There is an increase in oxidation state from 0 in Cl2 (aq) to +1 in ClO (aq)

    • The half-equation for the oxidation reaction is:

½Cl2 (aq) + 2OH (aq) → ClO (aq) + H2O (l) + e

  • Chlorine gets reduced

    • There is a decrease in oxidation state from 0 in Cl2 (aq) to –1 in Cl (aq)

    • The half-equation for the reduction reaction is:

½Cl2 (aq) + e → Cl (aq)

  • Again, this shows the reaction is disproportionation

Water Treatment

  • We can clean water and make it safe to drink by adding either chlorine or chlorate(I)

Adding chlorine

  • When chlorine reacts with water, chloric(I) acid, and hydrochloric acid are formed

  • Chloric(I) acid, HClO, sterilises water by killing bacteria

  • In shallow swimming pools, chlorine is rapidly lost from the water due to its exposure to sunlight

  • In sunlight, the following reaction occurs:

2Cl2 (g) + 2H2O (l) → 4HCl (aq) + O2 (g)

  • The chlorine has to be topped up on a regular basis increasing costs

Adding sodium chlorate(I)

  • An alternative to directly adding chlorine to water is to add chlorate ions, ClO- in the form of sodium or calcium chlorate(I)

  • The reaction is:

NaClO (aq) + H2O (l) → Na+ (aq) + OH- (aq) + HClO- (aq)

  • ClO- (aq) acts as a sterilising agent cleaning the water

Issues with chlorine

  • When deciding whether to add a chemical to water supplies, the advantages and disadvantages are assessed

  • Chlorine is a toxic substance so the levels of chlorine in water and swimming pools have to be carefully monitored

  • There needs to be enough chlorine present to kill bacteria and sterilise the water without causing any harm to us

  • Generally, the benefits to our health of adding chlorine to water outweigh its toxic effects and possible risks from the formation of chlorinated hydrocarbons

Chlorine-in-swimming-pools
Chlorine levels need to be carefully monitored in swimming pools

Photo by Jason Hawke on Unsplash 

Examiner Tips and Tricks

Make sure you can determine the oxidation state of the substances in each equation.

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.