A LevelChemistryAQARevision NotesInorganic ChemistryGroup 7 (17), the HalogensUses & Reactions of Chlorine

Uses & Reactions of Chlorine (AQA A Level Chemistry): Revision Note

Exam code: 7405

Stewart Hird

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on

Reactions of Halide Ions

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

  • The reaction of chlorine with dilute alkali is an example of a disproportionation reaction

  • In these reactions, the chlorine gets oxidised and reduced at the same time, forming sodium chloride, sodium chlorate(I), and water

  • Different reactions take place at different temperatures of the dilute alkali

Chlorine in cold alkali (15 oC)

  • The reaction that takes place is:

Chemical equation showing Cl₂(aq) reacting with 2NaOH(aq) to form NaCl(aq), NaClO(aq) and H₂O(l) on a white background.

  • The ionic equation is:

Chlorine disproportionation in alkali: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O, with oxidation states 0 to −1 (reduction) and 0 to +1 (oxidation).

  • The ionic equation shows that the chlorine gets both oxidised and reduced

  • Chlorine gets oxidised as there is an increase in oxidation. no. from 0 to +1 in ClO-(aq)

    • The half-equation for the oxidation reaction is:

Redox half-equation showing chlorine and hydroxide forming hypochlorite, water and an electron, with chlorine oxidation states 0 and +1 marked in red

  • Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)

    • The half-equation for the reduction reaction is:

Half equation showing reduction: one-half Cl₂ plus one electron forms chloride ion, with oxidation states changing from 0 to –1 shown in red.

Chlorine

  • Chlorine can be used to clean water and make it drinkable

  • The reaction of chlorine in water is a disproportionation reaction in which the chlorine gets both oxidised and reduced

  • This reaction is important in the purification of water for drinking and swimming pools

Equation showing Cl₂ + H₂O → HCl + HClO, with oxidation states 0 to −1 and +1, labelled to illustrate simultaneous oxidation and reduction of chlorine.
The disproportionation reaction of chlorine with water in which chlorine gets reduced to HCl and oxidised to HClO

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

  • Chloric(I) acid can further dissociate in water to form ClO- (aq):

HClO (aq) ⇌ H+ (aq) + ClO- (aq)

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

  • Chlorine is used in water treatment to kill bacteria

    • It has been used to treat drinking water and the water in swimming pools

  • The benefits to health of water treatment by chlorine, by its killing of bacteria, outweigh its risks of toxic effects and possible risks from the formation of chlorinated hydrocarbons

    • Chlorine is used in small, controlled amounts

  • In shallow swimming pools, chlorine is rapidly lost from the water, so the chlorine has to be topped up regularly, adding to the cost

    • This is because it is more exposed to sunlight

  • In sunlight, the following reaction occurs:

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

Related topics

Examiner Tips and Tricks

Adding chlorine to a water supply is an effective way to make the water safe to drink, as it forms the sterilising agent HClO, which in turn dissociates in water into another sterilising agent, ClO-(aq)

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

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.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.