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2.3.4Physical Chemistry

Uses and Reactions of Chlorine — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 63 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Chlorine and Water

Chlorine dissolves in water and reacts reversibly: Cl₂(g) + H₂O(l) ⇌ HClO(aq) + HCl(aq).

Products
  • Products Chloric(I) acid (HClO) and hydrochloric acid form, so the solution is acidic and pale green at first.
Disproportionation
  • Disproportionation Chlorine is both oxidised (0 to +1 in HClO) and reduced (0 to −1 in HCl) in the same reaction.
Disinfection
  • Disinfection Chlorate(I) ions, ClO⁻, are the active species that kill bacteria in drinking water and swimming pools.
Bleaching
  • Bleaching Damp litmus paper is first turned red by the acid and then bleached white by chlorate(I).

Part 2 of 4Chlorine and Alkali

The reaction with cold, dilute aqueous sodium hydroxide produces household bleach.

1
The equation is Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l).
2
Chlorine changes oxidation state from 0 to −1 in sodium chloride, so it is reduced.
3
Chlorine also changes from 0 to +1 in sodium chlorate(I), so it is oxidised in the same reaction.
4
A reaction in which one element is simultaneously oxidised and reduced is called disproportionation.
5
The mixture of NaCl and NaClO is sold as bleach and is used to disinfect surfaces and bleach fabrics.

Part 3 of 4Benefits and Risks of Water Treatment

Benefits and Risks of Water Treatment
Overall judgement
  • Overall judgement The reduction in waterborne disease is usually judged to outweigh the small risk from by-products.
Free chlorine
  • Free chlorine Residual chlorine remains in the water and continues protecting it as it travels through pipes.
Alternatives
  • Alternatives Other disinfectants such as ozone or ultraviolet light exist, but chlorine is cheap and long-lasting.
BenefitRisk
Kills bacteria, preventing diseases such as cholera and typhoidChlorine is toxic and corrosive, so it is hazardous to store and handle
Reduces algae and unpleasant tasteChlorine can react with organic matter to form chlorinated hydrocarbons, some linked to cancer

Part 4 of 4Exam Checklist & Tips

2.3.4 – Key Points for the Exam

  • Disproportionation A reaction in which the same element is both oxidised and reduced.
  • Chlorate(I) ion ClO⁻, an ion containing chlorine in the +1 oxidation state, responsible for disinfection.
  • Bleach A solution containing sodium chlorate(I) and sodium chloride made from chlorine and cold dilute NaOH.
  • Water treatment Adding chlorine to kill microorganisms in drinking water.

Common Mistakes

  • Saying chlorine is only reduced when it reacts with water or alkali.
  • Writing the formula of bleach as NaClO₃, which is chlorate(V), not chlorate(I).
  • Forgetting to state the oxidation state changes when asked to show disproportionation.
  • Describing chlorine in water treatment as risk-free, ignoring by-products and its toxicity.

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