Interpret chlorine with cold dilute sodium hydroxide
Write the cold alkali reaction and identify chlorine disproportionation from oxidation numbers.
Section 11 · Group 17
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What oxyanion is formed with cold dilute NaOH?
What you will learn
- Balance the cold dilute chlorine–alkali equation
- Assign chlorine oxidation numbers
- Recognise simultaneous oxidation and reduction
Cold dilute alkali forms chloride and hypochlorite
Chlorine reacts with cold, dilute aqueous sodium hydroxide: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. The corresponding ionic equation is Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O.
In Cl₂ chlorine is 0. It becomes −1 in Cl⁻ and +1 in ClO⁻. The same element is reduced and oxidised, so this is disproportionation.
Keep the condition with the product set
Cold, dilute NaOH gives hypochlorite, ClO⁻ (or NaClO). Hot, concentrated alkali gives chlorate instead; these conditions and products must not be swapped.
Hypochlorite is an oxidising bleach species. Do not turn the syllabus equation into household chemical-mixing advice; mixing cleaners can release hazardous gases.
Pause and explain
Why is chlorine with cold NaOH a disproportionation reaction?
Worked example
Balance chlorine with cold dilute sodium hydroxide and show both oxidation-number changes.
Show the worked solution
- The products are chloride and hypochlorite.
- Use two NaOH to balance sodium and hydroxide atoms.
- Chlorine changes from 0 to −1 and +1.
Answer Cl₂ + 2NaOH → NaCl + NaClO + H₂O; chlorine is reduced 0 → −1 and oxidised 0 → +1.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the balanced equation for Cl₂ reacting with cold dilute NaOH.
Give me a hint
Use chloride, sodium hypochlorite and water as products.
Compare my reasoning
- Cl₂ supplies two chlorine atoms.
- Make one NaCl and one NaClO.
- Two NaOH balance sodium, hydrogen and oxygen with one H₂O.
Cl₂ + 2NaOH → NaCl + NaClO + H₂O.
Look for these in your work
- Condition attached
- Products correct
- Atoms balanced
Assign oxidation numbers to chlorine in Cl₂, NaCl and NaClO.
Give me a hint
Use elemental form, chloride charge, and the usual oxygen/sodium values.
Compare my reasoning
- Cl₂ is elemental, so Cl is 0.
- In NaCl, Na is +1, so Cl is −1.
- In NaClO, Na is +1 and O is −2, so Cl is +1.
Cl is 0 in Cl₂, −1 in NaCl and +1 in NaClO.
Look for these in your work
- Element oxidation number correct
- Chloride value correct
- Hypochlorite value charge-balanced
Explain why both chloride and hypochlorite form from Cl₂ in cold alkali.
Give me a hint
Track two chlorine atoms in oxidation states 0, −1 and +1.
Compare my reasoning
- One chlorine atom gains electrons to become Cl⁻.
- Another chlorine atom loses electrons to become ClO⁻.
- Simultaneous oxidation and reduction is disproportionation.
Chlorine is reduced to Cl⁻ and oxidised to ClO⁻, so the reaction is disproportionation.
Look for these in your work
- Both products' redox directions identified
- Electron transfer explained
- Reaction class named
Common mistakes
- Using chlorate as the cold product
- Calling the reaction only oxidation or only reduction
- Forgetting water
- Omitting cold and dilute conditions
What can you explain now?
What oxyanion is formed with cold dilute NaOH?
Compare with the explanation
Hypochlorite, ClO⁻
Cold dilute conditions produce hypochlorite.
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Make it stick
What products and chlorine oxidation numbers identify the cold, dilute chlorine–alkali disproportionation?
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