In sodium hydroxide, some chlorine atoms gain electrons while others lose them. That disproportionation makes household bleach, and a similar reaction with water is why chlorine keeps drinking water safe.
Disproportionation is a redox reaction in which the same element is simultaneously oxidised and reduced. Chlorine (oxidation number 0) disproportionates in sodium hydroxide: in cold, dilute NaOH it forms NaCl (−1) and NaClO (+1), the mixture used as bleach; in hot, concentrated NaOH it forms NaCl (−1) and NaClO3 (+5). With water, Cl2 + H2O ⇌ HCl + HClO, and the chloric(I) acid kills bacteria, which is why chlorine is used to treat drinking water.
Fig. 1: oxidation numbers of chlorine in its reactions with sodium hydroxide
Section 01
In alkali and in water.
Bubble chlorine into cold dilute and hot concentrated sodium hydroxide in the simulator, then into water, and track the oxidation number of chlorine in each product.
Conditions
Equation
Oxidation numbers of Cl
cold, dilute NaOH
Cl2 + 2NaOH → NaCl + NaClO + H2O
0 → −1 and +1
hot, concentrated NaOH
3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
0 → −1 and +5
water
Cl2 + H2O ⇌ HCl + HClO
0 → −1 and +1
Bleach: sodium chlorate(I), NaClO, is the active ingredient of household bleach.
Balancing the hot reaction: five Cl atoms gain one electron each (−5) while one loses five (+5), so the electrons balance.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
What is disproportionation?
Quick check+10 XP
What are the chlorine-containing products when chlorine reacts with cold, dilute sodium hydroxide?
Quick check+10 XP
What is the oxidation number of chlorine in NaClO3?
Quick check+10 XP
Why is chlorine added to drinking water?
Section 02
Treating water.
Adding a small amount of chlorine to drinking water saves millions of lives, but it is not without risk.
Benefit: HClO and ClO− kill bacteria and other pathogens, preventing diseases such as cholera and typhoid; residual chlorine keeps the water safe in the pipes.
Risks: chlorine is toxic in large amounts, and it can react with organic matter in water to form chlorinated organic compounds, some of which may be harmful.
Balance: the concentration is kept very low; the health benefits are judged to far outweigh the risks.
Examiner trap
In disproportionation the same element is both oxidised and reduced, so chlorine appears in two products with different oxidation numbers. Do not mix up the two NaOH conditions: cold dilute gives chlorate(I), NaClO; hot concentrated gives chlorate(V), NaClO3.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
In 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O, which statement is correct?
Exam style+20 XP
What is the oxidation number of chlorine in chloric(I) acid, HClO?
Exam style+20 XP
Which is a risk of chlorinating drinking water?
Exam style+20 XP
0.710 g of Cl2 (Mr 71.0) reacts with excess cold dilute NaOH. What mass of NaClO (Mr 74.5) forms?
Skill unlocked
Chlorine chemistry, mastered.
This skill is now recorded on your profile. Keep the chain going.