Reactions of chlorine
Reactions of chlorine
- Chlorine reacts with sodium hydroxide in two ways.
- In both, chlorine is disproportionated.
- A little chlorine also makes water safe to drink.
With sodium hydroxide
- Cold, dilute NaOH → chloride + chlorate(I):
$$\text{Cl}_2 + 2\text{NaOH} \rightarrow \text{NaCl} + \text{NaClO} + \text{H}_2\text{O}$$
- Hot, concentrated NaOH → chloride + chlorate(V):
$$3\text{Cl}_2 + 6\text{NaOH} \rightarrow 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}$$
- In both, chlorine's oxidation number goes both up and down from 0 — disproportionation.
Practice
Chlorine with cold, dilute sodium hydroxide forms:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O (chlorate(I)); hot concentrated NaOH gives chlorate(V) instead.
Practice
These reactions of chlorine with NaOH are disproportionation because chlorine:
Some chlorine goes to −1 (chloride) and some to +1 or +5 (chlorate) — the same element both up and down.
Practice
In the reaction of chlorine with sodium hydroxide, chlorine is both oxidised and reduced.
That is what makes it a disproportionation reaction.
Water purification
- A little chlorine is added to water:
$$\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{HCl}$$
- The active species HOCl and ClO⁻ kill bacteria, making the water safe to drink.
Practice
Chlorine makes water safe to drink because:
Cl₂ + H₂O ⇌ HOCl + HCl; the HOCl and ClO⁻ kill bacteria.
You've got it
Key idea
- chlorine + cold dilute NaOH → NaCl + NaClO (chlorate I); + hot conc NaOH → NaCl + NaClO₃ (chlorate V)
- both are disproportionation (Cl goes up and down from 0)
- chlorine purifies water: $\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{HCl}$; HOCl kills bacteria