The reactions of chlorine

AS Level Chemistry · Group 17. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In disproportionation the same element is oxidised and reduced in one reaction. Chlorine does this with water and with aqueous sodium hydroxide. It starts at oxidation number 0. Some chlorine atoms are reduced to −1 in chloride, and the others are oxidised to a positive oxidation number.

The temperature decides how far the oxidation goes. With cold dilute alkali the oxidised product is chlorate(I), ClO−, with chlorine at +1. With hot concentrated alkali it is chlorate(V), ClO3−, with chlorine at +5.

Chlorine is added to drinking water because its reaction with water makes HOCl, chloric(I) acid. HOCl and the ClO− ion kill bacteria.

Rules to remember

Common mistake

Students often give the products of the cold reaction for the hot one. Cold dilute alkali gives NaClO (chlorine +1); hot concentrated alkali gives NaClO3 (chlorine +5).

Worked example

0.060 mol of chlorine reacts completely with excess hot concentrated aqueous sodium hydroxide. How many moles of sodium chlorate(V) and of sodium chloride are formed?

  1. Equation: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O.
  2. 3 mol of Cl2 give 1 mol of NaClO3: 0.060 ÷ 3 = 0.020 mol.
  3. 3 mol of Cl2 give 5 mol of NaCl: 0.060 × 5 ÷ 3 = 0.100 mol.
  4. Check chlorine atoms: 0.020 + 0.100 = 0.120 mol = 2 × 0.060 mol.

Answer: 0.020 mol of NaClO3 and 0.100 mol of NaCl.

Practice questions

Try each one, then open the answer.

1. Which equation shows the reaction of chlorine with water that produces the species used to kill bacteria in water treatment?

  1. A
    Cl2 + H2O ⇌ 2HOCl
  2. B
    Cl2 + H2O ⇌ HCl + HOCl
  3. C
    Cl2 + H2O ⇌ Cl2O + H2
  4. D
    Cl2 + 2H2O ⇌ 2HCl + H2O2
Show answer

Answer: B. Chlorine disproportionates in water to HCl and HOCl, chloric(I) acid. HOCl and the ClO− ion it forms are the active species; the equation giving 2HOCl is not balanced for hydrogen.

2. In treated water, HOCl partly ionises: HOCl ⇌ H+ + ClO−. What is the oxidation number of chlorine in HOCl and in ClO−?

  1. A
    +1 in HOCl and −1 in ClO−
  2. B
    −1 in both
  3. C
    +1 in both
  4. D
    +2 in HOCl and +1 in ClO−
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Answer: C. In HOCl: +1 (H) − 2 (O) + x = 0, so x = +1. In ClO−: x − 2 = −1, so x = +1. The ionisation is an acid–base change, not a redox change.

3. In the reaction 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O, what is the ratio of chlorine atoms reduced to chlorine atoms oxidised?

  1. A
    1 : 5
  2. B
    1 : 1
  3. C
    5 : 1
  4. D
    3 : 1
Show answer

Answer: C. Five Cl atoms go from 0 to −1 (reduced, gaining 5 electrons in total) and one goes from 0 to +5 (oxidised, losing 5 electrons), so the ratio is 5 : 1 and the electrons balance.

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