Uses of Chlorine & Chlorate(I) (Oxford AQA International A Level (IAL) Chemistry): Revision Note

Reactions of Chlorine

• A disproportionation reaction is a reaction in which the same species is both oxidised and reduced

• Examples of these reactions are:

  • The reaction of chlorine with water

  • The reaction of chlorine with cold, dilute alkali

• In both cases, the chlorine gets oxidised and reduced at the same time

Reaction with water

• When chlorine reacts with water, chloric(I) acid, and hydrochloric acid are formed

• This is a redox reaction because:

  • The oxidation state of one chlorine atom increases from 0 to +1

  • The oxidation state of the other decreases from 0 to -1

• Due to the same element being oxidised and reduced, it is a disproportionation reaction

Reaction with cold alkali (15 ^oC)

• When chlorine reacts with cold, dilute sodium hydroxide, sodium chlorate(I) is formed

Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H₂O (l)

• The ionic equation is:

Cl₂ (aq) + 2OH^– (aq) → Cl^– (aq) + ClO^– (aq) + H₂O (l)

• Chlorine gets oxidised

  • There is an increase in oxidation state from 0 in Cl₂ (aq) to +1 in ClO^– (aq)

  • The half-equation for the oxidation reaction is:

½Cl₂ (aq) + 2OH^– (aq) → ClO^– (aq) + H₂O (l) + e^–

• Chlorine gets reduced

  • There is a decrease in oxidation state from 0 in Cl₂ (aq) to –1 in Cl^– (aq)

  • The half-equation for the reduction reaction is:

½Cl₂ (aq) + e^– → Cl^– (aq)

• Again, this shows the reaction is disproportionation

Water Treatment

• We can clean water and make it safe to drink by adding either chlorine or chlorate(I)

Adding chlorine

• When chlorine reacts with water, chloric(I) acid, and hydrochloric acid are formed

• Chloric(I) acid, HClO, sterilises water by killing bacteria

• In shallow swimming pools, chlorine is rapidly lost from the water due to its exposure to sunlight

• In sunlight, the following reaction occurs:

2Cl₂ (g) + 2H₂O (l) → 4HCl (aq) + O₂ (g)

• The chlorine has to be topped up on a regular basis increasing costs

Adding sodium chlorate(I)

• An alternative to directly adding chlorine to water is to add chlorate ions, ClO^- in the form of sodium or calcium chlorate(I)

• The reaction is:

NaClO (aq) + H₂O (l) → Na⁺ (aq) + OH^- (aq) + HClO^- (aq)

• ClO^- (aq) acts as a sterilising agent cleaning the water

Issues with chlorine

• When deciding whether to add a chemical to water supplies, the advantages and disadvantages are assessed

• Chlorine is a toxic substance so the levels of chlorine in water and swimming pools have to be carefully monitored

• There needs to be enough chlorine present to kill bacteria and sterilise the water without causing any harm to us

• Generally, the benefits to our health of adding chlorine to water outweigh its toxic effects and possible risks from the formation of chlorinated hydrocarbons

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