Reactions of chlorine with water and sodium hydroxide.

Reaction with water

Chlorine reacts slowly with cold water to form a mixture of

hydrochloric and chloric (i) acids.

Cl2 + H2O ↓

HCl + HOCl

Two reactions are going on;

1) ½Cl2 + H2O → HCl + OH-

Oxidation No of chlorine in chlorine molecules is O.

Oxidation No of chlorine in the chloride ion is -1.

As the oxidation No has become less positive chlorine has been reduced.

½Cl2 + e- → Cl-

2) ½Cl2 + H2O → HOCl + H+

Oxidation No of chlorine in

chlorine molecules is O.

Oxidation No of chlorine in the chlorate ion is +1.

As the oxidation number has become more positive chlorine has been oxidised.

½Cl2 + OH- → HOCl + e-

As chlorine has been both oxidised and reduced it is said to have disproportionated.

Chloric (i) acid decomposes on standing;

2HOCl → 2HCl + O2

Oxidation No of chlorine in chlorate ions is +1.

Oxidation No of chlorine in the chloride ion is -1.

As the oxidation No has become less positive chlorine has been reduced.

Oxidation No of oxygen in chlorate ion is -2.

Oxidation No of oxygen in oxygen

molecules is 0.

As the oxidation No becomes more positive oxygen has been oxidised.

Water purification

• Chloric (i) acid is the reason why chlorine is used in water purification.

• When it breaks down reactive oxygen atoms are produced that kill bacteria.

• HOCl → HCl + [O]

• The potential to kill bacteria must last right up the point of delivery to the consumer.

• So ammonia is also added, producing chloroamines;

• Eg; NH3 + HOCl → NH2Cl + H2O• Chloroamines then slowly release

chlorate (i) as they are hydrolysed.

• NH2Cl + H2O → NH3 + HOCl

Pros and cons of water purification

• Chlorine also reacts with organic matter to produce chlorinated organic compounds which increase the risk of cancer.

• But this increased risk is insignificant when placed against that of consuming contaminated water.

Reaction with sodium hydroxide.

Chlorine reacts with cold, dilute NaOH to form a mixture of hydrochloric and chloric (i) acids.

Cl2 + 2OH- → Cl- + OCl- + H2O

Two reactions are involved;

Two reactions are going on;

1) ½Cl2 + H2O → HCl + OH-

Oxidation No of chlorine in chlorine molecules is O.

Oxidation No of chlorine in the chloride ion is -1.

As the oxidation No has become less positive chlorine has been reduced.

½Cl2 + e- → Cl-

2) ½Cl2 + H2O → HOCl + H+

Oxidation No of chlorine in

chlorine molecules is O.

Oxidation No of chlorine in the chlorate ion is +1.

As the oxidation number has become more positive chlorine has been oxidised.

½Cl2 + OH- → HOCl + e-

As chlorine has been both oxidised and reduced it is said to have disproportionated.

Bleach

The reaction with sodium hydroxide is used commercially to manufacture household bleach.

• This is an equimolar solution of sodium chloride and sodium chlorate (i).

Action of bleach

• Chloric (i) acid is the reason why chlorine is used in bleach.

• When it breaks down reactive oxygen atoms are produced that react with, and bleach, coloured compounds.

• HOCl → HCl + [O]

Reaction with hot, concentrated NaOH.

With hot, concentrated sodium hydroxide chlorate (v) ions are produced instead of chlorate (i).

3Cl2 + 3OH- → ClO3- + 5Cl- + 3H+

Again two reactions are going on at the same time;

1) Chlorine is reduced;

½Cl2 + e- → Cl-

Oxidation No of chlorine in

chlorine molecules is O.

Oxidation No of chlorine in the chlorate ion is +5.

As the oxidation number has become more positive chlorine has been oxidised.

2) ½Cl2 + 3OH- → ClO3- + 3H+ + 5e-

As chlorine has been both oxidised and reduced in this reaction it is said to have disproportionated.

Chlorate (v) ions are also produced slowly when chlorate (i) ions are allowed to stand, or more rapidly when they are heated.

3OCl- → ClO3- + 2Cl-

Again chlorine is being both oxidised and reduced, so it is disproportionated.