Download Reactions of chlorine with water and sodium hydroxide

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 + OHOxidation 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 + eAs 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 + OHOxidation 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 + eAs 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-
2) ½Cl2 + 3OH- → ClO3- + 3H+ + 5eOxidation 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.
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- + 2ClAgain chlorine is being both oxidised and reduced, so
it is disproportionated.