Transcript Unit 33 Bleach

Unit 33
Bleach
Types of Bleaches
 Household chlorine
bleach – a solution of
chlorine in sodium
hydroxide solution
 Cl2(aq) + 2NaOH(aq)  NaCl(aq)
+ NaOCl(aq) + H2O(l)
Chlorine bleach – active ingredient
Active ingredient: sodium
hypochlorite /
hypochlorite ion
Types of Bleaches
 Sulphur dioxide / Sulphite industrial bleach
 Wet / moist SO2 - a source of sulphite ions
 Moist SO2 - a bleaching agent
How to obtain chlorine from brine ?
 Brine – sodium chloride (NaCl) solution
 By electrolysis of concentrated brine using
inert electrodes (carbon / platinum)
 Chlorine gas is given out at one of the
electrode (Which ?)
 (negative / positive electrode)
 Sodium hydroxide is formed as the byproduct.
Electrolysis of brine
wooden piece
supported across
wide tube
rubber
bands
small test
tube
brine
graphite
electrodes
partition to
prevent short
circuit
e-
e-
- +
6V
Preparation of chlorine in laboratory
 By the action of dilute acid on bleaching
solution.
 Chemical Equation ?
 Cl-(aq) + OCl-(aq) + 2H+(aq)  Cl2(g) + H2O(l)
 Precaution ?
 Should be carried out in the fume cupboard
as chlorine gas is poisonous.
 Should be kept away from children and not to
be stored together with acid (such as vinegar).
Physical Properties of Chlorine Gas
 A greenish-yellow gas with an irritating smell.
 A poisonous and acidic gas
 Slightly soluble in water and quite soluble in
1,1,1-trichloroethane (an organic solvent / a
non-polar solvent).
 Cl2(g) + H2O(l)  HCl(aq) + HOCl(aq)
 Denser than air.
Chemical Properties of Chlorine
 Dissolves in water to form a yellowish –green
solution – chlorine solution / water.
 2 acids are formed – hydrochloric and
hypochlorous acid.
 Cl2(g) + H2O(l)  HCl(aq) + HOCl(aq)
 Hypochlorous acid – unstable
 Decomposes by diffused sunlight to form
hydrochloric acid and oxygen.
 2HOCl(aq)  2HCl(aq) + O2(g)
Bleaching action of chlorine
 Due to the oxidizing action of hypochlorite ion
 Hypochlorite in gives up oxygen to the
coloured dye.
 OCl-(aq) + [dye]  Cl-(aq) + [dye+O]
 Bleaching of chlorine – permanent
 Does not work well in acidic medium /
solution. Why ?
Oxidizing action of chlorine
 Chlorine oxidizes others by gaining electrons.
 Cl2 + 2e-  2Cl Reacts with sulphur dioxide / sulphite ions
 Reacts with other halide ions (halogen
displacement reaction)
Reaction with sodium sulphite /
sulphur dioxide
 Yellowish green chlorine water / greenish
yellow chlorine gas is decolorized / reduced
by sulphur dioxide.
 Reddish brown bromine vapour / orange
bromine water is decolorized.
 Cl2 + 2e-  2Cl SO2 + 2H2O  SO42- + 4H+ + 2e SO2 + Cl2 + 2H2O  2HCl + H2SO4
Halogen displacement reaction
 A stronger oxidizing agent can displace a
weaker oxidizing agent from its aqueous salt
solution.
 When yellowish green chlorine water is
added into a colourless bromide solution, the
solution turns orange / reddish brown.
 Cl2(aq) + 2Br-(aq)  2Cl-(aq) + Br2(aq)
 When shaken with 1,1,1-trichloroethane, an
orange bottom oily organic layer is formed.
Halogen displacement reaction
 When chlorine water is added into colorless
iodide solution, the solution turns dark brown.
 Cl2(aq) + 2KI(aq)  2KCl(aq) + I2(aq)
 When shaken with 1,1,1-trichloroethane, a
violet / purple oily bottom organic layer is
formed.
Test for chlorine gas
 Test with wet (moist) blue litmus paper.
 The wet blue litmus paper first turns red, and
then quickly turns white (i.e., bleached by the
hypochlorite ions).
Moist sulphur dioxide / sulphite bleach
 A mild bleaching agent.
 Drop a piece of wet coloured flower petal into
a gas jar of sulphur dioxide.
 What do you see?
 The flower petal turns white after half an hour.
Bleaching action of sulphur dioxide
 Oxidation number of sulphur in sulphur
dioxide ??
 Is it a reducing agent / an oxidizing agent?
 Bleaching in steps:
 Dissolving in water to form sulphurous acid.
 SO2(g) + H2O(l)  H2SO3(aq)
 As a source of sulphite ions
 H2SO3(aq)  2H+(aq) + SO32-(aq)
Bleaching action of sulphur dioxide
 Sulphite ions bleach by reduction (removing
oxygen from the coloured dye).
 SO32-(aq) + [dye – o]  SO42-(aq) + [dye]
 Not permanent
 Give one example ??
Reducing action of sulphur dioxide /
sulphite ion
 Half equation for the oxidation of sulphur
dioxide ?
 Half equation for the oxidation of sulphite ion ?
 SO2
 SO42??
 SO32 SO42??
 SO2 + 2H2O
 SO42- + 4H+ + 2e SO32- + H2O  SO42- + 2H+ + 2e-
Common oxidizing agents react with
sulphur dioxide
 ???
 Acidified potassium permanganate
 Acidified potassium dichromate
 Halogen (bromine / chlorine)
Reaction with acidified potassium
permangante
 What do you see?
 Purple acidified permanganate solution turns
colourless.
 Equation ??
 MnO4- + SO2  ???
 MnO4- + SO32-  ???
Reaction with acidified potassium
dichromate
 What do you see??
 Orange acidified dichromate solution turns
green.
 What is the reduction product of dichromate
ion ??
Equation ??
Reaction with bromine
 What do you see?
 Orange /reddish brown bromine water is
decolorized by sulphite ion.
 Equation ??
Test for sulphur dioxide
 Put a piece of filter paper moistened with
acidified potassium dichromate solution near
a gas jar of sulphur dioxide.
 What do you see?
 Sulphur dioxide turns orange acidified
potassium dichromate solution green.
Comparison of the bleaching action
of sulphite & chlorine bleaches
Sulphur dioxide
Chlorine Bleach
bleach by
reduction
oxidation
rate of bleaching
slower and milder
faster and more vigorous
Bleaching effect
Not permanent
Original colour of the bleached
substance will be restored by
the oxidation of air (eg.,
Paper turns yellow on
standing in air)
Permanent
Bleaching effect can last for
a longer time.
Used to bleach
More delicate materials, e.g.., silk, cotton, linen, and plant
wool and paper. Can be used
fibers
to whiten food such as flour
and cheese.
Sterilizing action of chlorine and
sulphur dioxide
 Chlorine – as a
germicide for killing
germs / bacteria. For
sterilizing drinking water
and swimming pool
water.
 Sulphur dioxide – used
as preservatives for
food such as jam and
fruit juice.
Problems associated with the use of
chlorine and sulphur dioxide
 Cause skin irritation – Avoid contact with skin
and eyes.
 When discharging into rivers and streams
and mixing with acids, toxic gases, chlorine
and sulphur dioxide, will be given out. –
causing water pollution.
 Chlorine water is photo-sensitive. Should be
stored away from the sun.