Bleach

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Unit 33

• Bleach

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Types of Bleaches

• Household chlorine bleach a solution of
  chlorine in sodium hydroxide solution
• Cl2(aq) 2NaOH(aq) ? NaCl(aq) NaOCl(aq)
  H2O(l)

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Chlorine bleach active ingredient
Active ingredient sodium hypochlorite /
hypochlorite ion
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Types of Bleaches

• Sulphur dioxide / Sulphite industrial bleach
• Wet / moist SO2 - a source of sulphite ions
• Moist SO2 - a bleaching agent

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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 by-product.

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Electrolysis of brine
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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).

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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.

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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)

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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) dyeO
• Bleaching of chlorine permanent
• Does not work well in acidic medium / solution.
  Why ?

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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)

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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

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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.

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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.

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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).

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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.

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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)

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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 ??

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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-

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Common oxidizing agents react with sulphur dioxide

• ???
• Acidified potassium permanganate
• Acidified potassium dichromate
• Halogen (bromine / chlorine)

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Reaction with acidified potassium permangante

• What do you see?
• Purple acidified permanganate solution turns
  colourless.
• Equation ??
• MnO4- SO2 ? ???
• MnO4- SO32- ? ???

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Reaction with acidified potassium dichromate

• What do you see??
• Orange acidified dichromate solution turns green.
• What is the reduction product of dichromate ion
  ??Equation ??

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Reaction with bromine

• What do you see?
• Orange /reddish brown bromine water is
  decolorized by sulphite ion.
• Equation ??

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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.

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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, wool and paper. Can be used to whiten food such as flour and cheese. cotton, linen, and plant fibers
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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.

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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.