Unit 33 Bleach

Types of Bleaches Household chlorine bleach – a solution of chlorine in sodium hydroxide solution Cl 2(aq) + 2NaOH (aq)  NaCl (aq) + NaOCl (aq) + H 2 O (l)

Chlorine bleach – active ingredient Active ingredient: sodium hypochlorite / hypochlorite ion

Types of Bleaches Sulphur dioxide / Sulphite industrial bleach Wet / moist SO 2 - a source of sulphite ions Moist SO 2 - 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 by- product.

Electrolysis of brine wooden piece supported across wide tube brine partition to prevent short circuit rubber bands graphite electrodes 6 V small test tube +  ee ee

Preparation of chlorine in laboratory By the action of dilute acid on bleaching solution. Chemical Equation ? Cl - (aq) + OCl - (aq) + 2H + (aq)  Cl 2(g) + H 2 O (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). Cl 2(g) + H 2 O (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. Cl 2(g) + H 2 O (l)  HCl (aq) + HOCl (aq) Hypochlorous acid – unstable Decomposes by diffused sunlight to form hydrochloric acid and oxygen. 2HOCl (aq)  2HCl (aq) + O 2(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. Cl 2 + 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. Cl 2 + 2e -  2Cl - SO 2 + 2H 2 O  SO H + + 2e - SO 2 + Cl 2 + 2H 2 O  2HCl + H 2 SO 4

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. Cl 2(aq) + 2Br - (aq)  2Cl - (aq) + Br 2(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. Cl 2(aq) + 2KI (aq)  2KCl (aq) + I 2(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. SO 2(g) + H 2 O (l)  H 2 SO 3(aq) As a source of sulphite ions H 2 SO 3(aq)  2H + (aq) + SO 3 2- (aq)

Bleaching action of sulphur dioxide Sulphite ions bleach by reduction (removing oxygen from the coloured dye). SO 3 2- (aq) + [dye – o]  SO 4 2- (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 ? SO 2  SO 4 2- ?? SO 3 2-  SO 4 2- ?? SO 2 + 2H 2 O  SO H + + 2e - SO H 2 O  SO H + + 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 ?? MnO SO 2  ??? MnO SO 3 2-  ???

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 dioxideChlorine Bleach bleach by reductionoxidation rate of bleaching slower and milderfaster 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

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.