What is a bleach?

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21
Bleach
What is a bleach?
What has caused this colour change?
Why is chlorine added to water in a swimming pool?
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Bleach
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(No Transcript)
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21.1 What are household bleaches?
Bleach is used in two ways
1 to clean and bleach fabrics 2 to kill germs
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21.2 Production of bleach
The bleach we use at home smells like chlorine
gas.
Thus, we call it a chlorine bleach. It is made
by dissolving chlorine gas in a solution of
sodium hydroxide.
Cl2(g) 2NaOH(aq) ? NaOCl(aq) NaCl(aq)
H2O(l)
bleaching solution
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The laboratory preparation of bleach
Bleach is prepared during the electrolysis of
brine (a concentrated solution of sodium
chloride).
During electrolysis, three products are formed
1 chlorine gas 2 hydrogen gas 3 sodium
hydroxide solution
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The chlorine then reacts with the sodium
hydroxide solution to form a dilute solution of
bleach.
The ionic equation for the reaction is Cl2(g)
2OH-(aq) ? Cl-(aq) OCl-(aq) H2O(l) Thus, the
bleaching solution contains the hypochlorite ion
OCl-.
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21.3 Preparation and properties of chlorine
Obtaining chlorine from bleach
Chlorine is conveniently obtained from a chlorine
bleach by the action of a dilute acid.
For example Cl-(aq) OCl-(aq) 2H(aq) ?
Cl2(g) H2O(l)
from bleaching solution
from acid
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Properties of chlorine

• is a greenish-yellow gas

• is denser than most gases
• has a choking smell
• is poisonous
• is slightly soluble in water
• turns moist blue litmus paper red then bleaches it

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Chlorine water
When chlorine dissolves in water, it reacts to
form hydrochloric acid HCl and hypochlorous acid
HOCl
chlorine water Cl2(aq)
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Chlorine water is a convenient source of chlorine.
Hypochlorous acid is unstable. It slowly
decomposes into oxygen and hydrochloric
acid 2HOCl(aq) ? 2HCl(aq) O2(g) This
decomposition is faster in sunlight.
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Chlorine as an oxidizing agent
When chlorine molecules react, they gain
electrons and form stable chloride ions
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Cl2 2e- ? 2Cl-
Therefore, chlorine is an oxidizing agent.
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For example, when chlorine water is added to a
solution of potassium bromide, the colourless
solution changes to pale brown.
Cl2(aq) 2e- ? 2Cl-(aq) reduction - gain of
electrons
very pale green
colourless
2Br-(aq) ? Br2(aq) 2e- oxidation - loss of
electrons
colourless
pale brown
The overall ionic change is Cl2(aq) 2Br-(aq) ?
2Cl-(aq) Br2(aq)
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Extraction of bromine
Seawater contains a small percentage of sodium
bromide NaBr.
Bromine is extracted from seawater using chlorine
gas.
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Test for chlorine
Chlorine turns moist blue litmus paper red and
then bleaches it.
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21.4 The bleaching action of chlorine
Both chlorine water and household bleach are
bleaching agents.
They are used to bleach dyes from plants and
fabrics made from plant fibres. Therefore they
bleach

• flowers and litmus (a dye obtained from plants)

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• stains made by fruit juice, tea and wine

• cotton, linen and other fabrics made from plant
  fibres

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How chlorine bleach works
Chlorine water contains hypochlorous acid HOCl.
Household bleach contains sodium hypochlorite
NaOCl. Both solutions contain the hypochlorite
ion OCl-. The hypochlorite ion gives up its
oxygen to the coloured dye, thus making it
colourless as follows
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OCl-(aq) dye(aq) ? Cl-(aq) (dye O)
coloured
colourless
As oxygen is added to the dye, chlorine bleaches
by oxidation.
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21.5 Sulphur dioxide - an alternative bleach
An alternative to chlorine bleach is sulphur
dioxide bleach. This bleach is mainly for
industrial use.

• Bleaching delicate materials such as wool, silk
  and paper that are damaged by the more powerful
  chlorine bleaches.
• Paper making
• To give foods such as flour and some cheeses a
  white appearance.

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How sulphur dioxide bleach works
Sulphur dioxide only bleaches in moist conditions.
The gas dissolves in the water to form sulphurous
acid
sulphurous acid
The acid contains the sulphite ion
SO32-(aq). This ion removes oxygen from the
coloured dye, leaving it colourless SO32-(aq)
dye ? SO42-(aq) (dye - O)
coloured
colourless
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As oxygen is removed from the dye, sulphur
dioxide bleaches by reduction.
The bleaching action of sulphur dioxide is not
permanent. On exposure to air and light, the
original colour slowly returns. This is because
the air supplies the oxygen removed during
bleaching.
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Sulphur dioxide as a reducing agent
Sulphur dioxide in the presence of water is a
good reducing agent.
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SO32- (aq) ? SO42- (aq)
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The half equation is written
SO32- (aq) H2O(l) ? SO42- (aq) 2H(aq) 2e-
The oxidation number of sulphur increases from 4
to 6.
Here are three examples.
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1 Iron(III) ions
Fe3(aq)
Fe2(aq)
Reaction of aqueous sulphur dioxide with a
solution of iron(III) ions.
pale green
yellow/brown
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2 Acidified potassium permanganate solution
MnO4-(aq)
Mn2(aq)
Reaction of aqueous sulphur dioxide with a
solution of acidified potassium permanganate.
MnO4-(aq) 8H(aq) 5e- ?? Mn2(aq) 4H2O(l)
colourless
purple
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3 Bromine solution
Br-(aq)
Br2(aq)
Reaction of aqueous sulphur dioxide with bromine
water.
Br2(aq) 2e- ?? 2Br-(aq)
brown
colourless
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Tests for sulphur dioxide

• Sulphur dioxide has a characteristic choking
  smell.

• Filter paper moistened with acidified potassium
  dichromate solution changes from orange to green
  in the presence of sulphur dioxide.

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21.6 Comparing bleaches
involves the sulphite ion SO32-(aq)
involves the hypochlorite ion OCl-(aq)
bleaches by reduction
bleaches by oxidation
bleaching action is slow and mild
bleaching action is faster and more vigorous
bleaching action is not permanent and the
original colour will slowly be restored
bleaching action can last for a longer time
used with wool, silk, paper, some foods
used with cotton, linen, some food stains
A comparison of sulphur dioxide and chlorine
bleaches.
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21.7 Advantages and problems with the use of
bleaches

• Advantages

• The use of chlorine in drinking water has
  prevented the spread of certain diseases.
• Bleaches have improved hygiene in the home.
• Bleaches have helped to improve the quality of
  food and clothes.

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• Some problems

• Chlorine and sulphur dioxide are toxic
  substances.
• Household bleaches can harm the eyes and skin.
• Sulphur dioxide in food can cause severe
  breathing difficulties in some people.

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21.8 Use of chlorine to sterilize water
Chlorine in water can kill bacteria.
In the Hong Kong SAR, chlorine gas is added to
our drinking water at water treatment plants. It
is also added to the water in swimming pools
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21.9 Setting up a chlorine plant in the Hong Kong
SAR
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21.2 Production of bleach
More on sodium hypochlorite and its uses.
http//www.chlor-chemicals.com
21.3 Preparation and properties of chlorine
Uses of chlorine
http//c3.org/newsroom/brochures/enhancing.html
http//www.chlor-chemicals.com/
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21.8 Use of chlorine to sterilize water
How does chlorine kill harmful micro-organisms
http//www.sciam.com/askexpert/environment/environ
ment22/environment22.html
The use of sodium hypochlorite for the treatment
of swimming pool water.
http//www.chlor-chemicals.com/
Basics of water purification - main steps
http//c3.org/aol/newsroom/factsheets/watertreat.h
tml
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Summary
1 Make a written summary of this chapter by
answering these questions
What is household bleach used for? How is
household bleach manufactured? How is chlorine
prepared in the laboratory? What are some
properties of chlorine?
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How do the bleaching action of sulphur dioxide
bleach and chlorine bleach differ?
What are some properties of sulphur
dioxide? What are some industrial uses of
bleaches? 2 Using the answers to your questions,
prepare a table to compare chlorine and sulphur
dioxide bleaches.
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Misconception Analysis
Think carefully about the following statements
and then mark them TRUE or FALSE. Check your
answers at the back of the book to see whether or
not you have any misconceptions.
1Q Domestic bleach is usually prepared from
chlorine and sodium hydroxide obtained from
electrolysis of brine.
1A True
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2Q The active ingredient in household bleach is
hypochlorous acid HOCl.
2A False
3Q A piece of red litmus paper will turn blue
then white when dipped into a solution of sodium
hypochlorite.
3A False
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4Q Chlorine water is a solution of chlorine
molecules in water.
4A False
5Q Bleaches work by oxidation.
5A False