What is a bleach?
What has caused this colour change?
Why is chlorine added to water in a swimming
pool?
21.1 What are household bleaches?
21.2 Production of bleach
21.3
21.4
21.5
21.6
Preparation and properties of chlorine
The bleaching action of chlorine
Sulphur dioxide - an alternative bleach
Comparing bleaches
21.7 Advantages and problems with the use of bleaches
21.8 Use of chlorine to sterilize water
21.9 Setting up a chlorine plant in the Hong Kong SAR
IT on the net
Summary
Misconception analysis
21.1
What are household
bleaches?
Bleach is used in two ways:
Class
Practice
1
to clean and bleach fabrics
2
to kill germs
Activity 21.1 B
Testing the bleach solution.
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
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
Activity 21.1 C
How can we prepare bleach solution?
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-.
Class
Practice
IT ON THE NET
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
Class
Practice
Activity 21.2 A
How can chlorine be prepared?
from acid
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:
Cl2(g) + H2O(l)
HCl(aq) + HOCl(aq)
‘chlorine water’
Cl2(aq)
Activity 21.2 B
Investigationg chlorine water.
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.
Class
Practice
Chlorine as an oxidizing agent
When chlorine molecules react, they gain electrons
and form stable chloride ions:
reduction
0
-1
Cl2 + 2e-  2ClTherefore, chlorine is an oxidizing agent.
Activity 21.2 C
Is chlorine an oxidizing or a reducing agent?
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)
very pale green colourless
reduction - gain of electrons
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)
Extraction of bromine
Seawater contains a small percentage of sodium
bromide NaBr.
Bromine is extracted from seawater using chlorine
gas.
Test for chlorine
Chlorine turns moist blue litmus paper red and then
bleaches it.
Class
Practice
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)
•
stains made by fruit juice, tea and wine
•
cotton, linen and other fabrics made from
plant fibres
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:
OCl-(aq) + dye(aq)  Cl-(aq) + (dye + O)
coloured
colourless
As oxygen is added to the dye, chlorine bleaches by
oxidation.
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.
How sulphur dioxide bleach works
Sulphur dioxide only bleaches in moist conditions.
The gas dissolves in the water to form sulphurous
acid:
SO2(g) + H2O(l)
H2SO3(aq)
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
Activity 21.3 A
How does sulphur dioxide affect coloured substances?
colourless
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.
Class
Practice
Sulphur dioxide as a reducing agent
Sulphur dioxide in the presence of water is a good
reducing agent.
oxidation
+4
+6
SO32- (aq)  SO42- (aq)
Activity 21.3 B
Investigating reactions of aqueous sulphur dioxide.
The half equation is written:
SO32- (aq) + H2O(l)  SO42- (aq) + 2H+(aq) +2eThe oxidation number of sulphur increases from +4
to +6.
Here are three examples.
1 Iron(III) ions
SO2(aq)
Fe3+(aq)
Fe2+(aq)
Reaction of aqueous sulphur dioxide with a
solution of iron(III) ions.
Fe3+(aq)
yellow/brown
+ e-
reduction

Fe2+(aq)
pale green
2 Acidified potassium permanganate solution
-
MnO4 (aq)
SO2(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
3 Bromine solution
SO2(aq)
Br2(aq)
Br-(aq)
Reaction of aqueous sulphur dioxide with
bromine water.
Br2(aq) + 2e-  2Br-(aq)
brown
colourless
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.
Class
Practice
21.6
Comparing bleaches
Sulphur dioxide bleach
Chlorine bleach
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
used with wool, silk, paper, some
foods
bleaching action can last for a
longer time
used with cotton, linen, some
food stains
A comparison of sulphur dioxide and chlorine bleaches.
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.
•
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.
Class
Practice
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
Class
Practice
IT ON THE NET
21.9
Setting up a chlorine plant in the
Hong Kong SAR
Class
Practice
IT on the net
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/
IT on the net
21.8 Use of chlorine to sterilize water
How does chlorine kill harmful micro-organisms
http://www.sciam.com/askexpert/environment/environment
22/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.html
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?
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.
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
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
4Q:
Chlorine water is a solution of chlorine
molecules in water.
4A:
False
5Q:
Bleaches work by oxidation.
5A:
False