CHAPTER 01: OXYGEN

Occurrence of oxygen
Oxygen is a gas that forms about 21% by volume of the air.
Since oxygen is reactive, it is found in form of compounds like sulphates, carbonates, hydroxides,
nitrates and oxides.
Oxygen is life giving and it is essential element for chemical processes like combustion, rusting
and respiration.
Oxygen is called “fire air” because it is needed for fire to burn.

Stages of preparing gases.

There are mainly four stages which are involved in preparing gases, these are;
a. Production stage – involves adding the reactants in the reacting vessel to produce a gas.
b. Removal of impurities- involves removing of all impurities that come along with the prepared
gas
c. Drying stage- involves the removal of any moisture that has come along with the gas by
passing the moist gas through drying agents such as concentrated sulphuric acid and
anhydrous calcium chloride.
d. Collection stage – there are different methods of collecting gases depending on their
properties.
The method used to collect a particular gas mostly depend on:
▪ Solubility- its ability to dissolve in water.
▪ Density- how ‘heavy’ it is as compared to air.
There are three common methods to collect a sample of gas:
i. Downward displacement of water- This method is suitable for collecting gases which
are insoluble or only slightly soluble in water. As the gas cannot dissolve in water and are
lighter in density than water, they would rise to the top of the gas jar and be collected
there. Some examples of gases collected through this way include; hydrogen, oxygen and
carbon dioxide.
ii. Upward delivery method- This method is used to collect gases which are less dense than
air. Some examples of gases collected through this way include; hydrogen, chlorine,
hydrogen chloride gas and sulphur dioxide.
iii. Downward delivery method- This method is used to collect gases which are heavier than
air. Some examples of gases collected through this way include ammonia.

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LABORATORY PREPARATION OF OXYGEN.
There are several methods of preparing oxygen gas in the laboratory but the common ones are;
i. Decomposition of hydrogen peroxide
ii. Decomposition of potassium chlorate in the presence of catalyst (manganese (IV) oxide)
iii. Heating compound rich in oxygen like mercury oxide, potassium permanganate and some
metal nitrates
iv. Electrolysis of water
From the listed methods, the frequently used methods are; Decomposition of hydrogen peroxide
and Decomposition of potassium chlorate in the presence of catalyst (manganese (IV) oxide).
Other compounds that contain oxygen can be used to prepare oxygen gas but this method is not
commonly used in the laboratory preparation of oxygen gas due to some associated challenges
such as;
▪ High energy requirement e.g. Electrolysis of water and decomposition of potassium
permanganate
▪ Explosion e.g. Decomposition of potassium chlorate
▪ Health problems e.g. Decomposition of mercury oxide which is toxic and produce fume
which irritates the eyes, skin and respiratory tract.

Important terms used;

▪ Decomposition: is the breakdown of a chemical compound into its constituent elements or
smaller compounds.
▪ Catalyst: is the substance that speeds up the rate of chemical reaction but remain chemically
unchanged at the end of the reaction.

i. Preparation of oxygen gas by decomposition of hydrogen peroxide.

Hydrogen peroxide decomposes to give oxygen gas and water in the presence of catalyst
(manganese (IV) oxide).
At room temperature hydrogen peroxide decomposes very slowly, under this circumstance it
would take very long time to collect even one gas jar of oxygen, so manganese (IV) oxide is added
to speed up the decomposition reaction.
Word equation:
𝑀𝑎𝑛𝑔𝑎𝑛𝑒𝑠𝑒 (𝐼𝑉)𝑜𝑥𝑖𝑑𝑒
Hydrogen peroxide → Water + Oxygen

Molecular equation
𝑀𝑛𝑂2
2H2O2 (aq) → 2H2O (l) + O2 (g)

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 Diagram:

Procedures for preparation of oxygen gas using hydrogen peroxide.

1. Measure about 3g of manganese (IV) oxide and add it to the flask.
2. Connect your apparatus as shown on the diagram above.
3. Add hydrogen peroxide to the thistle funnel and gradually open the tap.
4. Allow the first few bubbles of the gas to escape before collecting.
5. Collect the gas jars of oxygen.
NOTE: The end of the thistle funnel should be below the surface of the solution in the flat-
bottomed flask to avoid the gas evolved to push back the solution of hydrogen peroxide.

ii. Preparation of oxygen gas by decomposition of potassium chlorate

Potassium chlorate is oxygen- rich compound, when heated decompose to produce potassium
chloride and oxygen gas.
Decomposition of potassium chlorate is also speeded up by catalyst manganese (IV) oxide.
When preparing oxygen gas using this method make sure that manganese (IV) oxide to be used
is not contaminated, since when it is contaminated, there is a risk of explosion.

Word equation
𝑀𝑎𝑛𝑔𝑎𝑛𝑒𝑠𝑒 (𝐼𝑉)𝑜𝑥𝑖𝑑𝑒
Potassium chlorate → Potassium chloride + Oxygen

Molecular equation
𝑀𝑛𝑂2
2KClO3(s) → 2KCl (s) + 3O2 (g)

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 Diagram

The mouth of the test tube should be slanting downwards so that water vapour if any does not
condense and flow backwards into the test tube and prevent breaking of the test tube.

REMEMBER:
▪ During preparation of oxygen gas, it is necessary to fill a gas jar with water because makes easier to
collect the gas also determination of volume of the gas collected, when you’re drawing the diagram
don’t forget to fill the beehive shelf with water.
▪ H2O2 is preferred for laboratory preparation of oxygen because of the following reasons;
i. No heating is required
ii. The rate of evolution of oxygen gas is moderate and can be controlled.
iii. H2O2 is a safe chemical

Uses of apparatus.
• Thistle funnel- used to transfer solution into the flat-bottomed flask.
• Flat bottomed flask- used to mix solution (where the reactants are introduced) e.g. Hydrogen
peroxide and manganese (IV) oxide
• Delivery tube- used to transfer gas into beehive shelf.
• Beehive shelf- used to support the gas jar.
• Gas jar- used to collect the gas
• Trough- used to hold water.

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Method of collecting oxygen gas.
(a) Oxygen gas is collected over water by the process called “downward displacement of water”
this is possible because oxygen is only slightly soluble in water and lighter than water, thus,
during its preparation it easily displaces water and is collected over water.
(b) Oxygen gas can also be collected by downward delivery method. This method is possible
because oxygen gas is heavier than air (denser than air).
During collection of oxygen gas, the first few bubbles should not be collected because they contain
air mixed with the gas.
After collecting oxygen gas store the covered bottles of oxygen gas mouth up rather than down
because oxygen gas is denser than air, so it sinks to the bottom of the flask and will not readily
leak out the top.

Method of drying oxygen gas.

Dry oxygen gas can be obtained after passing the gas collected through drying agents like
phosphorus pentoxide, dry calcium chloride or by bubbling the gas through concentrated sulphuric
acid

Chemical test for oxygen gas

Oxygen gas can be tested by introducing a glowing wooden splint into a gas jar of oxygen, if
oxygen is present a glowing wooden splint light up (relights).
That is “oxygen gas relights or lights up a glowing wooden splint”.
This is similarly when a lit candle is lowered into a gas jar containing oxygen gas it burns more
brightly. This confirms that oxygen supports combustion.

Physical properties of oxygen gas

i. It is colourless,
ii. It is tasteless
iii. It is odourless
iv. It is slightly soluble in water
v. Its 1.1 denser than air
vi. It boils at -183℃
vii. It freezes at -218℃

Chemical properties of oxygen gas

i. It supports combustion
ii. It is a very strong oxidizing agent
iii. Oxygen gas is neutral to litmus paper.
iv. React with metal to form basic oxides.
E.g. Sodium + oxygen→ sodium oxide
v. React with non-metal to form acidic oxides
E.g. Carbon + oxygen→ Carbon dioxide

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vi. It reacts with hydrogen to form water.
E.g. Hydrogen + oxygen→ Water
vii. It reacts with iron in the presence of water to form rust.
E.g. Iron + oxygen + water → hydrated iron (III) oxide
(rust)

Basic nature of metal oxides.

Many metals burn in oxygen to produce basic oxide. These oxides are basic because reacts with
water to form basic solution or with acid to form salt and water.
Basic solutions turn red litmus paper blue, for example, magnesium burns to form magnesium
oxide whose solution turns red litmus paper blue.
Example
Sodium + Oxygen→ Sodium oxide
Sodium oxide + Water→ Sodium hydroxide
(basic solution)
Consider the table below summarizes the reaction of some metals with oxygen.

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Acidic nature of non- metal oxides.
Non- metals burn in oxygen to produce acidic oxide. These oxides are acidic because reacts with
water to form acidic solution which turns blue litmus paper red, for example, carbon burns to
form carbon dioxide whose solution turns blue litmus paper red.
Example
Carbon + Oxygen → Carbon dioxide
Carbon dioxide + Water → Carbonic acid (acidic solution).

Consider the table below summarizes the reaction of some metals with oxygen.

Industrial manufacture of oxygen.

Various methods can be used in industries to prepare oxygen gas but the commonly used method
used to produce oxygen in large scale is “Fractional Distillation of Liquefied Air” because this
method produces the highest amount of oxygen than other methods such as Electrolysis of water.

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Fractional Distillation of Liquefied Air
Fractional distillation is the method of separating liquid components with close boiling points
from the liquid mixture.
Fractional distillation of air starts with liquefaction of air followed by distillation of air.

❖ Liquefaction of air
Liquefaction of air involves filtration of air to remove dusts compressing and then cooling down
to -200℃ until it liquefies. During liquefaction the following occurs;
a) Water vapour condenses and is removed by using special filters.
b) At -78.5℃, carbon dioxide freezes and is removed.
c) At -183℃ oxygen liquefies.
d) Nitrogen liquefies at -196℃

❖ Distillation
At -200℃, there is still a mixture of some liquid nitrogen and liquid oxygen. The two liquids are
separated by fractional distillation due to their close boiling points.

NOTE:
The liquid mixture is passed into a fractionating column from the bottom. Since the column is
warmer at the bottom than at the top, the liquid nitrogen boils at the bottom of the column. The
gaseous nitrogen rises to the top where it is collected via the pipe to the storage tank, while the
liqiuid oxygen collects at the bottom of the column
Diagram

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Uses of oxygen
1. Oxygen is used as an aid to breathing in hospitals
2. Respiration in spacecraft and submarine.
3. Used in incubator for premature babies.
4. It is used in welding and cutting metals.
5. Used as an oxidizer for rocket fuel.
6. Used in incineration.
7. It is used for removing impurities from pig iron in order to produce steel.
8. Used in Pulp and Paper making
9. Used in treatment of sewage plant
10. Used in glass making
11. Used in manufacture of chemicals such as oxides.
The figure below summarizes the areas in which oxygen is used

Relationship between some uses of oxygen and its properties

a) Solubility in water
The usefulness of oxygen to aquatic organisms is due to its solubility in water. Oxygen is slightly
soluble in water, therefore, water contains some dissolved oxygen that is used by aquatic living
organisms.

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b) Combustibility
Oxygen supports combustion. Therefore, it is used in incineration or burning of substances,
welding and metal cutting.
In living organisms, oxygen is used to support burning of food in the body through respiration.

c) Reactivity with elements

Oxygen reacts with many elements. This makes it useful in industrial chemical processes such as
manufacturing of chemicals, glass, pulp and paper.

Sample questions with answers

Explain the following
a) Oxygen is called “fire air”
b) Oxygen is used in pulp and paper making
c) Oxygen is used in glass making
d) Oxygen is used for welding
e) Oxygen is used in space rockets
f) Oxygen is used in treatment of sewage plant

Answers
a) Oxygen is called “fire air” because it is needed for fire to burn.
b) Oxygen is used in pulp and paper making, since oxygen is added in alkaline extraction to
intensify the bleaching effect and hence decrease the volume of more expensive bleaching
agent required.
c) Oxygen is used in glass making because glass blowers rely on oxygen as their fuel for the
burners and torches, they use to melt the glass.
Oxygen is the only fuel source that will ensure the flame is hot enough for effectively and
strategically melt quartz and borosilicate glass.
d) Oxygen is used for welding because pure oxygen increases the flame temperature to allow
melting of the work piece materials like steel.
e) Oxygen is used in space rockets because is used to burn the fuel.
f) Oxygen is injected during secondary treatment to encourage growth of bacteria, which are used
to breakdown organic matter containing carbon to form carbon dioxide and water.

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 REVISION EXERCISE
1. How is oxygen prepared using the following chemicals
a) hydrogen peroxide b) Potassium chlorate

2. (a) During the laboratory preparation of oxygen in the laboratory, it was observed that bubbles of
oxygen was moving in the top of the inverted gas jar filled with water, at the end it was
spotted all of water was pushed out of gas jar.
i) Name this process?
ii) Give reason why it was possible for gas to push
water out of gas jar. (2 points)
(b) i) Explain how pure oxygen can be distinguished from ordinary air
ii) Why it was necessary to fill a gas jar with water?
(c) Write the names and chemical formula of two important chemical substance used in the
laboratory preparation of oxygen gas.

3. By giving one reason, explain the following facts:

a) During laboratory preparation of oxygen gas, little manganese dioxide is added to hydrogen
peroxide.
b) Fish can obtain oxygen for respiration although they spend their lives in water.
c) Oxygen gas can be used for welding activities although it does not burn.
d) Oxygen is used in treatment of sewage plant

4. One of the methods used for preparation of oxygen gas is by decomposition of hydrogen
peroxide as shown below in the following reaction;
𝑀𝑛𝑂2
2H2O2 (aq) → 2H2O (l) + O2 (g)
a) What is the main purpose of MnO2 in the reaction?
b) Briefly explain why oxygen is collected over water.
c) The formula of manganese (IV) oxide is MnO2 and that of hydrogen peroxide is H2O2,
which one produces oxygen?
d) Why it is not preferred to use potassium chlorate in preparation of oxygen.
5. You are required to demonstrate one of the chemical properties of oxygen. Your provided with
four gas jars of oxygen and four pieces of elements named A, B, C and D.
Procedure
Elements are placed in deflagrating spoon and inserted in the gas jars of oxygen one after
another
Observation
Element A + oxygen→ it burns with a bright white flame leaving white powder
Element B + oxygen→ it burns vigorously with a yellow flame leaving a pale yellow solid.
Element C + oxygen→ it melts and burns with a blue flame giving a misty (white gas).
Element D + oxygen →it burns slowly with yellowish white flame giving a colourless gas.

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 Questions
a. Identify elements A, B, C and D.
b. Name the products formed after elements A, B, C and D burned in oxygen.
c. Classify elements A, B, C and D into metals and non-metals.

6. When manganese dioxide is added to hydrogen peroxide solution, it speeds up the

decomposition of hydrogen peroxide to form a gas “P” which is collected by downward
displacement of water.
a) Name the gas “P”
b) Why gas “P” is collected by downward displacement of water?
c) Write a word equation for the decomposition of hydrogen peroxide.
d) Mention three physical properties of gas “P”
e) List four uses of gas “P”.

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