Oxygen is a gas that forms about 21% by volume of the air.
In combined state, oxygen occurs in many compounds such as
Water
Mineral ores
Protein molecules
Laboratory preparation of oxygen
The common methods of preparation of oxygen in the laboratory include the following:
Decomposition of hydrogen peroxide.
Thermal decomposition of potassium chlorate.
Heating compounds rich in oxygen.
Decomposition of hydrogen peroxide
Requirements:
Flat bottomed flask
Beehive shelf
Delivery tube
Water trough
Thistle funnel
Gas jar
Hydrogen peroxide
Manganese(IV) oxide
Water
Diagram Showing the Laboratory Preparation of Oxygen from Hydrogen Peroxide
Word equation: Hydrogen peroxide → water + oxygen Molecular/formula equation: 2H2O2 → 2H2O + O2
This reaction is very slow, but manganese (IV) oxide act as a catalyst to speed up the rate of chemical reaction. Catalyst is a substance that alters the rate of a chemical reaction but remains unchanged at the end of the reaction.
Decomposition is the reaction in which a chemical compound breaks down into its constituent elements or simpler compounds
For example AB → A + B
During collection, allow the first few bubbles to escape to ensure that the oxygen gas collected is pure.
Oxygen can be collected by downward displacement of water because it is only slightly soluble in water.
Heating Compounds Rich in Oxygen
Some of compounds containing oxygen can be used to prepare oxygen gas:
For example:
Potassium permanganate
Mercury oxide
However, this method of preparing oxygen is not commonly used because:
Potassium permanganate - require high amount of energy -It can explode
Mercury oxide - produces a mixture of highly toxic mercury fumes and oxygen which:
-irritate the eyes, skin and respiratory tract
-have effects on the kidney and can cause death.
Properties of Oxygen
Physical properties
Oxygen is colourless, tasteless and odourless.
It is slightly soluble in water.
It is denser than air. It is not available in air because it is heavier than air so sinks on the lower atmosphere.
It boils at -183◦C.
It freezes at -218◦C.
Chemical properties
It supports combustion.
It is a very strong oxidizing agent.
It reacts with metals to form basic oxides.
It reacts with non-metals to form acidic oxides.
Chemical test of oxygen/Identity test of oxygen gas
When a glowing splint of wood is introduced into the gas jar containing oxygen gas, the glowing splint relights or rekindles or bursts into flame.
It can simply stated:
Oxygen relights or rekindles a glowing splint.
Industrial manufacture of oxygen.
In industry oxygen is prepared by fractional distillation of liquefied air and electrolysis of water.
The Steps involved in the industrial preparation of oxygen by fractional distillation:
Step 1: Filtration and Compression of Air
First, air is filtered to remove dust and other particulate matter.
After filtration, the air is compressed using compressors to about 6-10 atmospheres. This prepares the air for cooling and liquefaction.
Step 2: Cooling and Liquefaction of Air
Compressed air is passed through a cooling system where it is gradually cooled to extremely low temperatures (below −200°C).
This cooling is achieved using a process called the Joule-Thomson Effect, where compressed gas expands rapidly, causing it to cool.
At these temperatures, air turns into a liquid.
Step 3: Separation of Components Based on Boiling Points
The liquefied air is transferred to a fractionating column.
The column is designed to gradually heat the liquid. As it warms:
Nitrogen (boiling point: −196°C) boils first and is separated.
Argon (boiling point: −186°C) boils next and is collected.
Oxygen (boiling point: −183°C) boils last and is collected separately.
Step 4: Collection of Oxygen and Other Gases
Oxygen is collected in its pure form and stored in high-pressure tanks for industrial or medical use.
Nitrogen and argon, by-products of this process, are also stored for other applications.
Uses of Oxygen
Oxygen is used in breathing and respiration.
Oxygen is used in industry to carry out various processes such as manufacture of steel and welding.
It is used in hospitals to help patients with breathing difficulties.
Liquid oxygen is used to burn fuel in rockets.
It is used by deep divers to supply oxygen.
It is used by mountain climbers to supply oxygen when they are at high attitudes.
Class Work
Why is hydrogen peroxide preferred over potassium chlorate in the laboratory preparation of oxygen?
Because it produces enough oxygen gas without the use of heat.
What is the role of manganese (IV) oxide in the laboratory preparation of oxygen? The role of manganese(IV) oxide is to speed up the rate of chemical reaction.
