Oxygen

Symbol: O
Molecular formula: O₂
Valency: 2
Position in periodic table: Group-VI A, Period-2nd
Electronic configuration: 2, 6 (1s², 2S² 2P⁴)
Atomic number: 8
Atomic weight: 16
Molecular weight: 32 amu
Freezing point:-219°C
Boiling point: -183°C

Occurrence

Oxygen is abundantly occurring element on the earth. It constitutes about 47.6% of the earth's crust.

General methods for the preparation of oxygen gas

From metallic oxides:

Oxides of metals give oxygen when heated.
2HgO → 2Hg + O₂↑

From metal peroxide:

When sodium peroxide is treated with water, oxygen is liberated.
2Na₂O₂ + 2H₂O → 4NaOH + O₂↑

From electrolysis of water:

Oxygen is evolved in the positive electrode during the electrolysis of acidic water in Hofmann Voltameter.
2H₂O→ 2H₂ +O₂

Laboratory preparation of oxygen gas

Oxygen gas is prepared in laboratory by the following two methods:

• By heating method
• Without heating method

By heating method

Principle:
When potassium chlorate is heated in the presence of manganese dioxide in the ratio of 3:1, it decomposes at 250°C into potassium chloride and oxygen.
2KClO₃ → 2KCl + 3O₂

Take a mixture of powdered potassium chlorate and manganese dioxide in the ratio of 3:1 in a hard glass test tube. Fit it with a cork and a delivery tube and adjust the test-tube in a stand as shown in the figure. Insert the other end of delivery tube into the bee-hive shelf. A gas jar is filled with water and inverted over the bee-hive shelf in the pneumatic trough. Now, heat the mixture. When potassium chlorate is decomposed, oxygen is liberated. The first formed oxygen gas is contaminated with the air inside the hard glass test tube and delivery tube and is allowed to escape. Then the oxygen is collected in gas jar by downward displacement of water.

Precautions

• The manganese dioxide (MnO₂) should be pure, otherwise the particles of carbon along with MnO₂ burn inside the tube and it may cause explosion.
• Apparatus should be made airtight.
• The test tube should be fixed in slightly slanting position.

Without heating method

Principle:
Oxygen gas can be prepared in the laboratory by hydrogen peroxide and manganese dioxide. In this reaction manganese dioxide is used as catalyst.
2H₂O₂ → 2H₂O + O₂↑

The apparatus is set up as shown in the figure. Hydrogen peroxide is poured in manganese dioxide and water having a conical flask with the help of a thistle funnel. Inside the flask, the decomposition of hydrogen peroxide is taking place in the presence of manganese dioxide and oxygen gas is liberated. The produced oxygen gas is collected in the gas jar by downward displacement of water.

Test of oxygen:
To test whether the produced gas is oxygen or not, introduce a glowing matchstick in the jar containing gas. This burns with bright light.

Manufacture of oxygen

From air:
By compression, cooling and sudden expansion of air, the air is changed into liquid state and is freed from moisture and carbon dioxide. The liquid air has nitrogen and oxygen only. The liquid air allowed to vaporize. Being the boiling point of nitrogen lower, it escapes first and is separately collected. The left liquid is nearly pure oxygen.

By electrolysis of water:
Oxygen can also be manufactured by the electrolysis of water as you are explained in hydrogen gas.

Properties of oxygen gas

Physical properties

• It is neutral to the indicator.
• It is non-combustible gas but it is combustion supporter.
• It is slightly heavier than air.
• It liquefies at -183°C and solidifies at -219°C.
• Oxygen is a poor conductor of heat and electricity.
• Oxygen is odorless gas.
• Oxygen is tasteless gas.

Chemical properties

• Metals like sodium, potassium, calcium and magnesium burn with a bright flame in oxygen to produce their oxides.
  4K + O₂ → 2K₂O (Potassium oxide)
  Ca + O₂ → 2CaO (Calcium oxide)

• Most of the non-metals like carbon, phosphorous and sulphur burn in oxygen to form oxides
  C + O₂ → CO₂ (Carbon dioxide)

• Ammonia burns in oxygen gas with greenish yellow flame and procedures nitrogen and water.
  4NH₃ + 3O₂ → N₂+6H₂O

• Iron changes into Ferro-ferric oxide when it is strongly heated in the presence of oxygen.
  4Fe + 2O₂ → Fe₂O₄ (Ferro-ferric oxide)
  But in the presence of moisture, iron reacts with oxygen to form rust. The process of formation of rust is called rusting.
  4Fe + 3O₂ + XH₂O → 2Fe₂O₃.XH₂O
• Various organic compounds like carbohydrate, ethyl- alcohol, oil, petrol, wax etc. burn in oxygen forming carbon dioxide.
  CH₄ + 2O₂→ CO₂ +2H₂O + Heat
  C₆H₁₂O₆ +6O₂→ 6CO₂ + 6H₂O + Heat
• We need oxygen for oxidation of our food as it oxidizes the food and releases energy during respiration which is used to perform body functions. Here, carbon dioxide and water are formed as a by- products of respiration.
  C₆H₁₂O₆ +6O₂→ 6CO₂ + 6H₂O + Energy
• Oxygen when passed through an electric aRC struck between two copper electrodes at 3000^oC, nitric oxide is formed. It is also produced during lightning.
  N₂ + O₂ → 2NO (Nitric Oxide)
  3000^oC

Uses of oxygen

• Oxygen is required for oxidation of food to produce energy during respiration.
• It is used in hospitals for the artificial respiration of pneumatic patients.
• The liquid form of oxygen is used as fuel in rockets.