Oxygen Gas

Symbol: O          

Molecular formula: O₂

Molecular weight: 16

Fig: Electronic configuration of oxygen

Discovery

Oxygen is one of the abundantly available gases in air. It makes about 21% of atmosphere by volume. It is important to all the life on earth. It was discovered by a Swedish chemist Scheele in 1772 by heating red oxide of mercury and by Priestley of England in 1774. Antony Lavoisier of France studied its properties and named as ‘Oxygen’ in 1776. The credit of discovery goes to Joseph Priestley because his papers were published earlier.

               Oxygen is available in free state in air. It forms a mixture with other gases to form air. In combined state, it is present in oxides. It combines with many elements to form compounds such as sugar, starch, fats and oils, acid, bases and salts. It is found in maximum quantity upon the earth in oxides of metals and non-metals.

Laboratory preparation of Oxygen (using heat)

Apparatus required:

1.      Hard glass test-tube.

2.      Delivery tube.

3.      Trough with water

4.      Gas jar

Chemical required:

1.      Potassium chlorate (KClO₃)

2.      Manganese dioxide (MnO₂)

Water

Molecular formula: H₂O

Molecular weight: 18

Introduction

            From the origin of the earth water is present on the earth. In Hindu mythology water is an element of constituent of life. But modern science has revealed that water is compound of Hydrogen and oxygen combined in 1:8 ratios.

Water has three states: solid, liquid and gas (vapour) according to the temperature. At normal temperature it is found in liquid state but when it is cooled less than 0 C, it becomes ice (solid state) and at higher temperatures it becomes vapour (gas state). Water has a special property. When heated from 0C to 4C its density increases and occupies less volume. This is known as anomalous property of water.

            Water is known as universal solvent since it dissolves many substances. Without water life is not possible. Water content in human body is 70%, in fishes 80%, in vegetables 90-95% by weight.

            Water is used in drinking, cleaning utensils, washing. Cooking, cooling in industries, etc. For different purposes water is used in pure form. For the chemical reactions in the lab, it is used in very pure form.

Nitrogen Gas

Fig:electronic configuration

Symbol: N

Molecular formula: N₂

Atomic number: 7

Atomic number: 14                                               

Position in the Periodic Table: Group 5

Electronic configuration: 2, 5  

Discovery

            Nitrogen gas was discovered in 1772 by Daniel Rutherford, a Scottish physician and chemist. He obtained it by burning phosphorous in enclosed air. Lavoisier studied its properties in detail and showed that it does not support combustion and respiration and for this reason called azote (Greek; a = no, zoe = life).

            Nitrogen is one of the gases found in nature in abundant quantity. It occupies about 78% by volume. It is also found in combined state in proteins (meat, fish, egg and pulses) and nitrate salts.

            Nitrogen is an essential constituent of animal and plant life. The atmospheric nitrogen goes in cycle so that the natural balance of atmosphere air is possible. The nitrogen cycle is given below.

Fig: nitrogen cycle

Preparation of Nitrogen gas

From air

Nitrogen gas can be prepared from air by burning phosphorous. A small crucible is placed on a cork floating in water. The phosphorous is ignited and the crucible covered immediately by a gas jar. The oxygen present in the jar combines with phosphorous to form phosphorous pentoxide which dissolves in water and the level of water rises in the jar. The gas remaining in the jar is nitrogen.

P₄ + 5H₂O = 2P₂O₅

P₂O₅ + 3H₂O = 2H₃PO₄

Nitrogen can be prepared by passing air over red hot copper which combines with the oxygen present in the air to form copper oxide and the nitrogen present can be collected over water.

Laboratory preparation of Nitrogen gas

           

Apparatus required:

1.Round bottomed flask

2.Burner

3.Delivery tube

4.Trough

5.Beehive shelf

6.Gas jar

Chemical required

1.Ammonium chloride (NH₄Cl)

2.Sodium nitrite (NaNO₂)

Principle

            A mixture of ammonium chloride and sodium nitrite when heated, nitrogen gas will be produced which can be collected by downward displacement of water.

            NH₄Cl + NaNO₂ = NaCl + N₂ + 2H₂O

          

  Procedure

The apparatus is fitted as shown in the figure. When the mixture of ammonium chloride and sodium chloride is heated it begins to decompose. Nitrogen bubbles displace water in the gas jar. Thus nitrogen gas can be collected by downward displacement of water.

Manufacture of Nitrogen gas

            At industrial level, nitrogen is manufactured from air. Air is a mixture of different gases where nitrogen has higher percentage by volume. This nitrogen can be separated from air.

            Atmospheric air is cooled down so that all gases are liquefied. The liquid air is cooled down so that nitrogen boils (nitrogen has lower boiling point than other gases). Nitrogen vapour is collected in the cylinders and sent for use. 

Properties of Nitrogen gas

            Physical properties:

1.Nitrogen is a colorless, odorless and tasteless gas.

2.It is slightly lighter than air. Its density is 14 while that of air is 14.4.

3.It is insoluble in water.

4.It is non-poisonous but animals die in an atmosphere of nitrogen for want of oxygen.

Chemical properties:

1.It is neutral to litmus.

2.Nitrogen is incombustible and a non-support of combustion but burning magnesium or aluminium continues burning in an atmosphere of nitrogen forming nitrides.

3Mg + N₂ = Mg₃N₂

2Al + N₂ = 2AlN

3.With oxygen it combines only in the presence of lighting discharge or when the mixture is passed through an electric arc.

N₂ + O₂ = 2NO

4.With hydrogen it combines under pressure (200 atmospheres) and in the presence of catalyst (finely divided iron + molybdenum at 500◦C).

N₂ + 3H₂ = 2NH₃

Use of Nitrogen gas

1.Nitrogen gas is used to prepare Ammonia, nitric acid and explosives.

2.Nitrogen gas is used in aeroplane to prevent formation of explosive mixture by combination of fuel and atmospheric air.

Physical change and Chemical change

Chemical change

             A chemical change is more or less permanent and gives rise to a new substance having composition and properties altogether different from those of the original substances.

For example, burning of magnesium, rusting iron, preparation of gases in the laboratory, etc.

Experiment

Take a little sugar in a test tube. Heat test tube slowly for sometime. You will see that sugar turns into black and some water droplets collect on its mouth. Sugar turns into carbon and water due to heat. Such a change is known as Chemical change.

Physical Change

Ice melting: an example of physical change

            A physical change is change of state only and is not accompanied by an alternation in composition, weight or chemical properties of substances. Examples, melting of ice, distillation of water, magnetising a needle, etc.

Experiment

            Take some water in a beaker. Put a watch glass on its mouth containing some water. Heat the beaker as shown in the figure. You will see that water vaporizes. Some water droplets will be seen on the inner side of water glass. This is due to water in the watch glass. Thus, when water is heated it turns into vapour and after cooling, again, it turns into water. Such a change is known as Physical change.

          From above discussion we come to conclude that chemical change cannot be reversed and physical change can be reserved.

            There are certain distinctions in between physical change and chemical change. The points of difference between physical and chemical changes are summarized in the table below.

           

Chemical change	Physical change
1. Change takes place in the composition of the  substance.	1. No alternation in the chemical composition.
2. A new substance is formed with entirely different properties.	2. The chemical properties remain the same. A change may take place only in some of the  physical properties like color, taste, etc.
3. The chemical can’t be reversed and is permanent.	3. The change is temporary and can be easily reversed back.

           

AMMONIA

Molecular formula: NH₃

Molecular weight: 17

Occurrence

            Small quantities of ammonia are found in a place where nitrogenous decay takes place. In combined form it is found in Ammonium chloride and Ammonium sulphate naturally.

            Ammonia is a compound and it has molecular weight and molecular formula which has been given above. It forms by combining 1 atom of nitrogen and 3 atoms of hydrogen.

Laboratory preparation of Ammonia gas

            Ammonia gas is prepared in the laboratory by heating a mixture of ammonium chloride and slaked lime (calcium hydroxide).

            2NH₄Cl + Ca (OH)₂ = CaCl₂ + 2NH₃ + 2H₂O

Experiment: Powdered ammonium chloride many be mixed with slaked lime (calcium hydroxide) at the ratio of 2:1. The mixture may be put in a flask or hard glass test tube. Fit a cork at the mouth of the test tube with a delivery tube directing upwards which leads the gas in a jar which is inverted. The gas is lighter than the air and it is collected by downward displacement of air. When the test tube is heated the gas is liberated and collected in a gas jar. To test whether the jar is filled bring a moist red litmus paper. This turns into blue.

Fig: Lab preparation of ammonia gas.

Manufacture of Ammonia

            In industrial scale, ammonia gas is prepared by Haber’s Synthetic process. By laboratory process of preparation of ammonia gas we cannot fulfill the demand as it is required in the production of nitrogen rich fertilizers such as Ammonium sulphate.

Fig: Haber's synthetic process of preparation of Ammonia gas.

            

 A mixture of one volume of nitrogen and three volumes of hydrogen in a dry state is passed under a pressure of 200 atmospheres through a chamber of packed catalyst consisting of finely divided iron with a little of molybdenum and maintained at about 500◦C . The two gases combine under these conditions to form about 10% of ammonia.

            N₂ + 3H₂ = 2NH₃ + 22,500 cals

            The resulting mixture is passed through a coil maintained a very low temperature to condense the ammonia and the remaining uncombined gases are again passed through the catalyst chamber.

            In conclusion, the following conditions are required for the preparation of ammonia by Haber’s process.

1.One volume of Nitrogen and three volume of hydrogen flow.

2.Pressure of 200 atmospheres.

3.Temperature of 500◦C .

4.Catalyst, finely divided iron with a little of molybdenum.

Properties of Ammonia Gas

            Physical properties:

1. Ammonia is colorless gas.

2. It has pungent smelling and brings tears into the eyes.

3. It is poisonous.

4. It is lighter than air.

5. It is highly soluble in water.

Chemical properties:

1. It is basic in nature and therefore turns red litmus paper into blue.

2. Ammonia dissolves in water and produces hydroxide (ammonium hydroxide).

Ammonium hydroxide is a base and when it is treated with sulphuric acid, produces ammonium sulphate, a chemical fertilizer.

2NH₄OH + H₂SO₄ = (NH₄OH)₂ SO₄ + 2H₂O

3. Ammonia reacts with hydrogen chloride and produces dense white fumes of ammonium chloride. This property of ammonia is the sure test of gas.

NH₃ + HCl = NH₄Cl

Uses of Ammonia gas

1. In the manufacture of Urea and ammonium sulphate (chemical fertilizer) ammonia is extensively used.

2. Ammonia is used in the manufacture of nitric acid (Ostwald process).

3. It is extensively used in the laboratory as a cleaning agent for removing grease.

4. Ammonia is used in the refrigerators.

5. In the development of blue print of maps ammonia is used.

6. It is also used in the preparation of artificial silk.