Chapter 5.2 - Preparation of Nitrogen

In the previous section, we completed a discussion on oxygen. In this section we will see nitrogen.

We have seen that nitrogen constitutes 78.08% of the atmospheric air. This is more than twice the quantity of oxygen (20.95%). In the experiment for producing oxygen in the lab by heating potassium permanganate, we saw that the glowing splinter burst into flames. That is., a small glow instantly became a large flame. This is because of the concentrated quantity of oxygen in the boiling tube. There was no element other than oxygen in that boiling tube. So, if in the atmosphere also, there is a huge amount of oxygen, we would not be able to control combustion. Thanks to the nitrogen which s present in greater quantity, the combustion is regulated.

Nitrogen is almost inert. That is., it does not take part in chemical reactions easily. This is because of the triple bond between two nitrogen atoms that is present in it's molecule. We have seen the details here. A large amount of energy is required to break the triple bond. So nitrogen does not normally take part in chemical reactions.

Preparation of nitrogen

In the laboratory, nitrogen can be prepared by heating a mixture of ammonium chloride and sodium nitrite. Let us write the steps:

• One molecule of ammonium chloride is NH₄Cl, and one molecule of sodium nitrite is NaNO₂. These will be written on the left side.

• The products are Ammonium nitrite and sodium chloride. One molecule of ammonium nitrite is NH₄NO₂. And one molecule of sodium chloride is NaCl. We will write these on the right side.

• So the skeletal equation is: NH₄Cl + NaNO₂ → NH₄NO₂ + NaCl. This equation is already balanced.

• Now, the ammonium nitrite (NH₄NO₂) formed in this way is unstable. So it decomposes immediately to become nitrogen gas and water. Thus we obtain free nitrogen.

• The skeletal equation for this decomposition is: NH₄NO₂ → N₂ + H₂O. This is not a balanced equation. The steps for writing the balanced equation are given below:

So the balanced equation is: NH₄NO₂ → N₂ + 2H₂O.

■ Industrially, nitrogen is prepared by the fractional distillation of liquefied air. Let us first see what 'fractional distillation' is:

• Consider a mixture of three different liquids. Liquid A, Liquid B and Liquid C. Let the boiling point of A be 110^o C, that of B be 125^o C and C be 145^o C . That means:

    ♦ at 110^o C, liquid A will begin to boil, and turn into gaseous state.

    ♦ at 125^o C, liquid B will begin to boil and turn into gaseous state.

    ♦ at 145^o C, liquid C will begin to boil and turn into gaseous state.

• Now, the mixture containing A, B and C is heated to such a temperature that, the three of them turn into gaseous state. We have to separate them from one another. 

• For that, we pass the gaseous mixture through a 'fractionating column'. This is a vertical column which is heated at the bottom. So the bottom portion will be hotter, and as we move up, the temperature will decrease. 

• When the gaseous mixture move up through the column, it will first reach the point at which the surrounding temperature is 145^o C. When the mixture moves further up, the surrounding temperature will be lower than 145^o C. This lower temperature is not sufficient to keep C in the gaseous state. So C will begin to turn back into liquid state. That is., C will begin to condense. So the liquefied C will fall to the bottom of the column, and is collected from there.

• Now, the gas is a mixture of A and B. When this gaseous mixture move up through the column, it will reach the point at which the surrounding temperature is 125^o C. When the mixture moves further up, the surrounding temperature will be lower than 125^o C. This lower temperature is not sufficient to keep B in the gaseous state. So B will begin to turn back into liquid state. That is., B will begin to condense. The condensed B is not allowed to fall back into lower portion of the column. In that case it will again turn to gaseous state. So B is collected out as soon as it becomes a liquid, through an outlet. 

• After this, the gas is no longer a 'mixture'. It contains only A. As it move up the column, it will reach a point at which the surrounding temperature is 110^o C. When it moves further up, the surrounding temperature will be lower than 110^o C. This lower temperature is not sufficient to keep A in the gaseous state. So A will begin to turn back into liquid state. That is., A will begin to condense. The condensed A is not allowed to fall back into lower portion of the column. In that case it will again turn to gaseous state. So A is collected out as soon as it becomes a liquid, through an outlet at the top most portion.

• Thus A, B and C are separated. A schematic diagram is shown in fig.5.3(a) below:

Fig.5.3

• In the above example, we considered a mixture of 3 liquids. We can use the method to separate two or more than two liquids.
■ Now we will see how fractional distillation is used to separate nitrogen from liquefied Air:
• First, air is cooled to very low temperatures. 

• At -183^o C, oxygen liquefies. 

• When it is further cooled, at -196^o C, nitrogen liquefies. 

• At -200^o C, the complete air becomes a liquid. 

• This liquid air is made to move up through a fractionating column. This column will be warmer at the bottom and cooler at the top. That is., as we move up, the temperature decreases. Starting from -185 at the bottom, it will become -186, -187, -188... and so on as we move upwards. 

• The liquid air at -200 enters the bottom of the column. There the temperature is -185. This is lower than the boiling point of oxygen. So oxygen remains as a liquid. 

• But for nitrogen, this -185 is too hot. So it will turn into the gaseous state, and will rise up. This gaseous nitrogen is collected at the top of the column.

• A schematic diagram is shown in fig.5.3(b) above. 
The method of fractional distillation is extensively used in Petroleum industry. We will learn about more detailed Scientific and Engineering procedures related to fractional distillation in higher classes.

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Nitrogen is an important element for the growth of plants. Although nitrogen is abundant in the atmosphere, plants cannot absorb it directly. Following are the methods by which plants obtain the required nitrogen:

• Biodegradation of the remains of plants and animals

• Fixation of nitrogen by bacteria

• Thunder and lightning

• Chemical fertilizers

We will now see each method in detail:

Biodegradation of the remains of plants and animals

Among the various elements present in the body of plants and animals, nitrogen constitutes a significant portion. When the remains of plants and animals decay, the nitrogen compounds in them mix with the soil. The roots of plants can absorb them

Fixation of nitrogen by legumes bacteria

• The bacterium Rhizobium is present in the soil. It will enter the roots of legumes, and live in those roots. Their presence can be detected by the nodules seen often in the roots of legumes. 

• These bacteria has the ability to convert atmospheric nitrogen into nitrates. These nitrates are stored in the nodules. The plant can absorb these nitrates. 

• Also, when the legumes decay, the nitrates in the nodules will mix with the soil, and thus they become available to other plants also.
■ This whole process is called 'Nitrogen fixation' because, the bacteria 'fixes' the atmospheric nitrogen into a substance which can be used by plants.

Thunder and lightning

When lightning strikes the nitrogen molecules, the triple bond will break, and individual nitrogen atoms will be formed. At that time, nitrogen can react with oxygen. This reaction will give nitric oxide. Let us write the equation:

• It is a reaction between nitrogen and oxygen. One molecule of nitrogen is N₂. One molecule of oxygen is O₂. So we will write them on the left side.

• The product is nitric oxide. One molecule of this is NO. We will write it on the right side.

• So the skeletal equation is: N₂ + O₂ → NO. This is not a balanced equation. The steps for balancing the equation are given below:

• Thus the balanced equation is: N₂ + O₂ → 2NO

■ The NO thus formed will combine with more oxygen to become Nitrogen dioxide NO₂. Let us write the equation:

• The reaction is between nitric oxide (NO) and oxygen (O₂ ). The product is nitrogen dioxide (NO₂).

• So the skeletal equation is: NO + O₂ → NO₂ . This is not a balanced equation. The steps for balancing the equation are given below:

• So the balanced equation is: 2NO + O₂ → 2NO₂ .

■ The nitrogen dioxide (NO₂) is a gas. It dissolves in rain water in the presence of oxygen and become nitric acid (HNO₃). Let us write the equation:

• The reaction is between nitric oxide (NO₂), water (H₂O) and oxygen (O₂ ). The product is nitric acid (HNO₃).

• So the skeletal equation is: NO₂ + H₂O + O₂ → HNO₃ . This is not a balanced equation. The reaction involves the formation of some intermediate products, which finally get converted to nitric acid. We will learn the steps for writing it's balanced equation in higher classes. 

• The balanced equation is: 4NO₂ + 2H₂O + O₂ → 4HNO₃.

• This nitric acid reacts with the minerals in the soil and become nitrate salts. Plants can absorb these salts. Thus they get nitrogen.

Use of chemical fertilizers

We have seen three methods by which nitrogen become available to plants. But by those methods, plants will get only small quantities of nitrogen. So farmers use chemical fertilizers. Let us see how it is done:

In the factories which produce the chemical fertilizers, nitrogen is first converted into ammonia. From this ammonia, many salts of nitrogen can be produced. These salts are excellent fertilizers. When they are added to soil, they mix with the soil, and the roots can extract the required nitrogen.

The chemical fertilizers have many advantages and disadvantages. An alternative to chemical fertilizers is the use of organic fertilizers. They can be produced by the decomposition of leaves, grass, kitchen wastes etc., A comparison between organic fertilizers and chemical fertilizers can be seen here.

Nitrogen cycle

We have seen that the nitrogen present in the atmosphere reaches the earth through different methods like nitrogen fixation, lightning etc., This nitrogen that reaches the earth is stored in plants and animals. When these plants and animals decompose, the nitrogen will be released again into the atmosphere. Also, by the decomposition of nitrogen containing salts, it will be released into the atmosphere. This forms a cycle, and is called the nitrogen cycle.

Uses of nitrogen

Nitrogen is used for the following purposes:

• In the manufacture of fertilizers

• For inflating tyres of vehicles

• Liquid nitrogen is used as a refrigerant

• To avoid the presence of oxygen in food packets

• In the manufacture of some medicines

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In the next section, we will discuss about Hydrogen. 

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Chapter 5.1 - Reaction of Oxygen with Metals

In the previous section, we saw some reactions of oxygen with non-metals. In this section we will see some reactions of oxygen with metals.

A freshly cut surface of metals like aluminium and iron will have a metallic lustre. But this lustre will fade away gradually. This is due to the formation of a new substance on the surface. This new substance is formed because of the reaction of the metal with oxygen.

Reaction with Aluminium:
Aluminium reacts with the oxygen in the atmosphere, and forms aluminium oxide. Let us write the equation:

• One molecule of aluminium is Al. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is Al₂O₃. We will write it on the right side

• So the skeletal equation is Al + O₂ → Al₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

So the balanced equation is: 4Al + 3O₂ → 2Al₂O₃

Reaction with Iron:

Iron reacts with the oxygen in the atmosphere, and forms iron oxide. Let us write the equation:

• One molecule of iron is Fe. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of iron oxide is Fe₂O₃. We will write it on the right side

• So the skeletal equation is Fe + O₂ → Fe₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

• So the balanced equation is: 4Fe + 3O₂ → 2Fe₂O₃

• The iron oxide is commonly known as 'rust'

Reaction with magnesium:

When magnesium burns, it reacts with oxygen. We can do a simple experiment to demonstrate this. [Note that all experiments should be done only under the supervision of teachers. All safety precautions should be taken. In the following experiment, avoid looking directly at the flash]

1. Take a magnesium ribbon about 2 cm long. Clean it's surface using sand paper. This is to remove any impurities. Because we want the reaction to take place directly between magnesium and oxygen. We do not want any impurities in between.

2. Hold the ribbon with a pair of tongs. Burn it using a spirit lamp or burner. Note that it should be burnt while keeping it as far away as possible from the eyes.

3. The ribbon will burn with bright white light. Do not look directly at the light. When the burning is complete, we will see that the ribbon has changed into a white powder. This powder is magnesium oxide.

• One molecule of magnesium is Mg. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is MgO. We will write it on the right side

• So the skeletal equation is Mg + O₂ → MgO . This is not a balanced equation.

• The steps for balancing the equation are given below:

So the balanced equation is: 2Mg + O₂ → 2MgO

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We have seen some of the reactions of oxygen with metals as well as non-metals. Next we will discuss about Ozone layer.

■ Oxygen (O₂) is found as a diatomic molecule. That is., each molecule of oxygen will have two atoms of oxygen. 

■ But some times, three atoms of oxygen will combine together to form a single molecule. That molecule is entirely different from oxygen in physical and chemical properties. It is called Ozone.

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■ Some elements are found in two or more physical forms. This is called allotropy. The different physical forms are called allotropes of the element. Example:

• Diamond and graphite are two allotropes of carbon.

• Oxygen (O₂) and Ozone (O₃) are two allotropes of oxygen

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Ozone is present mostly in the stratosphere of the atmosphere. Let us see the steps in the formation of Ozone:

1. The ordinary oxygen (O₂) molecules absorbs high energy ultraviolet radiation, and dissociates into individual oxygen atoms.

2. These individual oxygen atoms combine together to form molecules of ozone (O₃).

3. Ozone molecules so formed, absorb low energy ultraviolet radiations and decompose into oxygen molecules (O₂).

4. The O₂ molecules so formed will again undergo the process in step (1)

■ So we see that it is a cyclic process. The cycle is shown in the fig.5.2 below:

Fig.5.2

Let us see the important points to be noted in the cycle:

• In (1), the O₂ absorbs high energy ultraviolet radiation. But in (3), the O₃ absorbs low energy ultraviolet radiation.

• In (1), the O₂ dissociates into individual atoms. But in (3), the O₃ decomposes into O₂.

Importance of the ozone cycle:

We see that ultraviolet radiations are absorbed during the cyclic process. As a result, those harmful radiations do not reach the surface of the earth. Such radiations can cause diseases like cancer. So the ozone layer is very important for sustaining life on earth.

Ozone layer depletion

The proper functioning of Ozone layer is under threat. Let us see why:

• Compounds known as CFC or chlorofluorocarbons, are special compounds that contain chlorine, fluorine and carbon. 

• They have a special property: They are normally in the gaseous state. But they can be easily brought into liquid state by applying pressure. 

• Once they are in the liquid state, they absorb heat from the surroundings. Using this heat, they evaporate and gets converted back into gaseous state. 

• When they absorb heat, the surroundings are cooled. So they are used in refrigerators and air conditioners.

• After long periods of usage, the appliances like refrigerators, air conditioners etc., become worn out. They are then abandoned. 

• The CFC in them will escape into the atmosphere. They reach the stratosphere. 

• The molecules of CFC will break down under the action of ultraviolet radiations. When they break down, individual chlorine atoms become available. 

• These chlorine atoms react with ozone molecules. This reaction will cause the decomposition of ozone molecules. The ozone molecules thus decomposed will be out of the ozone cycle. 

• So if more CFC escapes into the atmosphere, more chlorine will become available, and so more molecules of ozone will be decomposed. Gaps will begin to appear in the ozone layer. This is known as ozone layer depletion.

•When more molecules of ozone is decomposed, only a lesser quantity of ozone will remain to take part in the cycle. This remaining ozone will not be able to absorb all the harmful radiations coming from the sun. As a result, much of those radiations will reach the surface of the earth.

■ In order to create awareness about the need to preserve the ozone layer, September 16^th is celebrated as International Ozone Day. 

■ Now a days many countries have introduced strict rules to avoid using harmful CFC. Safer substances should be used instead. We must all abide by the rules so that, the ozone layer can be restored back to it's original state.

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We have completed our present discussion on oxygen. In the next section, we will discuss about Nitrogen. 

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Chapter 5 - Non-Metals

In the previous section, we completed the discussion on the Periodic table. In this section we will discuss about Non-metals. We will discuss about non-metals like oxygen, hydrogen, nitrogen, chlorine etc., For each of these non-metals, we will see:

• Uses • Importance • Occurrence in nature • Preparation in the lab.

First we will see oxygen:

Oxygen is a gas. It is present in the atmosphere. But in the atmosphere, there are lots of other gases also. If we take a sample of air from the atmosphere,

• 78.08% of that sample will be nitrogen • 20.95% will be oxygen • 0.9% will be argon • 0.038% will be carbon dioxide

• The remaining portion, that is [100 -(78.08 +20.95 +0.9 +0.038)] 0.032% will consist of other gases.

So we see that oxygen has a significant presence. It is the breath of life. It is most essential for the existence of life. We know that oxygen used up by living beings is replenished by plants.

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Oxygen was discovered by a British scientist Joseph Priestly. It was identified as an element by a French scientist Antoine Lavoisier. The word 'oxygen' comes from a Greek word that means 'acid former'.

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Preparation of oxygen in the lab:

• In the lab, oxygen is prepared by heating crystals of potassium permanganate in a dry ‘boiling tube’. 

• Note that the ‘boiling tube’ must be dry. That is., there must not be any presence of water or other substances.

The diagram of the process is shown in fig.5.1 below. [It may be noted that, all experiments should be carried out under the supervision of teachers. All safety precautions should be taken]

Fig.5.1

■ So what happens when the crystals are heated?

• The crystals break down into three new substances:

(i) Potassium manganate: K₂MnO₄

(ii) Manganese dioxide MnO₂ and 

(iii) Oxygen: O₂

• So we see that oxygen is one of the ‘three new substances’. It is in the gaseous form, and so will rise towards the top of the tube.

• If we introduce a glowing splinter into the tube, it will burst into flame. This will tell us about the presence of oxygen. A video can be seen here.

■ So we tested and confirmed the presence of oxygen using the glowing splinter. How do we know the presence of the other two: K₂MnO₄ and MnO₂ ?

• There are various tests and procedures to determine the contents in the tube after the reaction. Such tests and procedures are used both in labs and in industries. We will learn about them in higher classes. For the present discussion, it is sufficient to know which are the 'three new substances'.

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So the crystals of potassium permanganate changed into three different substances. It is a decomposition reaction. As heat is required for the decomposition to take place, it is called a thermal decomposition reaction.

1. Let us write the equation for the reaction:

KMnO₄ + heat → K₂MnO₄ + MnO₂ + O₂

2. Let us analyse the above equation. When we take some crystals of potassium permanganate in the tube, we are taking some molecules of it.

3.Any one molecule of potassium permanganate is KMnO₄. So this molecule is placed on the left side of the equation in (1).

4. When heat is applied, we get three ‘new substances’.

• One molecule of a new substance is K₂MnO₄. So it is placed on the right side of the equation in (1)

• One molecule of another new substance is MnO₂. So it is also placed on the right side of the equation in (1)

• One molecule of the third new substance is O₂. So it is also placed on the right side of the equation in (1)

5. The equation seems to be complete. But it is not. Writing such an equation will not give us the ‘number of molecules’ of each substance involved in the reaction. [Equation in (1) is called the 'skeletal equation']. So we must write a balanced equation. Details here.

6. The steps for writing the balanced equation is shown below:

7. So the balanced equation is: 2KMnO₄ + heat → K₂MnO₄ + MnO₂ + O₂

From the balanced equation, we can see that, 2 molecules of potassium permanganate will be required to produce one molecule of oxygen.
8. Also, from balanced equations, we can calculate accurate quantities of substances taking part in a reaction. We will learn more about calculation of quantities in higher classes.

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Another method for the preparation of oxygen:

1. During electrolysis of water, we get hydrogen and oxygen. So it can also be used for the preparation of oxygen. Let us analyse the equation:

2. The original substance that we take is water. So we will write one molecule of it on the left side.

3. The new substances that we get are hydrogen and oxygen. So we will write one molecule of each on the right side. So the skeletal equation is:

H₂O → H₂ + O₂

4. Now we must balance it. The steps are shown below:

The balanced equation is: 2H₂O → 2H₂ + O₂

5. So we see that oxygen can be prepared by the electrolysis of water also.

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Chemical reactions in which oxygen is one of the reactants

Reaction of oxygen with other non-metals:
Take a small quantity of sulphur in a spatula and burn it. We will get the smell of sulphur dioxide.

Sulphur dioxide is a gas. It is formed when sulphur is burnt. During the burning, a chemical reaction takes place. The reaction is between sulphur and oxygen. 
■ A process of burning a substance in oxygen is called combustion.

So we see that sulphur and oxygen reacts together to form sulphur dioxide. Let us write the equation:
• One molecule of sulphur is S. One molecule of oxygen is O₂. We will write them on the left side.
• One molecule of sulphur dioxide is SO₂. We will write it on the right side.
• So the skeletal equation is: S + O₂ → SO₂ . This equation is already balanced.

■ Sulphur is a non-metal. So this is an example of the reaction of oxygen with a non-metal. There are more examples in which oxygen reacts with other non-metals:
■ Reaction with carbon: Carbon burns in oxygen to form carbon dioxide. When the burning takes place, the carbon is reacting with oxygen. 
• We will write one molecule each of carbon and oxygen on the left side
• We will write one molecule of the product, which is carbon dioxide, on the right side
• So the skeletal equation is: C + O₂ → CO₂ . This skeletal equation is already balanced.
■ Reaction with hydrogen: Hydrogen and oxygen reacts together to form water. But the reaction can be carried out only in special labs with special equipments. It is an explosive reaction, and so must be carried out only under authorised technical supervision.
• We will write one molecule each of hydrogen and oxygen on the left side
• We will write one molecule of the product, which is water, on the right side
• So the skeletal equation is: H₂ + O₂ → H₂O.
• This is not a balanced equation. The steps for writing the balanced equation is shown below:

• So the balanced equation is: 2H₂ + O₂ → 2H₂O. 
• From the balanced equation, we can see that 2 molecules of hydrogen will be required to react with one molecule of oxygen to produce water. Also note that, the reaction will give two molecules of water as the product.

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In the next section, we will discuss the reaction of oxygen with metals. 

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