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.
In the next section, we will discuss about Hydrogen.
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 110o
C, that of B be 125o
C and C be 145o
C . That means:
♦ at 110o
C, liquid A will begin to boil, and turn into gaseous state.
♦ at 125o
C, liquid B will begin to boil and turn into gaseous state.
♦ at 145o
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
145o
C. When the mixture moves further up, the surrounding temperature
will be lower than 145o
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 125o
C. When the mixture moves further up, the surrounding temperature
will be lower than 125o
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 110o
C. When it moves further up, the surrounding temperature will be
lower than 110o
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.
■ Now we will see how fractional distillation is used to separate nitrogen from liquefied Air:
• First, air is cooled to very low temperatures.
• At -183o
C, oxygen liquefies.
• When it is further cooled, at -196o
C, nitrogen liquefies.
• At -200o
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.
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:
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.
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.
■ 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
In the next section, we will discuss about Hydrogen.
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