In the previous section, we completed the discussion on Nitrogen. In this section we will see Hydrogen and Chlorine.
We have completed our present discussion on hydrogen and chlorine. In the next chapter, we will discuss about Acids, Alkalies and Salts.
We have seen that nitrogen is
the most abundant element in the atmosphere. But when we consider the
universe as a whole, hydrogen is the most abundant element. Hydrogen is the main component of Sun and other stars.
■ Hydrogen combines with other
elements to form various compounds. Some of it's important compounds
are:
• Water (H2O), • Sulphuric acid (H2SO4), • Hydrochloric acid (HCl), • Methane (CH4)
etc.,
Preparation of Hydrogen
■ Hydrogen is produced when
metals react with dilute acids. This property can be used to prepare
hydrogen in the lab. The following experiment will demonstrate the
procedure:
Take 5 ml of dilute
hydrochloric acid in a test tube. Add some zinc granules to it. The
zinc will react with hydrochloric acid to produce hydrogen. Let us write the equation:
• It is a reaction between zinc
and hydrochloric acid. One molecule of zinc is Zn. One molecule of
hydrochloric acid is HCl. So we will write it on the left side.
• The products are zinc chloride
and hydrogen. One molecule of zinc chloride is ZnCl2.
One molecule of hydrogen is H2.
We will write them on the right side.
• So the skeletal equation is: Zn
+ HCl →
ZnCl2
+ H2
.
This is not a balanced equation. The steps for balancing the
equation are given below:
So the balanced equation is:
Zn + 2HCl →
ZnCl2
+ H2
.
According to the above
equation, hydrogen will be produced in the test tube. But we have to
detect it's presence in the test tube. For that, bring a glowingspliter, or an incense stick to the mouth of the test tube. The flame
is put out with a pop sound. But the gas burns with the formation of
water droplets on the side of the test tube. A video showing a similar procedure can be seen here.
Another experiment:
Fix a balloon to the mouth of
the test tube in which hydrogen is produced. The hydrogen gas begins
to fill up the balloon. So it swells. Tie the mouth of the balloon
with a thread. Now release the balloon. It will rise high up in the
air. This is because hydrogen is lighter than air.
Water and Heavy water
We know that water is a
combination of oxygen and hydrogen. Now, hydrogen has two isotopes:
Deuterium and Tritium. We have seen the details here. Out of
these two isotopes, Deuterium combines with oxygen to form heavy
water.
• We know that hydrogen has no neutron. But deuterium has one neutron.
• So deuterium is heavier than hydrogen. We can write schematic equations:
• We know that hydrogen has no neutron. But deuterium has one neutron.
• So deuterium is heavier than hydrogen. We can write schematic equations:
♦ Hydrogen + Oxygen → Ordinary
water
♦ Heavy hydrogen (deuterium) +
oxygen → Heavy water
■ The above equations gives us
an idea about why 'Heavy' water is called so. Heavy water is indeed
heavy. It is 10 percent heavier than ordinary water. That is., if a
certain volume of ordinary water weighs 100 gm, the same volume of
heavy water will weigh 110 gm.
The chemical formula of heavy water is D2O. It is present in nature. But only in very small quantities. If we divide a given quantity of water into 6000 equal parts, one part will be heavy water.
The chemical formula of heavy water is D2O. It is present in nature. But only in very small quantities. If we divide a given quantity of water into 6000 equal parts, one part will be heavy water.
Uses of heavy water:
• As a moderator in nuclear
reactors • For the preparation of
deuterium • As a tracer in chemical
reactions
Uses of hydrogen:
• As a fuel • To fill balloons • To
prepare a fuel called 'water gas' • In the manufacture of fuel cells.
Hydrogen as a fuel
We know that, there are many
types of fuels. Some examples are: Methane, Petrol, Ethanol etc.,
Hydrogen is also a fuel. Of all the fuels available to us, hydrogen
has the highest calorific value. That means, if we take the same
quantity of various fuels, hydrogen will give the highest heat energy
value. Hydrogen is present in large quantities. It does not cause any
pollution while burning. Because, the only product of it's combustion
is water.
Though there are many merits,
we can not use hydrogen as a fuel very easily. This is because, it is
not easy to make hydrogen into usable form. It burns explosively in
air. It's storage and distribution is difficult.
Chlorine
Chlorine is an element that
is usually seen in combined form with other elements. This is
because, chlorine is highly reactive. It does not occur free in
nature.
Some important compounds of
chlorine are: NaCl, KCl, HCl, CaCl2
etc.,
Preparation of chlorine
In the laboratory, chlorine is
prepared by heating potassium permanganate (KMnO4)
with concentrated hydrochloric acid (HCl). The balanced equation is:
2KMnO4
+ 16HCl →
2KCl + 2MnCl2
+ 8H2O
+ 5Cl2.
The arrangement for
preparation of chlorine in the lab is shown in fig.5.4 below:
 |
| Fig.5.4 |
• The required quantity of HCl
is added to KMnO4
using a thistle funnel.
• This HCl can vapourise into gaseous state when the heat is applied.
• Also, from the equation, we can see that water is also a product. This will also be in the gaseos state as water vapour.
• So, the gas which comes out of the boiling flask will be a mixture of HCl, water vapour and chlorine gas. We have to separate the chlorine gas from the other two.
• For that, the delivery tube that comes out of the boiling flask is made to go into a conical flask containing water. This water will absorb the HCl.
• The resulting gas that comes out of the conical flask will be a mixture of chlorine gas and water vapour. So it is passed into another conical flask containing concentrated sulphuric acid.
• This sulphuric acid will absorb the water vapour. So the delivery tube that comes out of the second conical flask, will give pure chlorine gas.
• This HCl can vapourise into gaseous state when the heat is applied.
• Also, from the equation, we can see that water is also a product. This will also be in the gaseos state as water vapour.
• So, the gas which comes out of the boiling flask will be a mixture of HCl, water vapour and chlorine gas. We have to separate the chlorine gas from the other two.
• For that, the delivery tube that comes out of the boiling flask is made to go into a conical flask containing water. This water will absorb the HCl.
• The resulting gas that comes out of the conical flask will be a mixture of chlorine gas and water vapour. So it is passed into another conical flask containing concentrated sulphuric acid.
• This sulphuric acid will absorb the water vapour. So the delivery tube that comes out of the second conical flask, will give pure chlorine gas.
Bleaching action of chlorine
Chlorine has the ability to
bleach materials. That is., chlorine can remove colour of materials.
For the bleaching action, presence of moisture is essential. This is
because, water is a reactant in the 'chemical reaction that takes
place during bleaching'.
Place pieces of moistened
colour papers, petals, coloured cloth pieces etc., in a jar
containing chlorine gas. After some time, we will find that, the
colours have been removed from the materials. A video showing the bleaching action can be seen here.
Let us now see how the
bleaching action occurs. A product known as 'bleaching powder' is
available in the market. The main constituent of this powder is
chlorine. In a wet atmosphere, this powder will release chlorine.
This chlorine reacts with water to form hypochlorous acid. This acid
is unstable, and so will decompose to form ClO- ions. These ClO- ions
have the ability to bleach colours.
Uses of Chlorine:
• As a bleaching agent • To
produce bleaching powder • To produce insecticides • To remove stains
from fabrics • In the purification of water.
We have completed our present discussion on hydrogen and chlorine. In the next chapter, we will discuss about Acids, Alkalies and Salts.
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