Page 46 - Chemistry ICSE Class IX

34 ICSE Chemistry – 9
Some other reactions which take place in the presence of light are:
Š Photosynthesis: Green plants make their food from CO and H O
2
2
only in the presence of sunlight.
sunlight
6CO (g) + 6H O(l) o C H O 6 + 6O (g)
6
12
2
2
2

carbon dioxide water chlorophyll glucose oxygen
Š Fading of coloured clothes in sunlight: When bright-coloured
fabrics are spread in the sunlight, the colour fades away slowly.
This is because the dye on the fabric undergoes chemical changes
in the presence of sunlight.
Electricity
Certain reactions take place only when electricity is passed through the
reaction mixture. Such reactions are called electrochemical reactions.
For example, water (acidulated) decomposes to give hydrogen and
oxygen gases when electricity is passed through it.
electricity
2H O(l) o 2H (g) + O (g)
2
2
2
water (acidulated) hydrogen oxygen
Pressure
Certain chemical reactions take place only when pressure is applied.
For example, nitrogen and hydrogen gases react to form ammonia
under a pressure of 200-300 atm.
200-300 atm, 750 K
N (g) + 3H (g) o 2NH (g)
2
3
2
catalyst, promoter
nitrogen hydrogen ammonia
Catalyst
Many chemical reactions require a catalyst to proceed at an observable rate.
A substance which increases the rate of a reaction without itself undergoing
any chemical change is called a positive catalyst or simply as a catalyst.
For example, in the preparation of oxygen gas from potassium chlorate
(KClO ), manganese dioxide (MnO ) acts as a catalyst. This can be seen in
2
3
terms of the reactions given below.
Potassium chlorate (KClO ) when heated alone, decomposes very
3
slowly even at 600°C. In the presence of MnO , KClO decomposes much
3
2
faster even at lower temperature (at 400°C).
600°C
2KClO (s) o 2KCl(s) + 3O (g) (Very slow)
2
3
potassium chlorate oxygen
400°C
2KClO (s) + (MnO ) o 2KCl(s) + 3O (g) + (MnO ) (Fast)
3
2
2
2
potassium manganese oxygen
chlorate dioxide (catalyst)
There are certain catalysts which slow down reactions. Such catalysts
are called negative catalysts. For example, glycerine slows down the rate
of decomposition of hydrogen peroxide (H O ) without taking part in the
2
2
reaction. So, glycerine acts as a negative catalyst.
Some other reactions in which catalyst increases the rate of reaction are:
ƒ Manufacture of ammonia by Haber’s process: In Haber’s process
for the manufacture of ammonia from nitrogen and hydrogen, nely
divided iron is used as catalyst. A small quantity of molybdenum
(Mo) is also added to improve the ef ciency of catalyst. Here,
molybdenum (Mo) acts as the promoter.