Page 54 - Chemistry ICSE Class IX
P. 54
42 ICSE Chemistry – 9
What is an endothermic reaction
A reaction which proceeds with the absorption of heat is called an
endothermic reaction (endo – in and thermic – heat). Some typical
endothermic reactions are:
Reaction between nitrogen and oxygen. Nitrogen reacts with oxygen
at about 3000°C to form nitric oxide (NO).
3000°C
N (g) + O (g) + Heat o 2NO(g)
2
2
Methane is the principal nitrogen oxygen nitric oxide
constituent of natural gas
This is one of the major reactions which take place in the atmosphere
when lightning strikes.
Preparation of water gas. When steam [H O(v)] is passed over red
2
hot coke, water gas (CO + H ) is formed. This reaction is also an
2
endothermic reaction.
C(s) + H O(v) + Heat o CO(g) + H (g)
2
2
red hot coke steam water gas
Some other endothermic processes are:
Dissolution of ammonium chloride (NH Cl) in water
4
Exothermic oxidation reaction - Conversion of water to steam.
coal (carbon) burning
Reactions in Which Electrical Energy is Either
Absorbed or Evolved — Electrochemical Reactions
The reactions which occur with the absorption or liberation of electrical
energy are called electrochemical reactions.
For example,
Electrolysis of acidulated water. When electricity is passed through
acidulated water, it splits up to give hydrogen (at cathode) and
oxygen (at anode).
electric current
2H O(l) o 2H (g) + O (g)
2
2
2
acidulated hydrogen gas oxygen gas
water (at cathode) (at anode)
This reaction is called the electrolysis of water. In this reaction,
electrical energy is used up (or absorbed) to cause the splitting of
water into hydrogen and oxygen.
Working of a Daniell cell. The Daniell cell is a device which produces
electrical energy due to a chemical reaction. Such reactions in which
electrical energy is released are called electrochemical reactions.
A Daniell cell consists of
A zinc rod dipped into dilute sulphuric acid taken in a porous pot.
Zinc rod acts as the cathode.
A cylindrical copper container containing copper sulphate
solution. The copper container acts as the anode.
The chemical reaction that takes place in the Daniell cell is,
Zn(s) + CuSO (aq) o ZnSO (aq) + Cu(s)
4
4
Under standard conditions, Daniell cell produces electrical energy at
a voltage of 1.1 volts.
