Page 166 - Chemistry ICSE Class X

152 ICSE Chemistry – 10
The process of decomposition of an electrolyte with the help of
ƵƌŝŶŐ ĞůĞĐƚƌŽůLJƐŝƐ͕ ĞůĞĐƚƌŝĐĂů ĞŶĞƌŐLJ ŝƐ
ĐŽŶǀĞƌƚĞĚ ŝŶƚŽ ĐŚĞŵŝĐĂů ĞŶĞƌŐLJ͘ electricity is called electrolysis.
(QT GZCORNG YJGP GNGEVTKEKV[ KU RCUUGF VJTQWIJ CEKFKſGF YCVGT WUKPI
ͻഩ dŚĞ ĞůĞĐƚƌŽĚĞ ĐŽŶŶĞĐƚĞĚ ƚŽ ƚŚĞ
ƉŽƐŝƟǀĞ ƚĞƌŵŝŶĂů ŽĨ ƚŚĞ ďĂƩĞƌLJ platinum electrodes, it decomposes into hydrogen and oxygen.
electricity
ŐĞƚƐ ƉŽƐŝƟǀĞůLJ ĐŚĂƌŐĞĚ ĂŶĚ ŝƐ ĐĂůůĞĚ 2H O(l) o 2H (g) + O (g)
ĂŶŽĚĞ͘ 2 2 2
CEKFKſGF YCVGT J[FTQIGP ICU QZ[IGP ICU
ͻഩ dŚĞ ĞůĞĐƚƌŽĚĞ ĐŽŶŶĞĐƚĞĚ ƚŽ ƚŚĞ (at cathode) (at anode)
ŶĞŐĂƟǀĞ ƚĞƌŵŝŶĂů ŐĞƚƐ ŶĞŐĂƟǀĞůLJ The chemical equation given above describes the electrolysis of water.
ĐŚĂƌŐĞĚ ĂŶĚ ŝƐ ĐĂůůĞĚ ĐĂƚŚŽĚĞ͘
Why is water acidified before electrolysis
Pure water does not conduct electricity. So, a small quantity of dilute
sulphuric acid is added to make it conducting due to the presence of free
+
2–
H and SO ions. This permits the passage of electricity through water
4
and causes electrolysis of water.
How is electrolysis carried out
Electrolysis is carried out in an apparatus called electrolytic cell. An
electrolytic cell consists of a wide glass or plastic container and two
electrodes. Electrodes of material such as copper, carbon, silver, platinum,
etc. are commonly used.
The electrolytic solution is poured into the electrolytic cell, and the
two electrodes are placed into the solution. Then, one of the electrodes is
connected to the positive terminal, and the other to the negative terminal
of a battery through a key, and a battery bulb. The battery supplies direct
–
–
+
–
X o ½ X + e M + e o M
2
(oxidation at anode) (reduction at cathode) current (DC).
Fig. 6.4 Electrolysis of an electrolyte: 9JGP VJG MG[ KU KPUGTVGF VJG EWTTGPV UVCTVU ƀQYKPI VJTQWIJ VJG
When electrical potential is applied, the electrolytic solution and the bulb starts glowing. At this stage, cations
cations move towards cathode starts moving towards cathode, and anions towards anode.
(–ve electrode), and anions move towards
anode (+ve electrode). On reaching the At cathode, a cation gains one or more electrons from the electrode
surface of the respective electrode, these and gets reduced to form an atom. At anode, an anion loses one or more
ions get discharged.
electrons and gets oxidised (Fig. 6.4).
The products of electrolysis either get deposited on the electrode
surface or get evolved as a gas.
What is the mechanism of electrolysis
When electricity is passed through an electrolytic solution, the following
process occur:
ƒ The ions present in the solution/molten electrolyte start moving
towards respective electrodes.
^ŽŵĞƟŵĞƐ͕ ĂŶŝŽŶƐ ĂƌĞ ŶŽƚ ĞĂƐŝůLJ Š The cations (+ve ions) move towards cathode (the –vely charged
ŽdžŝĚŝƐĞĚ Ăƚ ƚŚĞ ĂŶŽĚĞ͘ /Ŷ ƐƵĐŚ ĐĂƐĞƐ͕ electrode).
ƚŚĞ ĞůĞĐƚƌŽĚĞ ŵĂƚĞƌŝĂů Žƌ ƚŚĞ ƐŽůǀĞŶƚ Š The anions (–ve ions) move towards anode (the +vely charged
ŝƚƐĞůĨ ŵĂLJ ŐĞƚ ŽdžŝĚŝƐĞĚ͘
electrode).
ƒ These ions get discharged at the respective electrode by losing or
gaining electrons.
At cathode: The cations gain electrons from the cathode and get
reduced.
–
M + + e o M (Reduction)
cation electron
reduction
At anode: The anions lose electrons to the electrode and get oxidised.