Page 173 - Chemistry ICSE Class X

Electrolysis 159
ƒ Electrolyte: #EKFKſGF UQNWVKQP QH EQRRGT UWNRJCVG %W51 ) in water.
4
+
2+
2–
–
The electrolyte solution contains, Cu (aq), SO , H (aq) and OH (aq)
4
ions.
ƒ Electrode reactions: See below.
ƒ Products of electrolysis: At cathode : Oxygen gas
At anode : Copper metal gets deposited
The solution around the anode after electrolysis is found to be acidic.
Fig. 6.8 Electrolysis of copper sulphate
Procedure: A concentrated solution of copper sulphate is taken in a solution using platinum anode
glass/plastic container. It is made acidic by adding some sulphuric acid.
A platinum foil is made anode and a copper plate is made cathode in the
electrolytic cell. The electrical circuit is completed by using a battery and
a key.
2+
2–
2+
The current is carried by Cu and SO in the solution. Cu ions go to
4
cathode and get reduced to metal copper.
2+
–
At cathode: Cu (aq) + 2e o Cu(s)
gets deposited on the cathode tŚĞŶ ĐŽƉƉĞƌ ƐƵůƉŚĂƚĞ ƐŽůƵƟŽŶ ŝƐ
2–
2–
The SO ions move towards anode. The SO ion does not get ĞůĞĐƚƌŽůLJƐĞĚ ƵƐŝŶŐ Ă ƉůĂƟŶƵŵ ĂŶŽĚĞ͕
4
4
–
oxidised easily. So, at anode, oxidation of OH ions (produced by the self- ĂŶĚ ĨŽƌ Ă ůŽŶŐ ƟŵĞ͕ ŝƚƐ ďůƵĞ ĐŽůŽƵƌ
ĨĂĚĞƐ ĂŶĚ ĮŶĂůůLJ ŵĂLJ ďĞĐŽŵĞ ĂůŵŽƐƚ
2–
ionisation of water) takes place in preference to SO . So, the anode reaction ĐŽůŽƵƌůĞƐƐ͘
4
is, dŚƵƐ͕ ĚƵƌŝŶŐ ƚŚĞ ĞůĞĐƚƌŽůLJƐŝƐ ŽĨ
+
–
At anode: 2H O(l) U 2H (aq) + 2OH (aq) Ƶ^K ƐŽůƵƟŽŶ͕ ĂůƚŚŽƵŐŚ ƚŚĞ ĐƵƌƌĞŶƚ
ϰ
2
Ϯн
Ϯʹ
ϰ
–
2OH (l) o H O(l) + 1 2 O (g) + 2e – ŝƐ ĐĂƌƌŝĞĚ ďLJ Ƶ ĂŶĚ ^K ŝŽŶƐ͕
ƚŚĞ ŝŽŶƐ ǁŚŝĐŚ ĂƌĞ ŝŶǀŽůǀĞĚ ŝŶ ƚŚĞ
2
2
Ϯн
ĞůĞĐƚƌŽĚĞ ƌĞĂĐƟŽŶƐ ĂƌĞ Ƶ ĂŶĚ
Net reaction at anode: H O o 2H + + 1 2 O (g) + 2e – K, ͘
ʹ
2
2
Electrolysis of copper sulphate solution using copper
electrodes
ƒ Electrolyte: Acidulated solution of copper sulphate in water.
ƒ Operating temperature: Room temperature to about 50°C.
ƒ Electrolytic cell: Glass/plastic/ceramic bath.
ƒ Electrodes: Copper rods or Copper plates.
2–
+
–
2+
ƒ Ion present in the electrolyte: Cu (aq), SO (aq), H (aq) and OH (aq)
4
ƒ Electrode reactions:
2+
At anode: Cu(s) o Cu (aq) + 2e –
from anode goes into the solution
–
2+
At cathode: Cu (aq) + 2e o Cu(s)
from solution gets deposited
at the cathode
Mechanism of the electrode reaction
2+
+
ƒ Both Cu (aq) and H (aq) migrate towards cathode
2+
and carry the current. The Cu (aq) ions get reduced in
+
preference to H (aq) because copper (Cu) occurs below
hydrogen (H ) in the activity series.
2
–
2–
ƒ Both sulphate (SO ) and hydroxyl (OH ) ions migrate
4
towards anode and carry the current. But none of these ions
is involved in the electrode reaction due to the attackable
nature of the anode material. As a result, copper of the Fig. 6.9 Electrolysis of CuSO solution using copper
4
2+
anode gets oxidised to give Cu ions. electrodes