Page 177 - Chemistry ICSE Class X

Electrolysis 163
that they look more attractive and beautiful (QT GZCORNG C ƀQYGT XCUG
of brass when electroplated with silver or gold looks more decorative or ĂƚŚƌŽŽŵ ĮƫŶŐƐ ;ƚĂƉƐ͕ ĞƚĐ͘Ϳ ĂƌĞ
attractive. ŵĂĚĞ ĨƌŽŵ ďƌĂƐƐ͘ ƌĂƐƐ ůŽƐĞƐ ŝƚƐ
ƐŚŝŶĞ ǀĞƌLJ ƐŽŽŶ͘ ^ŽŵĞƟŵĞƐ͕ ďƌĂƐƐ
ŽďũĞĐƚƐ ĂƌĞ ĨŽƵŶĚ ƚŽ ŚĂǀĞ Ă ŐƌĞĞŶŝƐŚ
For protection against corrosion ĚĞƉŽƐŝƚ ŽŶ ƚŚĞŵ͘ ƌĂƐƐ ŽďũĞĐƚƐ
More reactive metals tend to get rusted or corroded. To protect the objects ĂƌĞ ƉƌŽƚĞĐƚĞĚ ĂŐĂŝŶƐƚ ĐŽƌƌŽƐŝŽŶ ďLJ
made of such reactive metals, these are electroplated with less reactive ĐŚƌŽŵŝƵŵ ĞůĞĐƚƌŽƉůĂƟŶŐ͘
metals. For example, iron can be protected by electroplating it with nickel.

For repairing finer machine parts
Finer (small and precisely made) parts of certain machines cannot be repaired
by ordinary methods involving welding, etc. Such parts are repaired by
depositing the desired metal at the proper location electrolytically.

How is electroplating done

Cleaning and washing of the article/object
6JG CTVKENG QDLGEV VQ DG GNGEVTQRNCVGF KU ſTUV YCUJGF YKVJ CP CNMCNK UQNWVKQP
to remove any grease from its surface. Then, it is washed with dilute acid
solution to remove any oxide layer from its surface. The object is then
thoroughly washed in running water until it is free from the acid.

Setting up the equipment
The cleaned and washed object is made cathode in an electrolytic tank
containing the electrolyte (a solution of a salt of the metal to be deposited.)

A sheet of pure metal (to be electroplated) is made anode
With the passage of electric current (direct current), the anode goes on
dissolving, and the metal ions from the solution get deposited on the object
in the form of a thin layer of metal.
To illustrate the process of electroplating, we describe the electroplating
of silver and nickel.
How is electroplating of silver done

For silver plating, the following materials/apparatus are required:
ƒ Electrolyte.
Š Silver nitrate (AgNO UQNWVKQP CEKFKſGF YKVJ C UOCNN SWCPVKV[ QH
3
nitric acid (HNO ) is used as the electrolyte for silver electroplating.
3
Š Solution of sodium argentocyanide Na[Ag(CN) ]
2
ƒ Electrodes. Cathode: The object to be electroplated with silver.
Anode: A plate of pure silver.
ƒ Electrolytic cell. A glass/plastic or ceramic container ^ŽĚŝƵŵ ĂƌŐĞŶƚŽĐLJĂŶŝĚĞ Žƌ WŽƚĂƐƐŝƵŵ
ĂƌŐĞŶƚŽĐLJĂŶŝĚĞ ĐĂŶ ĂůƐŽ ďĞ ƵƐĞĚ
–
+
Š Electrode reactions. Cathode: Ag (aq) + e o Ag(s) ĂƐ ƚŚĞ ĞůĞĐƚƌŽůLJƚĞ ĨŽƌ ĞůĞĐƚƌŽƉůĂƟŶŐ
+
Anode: Ag(s) o Ag (aq) + e – ƐŝůǀĞƌ͘ Ƶƚ ƚŚŝƐ ŝŶǀŽůǀĞƐ ƚŚĞ ƵƐĞ
+
+
ƒ Ions which carry the current. Ag and H ions migrate towards ŽĨ ƐŽĚŝƵŵ ĐLJĂŶŝĚĞ Žƌ ƉŽƚĂƐƐŝƵŵ
cathode and carry current in that direction. ĐLJĂŶŝĚĞ ǁŚŝĐŚ ĂƌĞ ŚŝŐŚůLJ ƉŽŝƐŽŶŽƵƐ͘
dŚĞƌĞĨŽƌĞ͕ ƚŚĞƐĞ ĞůĞĐƚƌŽůLJƚĞƐ ƐŚŽƵůĚ
–
–
The NO (nitrate ion) and OH (hydroxyl ion, in very minute quantity) ďĞ ƵƐĞĚ ƵŶĚĞƌ ŽŶůLJ ƉƌŽĨĞƐƐŝŽŶĂů
3
migrate towards anode and carry the current in that direction. ƐƵƉĞƌǀŝƐŝŽŶ͘
ƒ Ions which take part in the electrode reaction.
+
Š At cathode : Ag ion.
+
Š At anode : Silver of the electrode get oxidised to form Ag .