Engineering Chemistry S Datta

224 ENGINEERING CHEMISTRY
Otherwise oxygen is liberated from water. If the anode is a metal (Cu, or Ag), atoms of
electrode turn into ions.
Cu(s) ⎯⎯→ Cu 2+ (aq) + 2e.
Q. 9. What is the effect of concentration of a salt solution during electrolysis?
Ans. The concentration of a salt solution has an effect on the product of electrolysis at
the anode. Chlorine is liberated at the anode during electrolysis of a concentrated solution of
NaCl. In electrolysis of a dilute solution of NaCl increasing amounts of oxygen are liberated.
Q. 10. (a) Draw a neat sketch of an electrolytic cell.
(b) Sodium is extracted by the electrolysis of a mixture of a molten NaCl
and CaCl 2
. In a commercial cell the current is 30,000 A. What mass of
Na is formed in an hour at the steel cathode?
Ans. (a)
– +
Positive ions gain
electrons at the
cathode (reduction)
and turn into
atoms
Cathode
–ve
ions
+ve
ions
Anode
Negative ions lose
electrons at the
anode (oxidation)
Electrolyte
Fig. 9.11. Electrolytic cell.
(b) The equation for liberation at the cathode is
Na + + e – ⎯⎯→ Na
Time of electrolysis
= 3600 sec.
Quantity of electricity in an hour = 30000 C/S × 3600 s.
= 108,000,000 C.
Amount of Na formed per hr. = 108,000,000 mol
96,480
108, 000,
000
Mass of Na formed per hr. = × 23 = 25,750 g
96,
480
= 25.75 kg.
Q. 11. What is the utility of electrolysis of brine?
Ans. Electrolysis of brine is the basis of the alkali and chlorine industry—a well known
electrochemical industry. Brine is a solution of NaCl in water. Electrolysis of brine is used
to manufacture chlorine, hydrogen and sodium hydroxide. During electrolysis chlorine is
liberated at the anode, hydrogen is liberated at the cathode and bubbles off, while the solution
turns to NaOH.