Page 169 - Chemistry ICSE Class X

Electrolysis 155
Preferential Discharge of Ions at the Electrodes

When two or more ions of the same type are present in the same solution,
they compete for the discharge at the same electrode. Under this condition,
one of them gets preferentially discharged. This behaviour can be predicted
from the relative position of the ions in electrochemical series (or activity
series) for ions. Some general rules based on the electrochemical series are
described in the following section.
What factors influence the preferential discharge of ions
during electrolysis

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electrolysis are:
ƒ Relative position of the ions in the electrochemical series
ƒ Concentration of ions in solution
ƒ Nature of electrode material
The effects of these factors are described below.
General rules based on the electrochemical series

For metal and their cations

ƒ Metals that occur at the top of electrochemical series have a very high
tendency to form their ions, i.e., to get oxidised.
ƒ The tendency to form cations decreases as we go from top to bottom
in the electrochemical series.
ƒ The cations that occur lower in the electrochemical series get
discharged (i.e., get reduced) more easily at the cathode during
electrolysis.
ƒ The tendency to gain electrons (or for reduction) increases as we go
down from top to the bottom in the electrochemical series.
These rules are illustrated below.

ƒ During the electrolysis of aqueous sodium chloride solution, the ions
+
+
+
available around cathode are Na and H . Since H occurs lower than
+
+
sodium ion (Na ) in the electrochemical series for cations, H gets
preferentially discharged (or reduced) at the cathode. KĨ ƚŚĞ ƚǁŽ ĐĂƟŽŶƐ ĐŽŵƉĞƟŶŐ ĨŽƌ
+
ƒ Let us consider a solution containing sodium ions (Na ), hydrogen ƌĞĚƵĐƟŽŶ ;Žƌ ĚŝƐĐŚĂƌŐĞͿ Ăƚ ƚŚĞ
2+
+
ions (H ), and mercuric ion (Hg ). ĐĂƚŚŽĚĞ͕ ƚŚĞ ĐĂƟŽŶ ǁŚŝĐŚ ŽĐĐƵƌƐ
2+
+
+
During electrolysis, H , Na and Hg will migrate towards cathode. lower ŝŶ ƚŚĞ ĞůĞĐƚƌŽĐŚĞŵŝĐĂů ƐĞƌŝĞƐ
ŐĞƚƐ ĚŝƐĐŚĂƌŐĞĚ ƉƌĞĨĞƌĞŶƟĂůůLJ͘
2+
2+
+
+
Since Hg ions appear lower relative to both Na and H , Hg will get
discharged preferentially during electrolysis.
For anions
Of the two anions competing for discharge (or oxidation) at the anode, the
anion which occurs above in the electrochemical series gets discharged
(or oxidised) preferentially.
For example,
–
ƒ The OH ion (hydroxyl ion) gets discharged (or oxidised) at the
–
anode in preference to Cl (chloride ion) because it lies above in the
electrochemical series of the anions.
2–
–
ƒ The OH ion gets preferentially discharged at the anode than SO ion
4
because it lies above in the electrochemical series of the anions.