Chemistry SS 3 Week: 7

Topic: Preferential Discharge of Ions during Electrolysis

There are certain factors that affect the discharge of electrons at electrodes.

Factors Affecting the Preferential Discharge of Ions in Electrolysis

The position of ion in electrochemical series.

The concentration of ion.

The nature of electrode.

Relationship between position of ions in the electrochemical series and the order of
discharge of ions.

a) For cations
Electrochemical series of cations:

K+   ——–> Least easy to discharge
Na+
Ca2+

Mg2+
Al3+
Zn2+
Fe2+

Sn2+
Pb2+
H+
Cu2+
Hg2+

Ag+    ———–> Most easy to discharge

 The cation has a lower position in the electrochemical series are preferentially discharged.

b) For anions

 Electrochemical series of anions

 OH   ——> Most easy to discharge
I
 Br
 Cl
  NO3
SO42-  ———> Least easy to discharge

 The anion has a higher position in the electrochemical series are preferentially discharged.

Concentration of the ions in the electrolyte
Irrespective of the position of the ions in the electrochemical series, there is a tendency to promote the discharge of the most concentrated ion present. For example, in concentrated sodium chloride solution (i.e. brine), the two cations present are the chlorine ion and the hydroxyl ion. Although the hydroxyl ion is more easily oxidized than the chlorine ion, it is the chlorine ion which will be discharged because its concentration is much greater than that of the hydroxyl ion. However, if the concentration of a particular ion is high, then this can alter the preferential discharge.
Also, if dilute hydrochloric acid is electrolyzed, hydrogen gas is produced at the cathode and oxygen gas at the anode. However, when concentrated hydrochloric acid is electrolyzed, hydrogen gas is still liberated at the cathode, but chlorine gas (and NOT oxygen gas) is now produced at the anode instead. The higher concentration of the chloride ions favour their discharge over the hydroxide ions

Nature of the Electrode: This is not as important as either of the other two factors, except in certain cases. For example in the electrolysis of molten sodium chloride using a mercury cathode, sodium ions are discharged in preference to hydrogen ions which are lower in the series.

Electrolysis of Specified Electrolytes

Electrolysis of Molten Sodium Chloride
When molten sodium chloride is electrolyzed the products obtained are, sodium metal at cathode and chlorine gas at anode. The electrolysis of molten sodium chloride is as follows:

These ions carry the current, Na+ ions move towards cathode, and Cl ions move towards anode. On reaching the respective electrodes, these ions get involved in electrode reactions as follows;

Electrolysis of molten sodium chloride

Electrolysis of Copper (II) Tetraoxosulphate (VI) Solution
When a solution of copper(II) tetraoxosulphate (VI) is electrolyzed using platinum electrodes, the products of electrolysis are copper at cathode and oxygen gas at anode. After electrolysis the solution around the anode is found to contain sulphuric acid. This can be explained as follows:

The electrolyte CuSO4 when dissolved in water dissociates to give Cu2+ and SO42-ions.