Electrolysis and Electrode Processes

The chemical reactions, which take place at the surface of electrodes are called electrode reaction (or electrode processes). According to the theory of ionization, electrolytes are present as ions in solution. These ions are directed towards the respective electrodes by the electricity supplied. The electrolytes can be electrolyzed only in the dissolved or molten state. Various types of electrode reactions are described below.

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.

Fig: 9.10 - Electrolysis of molten sodium chloride

Electrolysis of Copper Sulphate Solution

When a solution of copper sulphate 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 SO₄^2-ions.

Current is carried by Cu²⁺ and SO₄^2-in the solution. Cu²⁺ ions go to cathode and get reduced to metal copper.

The SO₄^2-ions move towards anode but is very difficult to get oxidized. So, at anode, oxidation of OH^- ions (produced by the self-ionization of water) takes place in preference to SO₄^2-. So, the anode reaction is,

Thus, during the electrolysis of CuSO₄ solution, although the current is carried by Cu²⁺ and SO₄^2-ions, but the ions, which are involved in the electrode reactions are Cu²⁺ and OH^-. Liberation of H⁺ around the anode makes the solution around it more acidic.

Electrolysis of Aqueous Sodium Chloride Solution

When sodium chloride solution is electrolyzed using platinum electrodes, the products are hydrogen gas at the cathode, and chlorine gas at anode. The mechanism of electrolysis involves the following reactions.

Thus, Na+ and Cl- ions carry the current. Na+ ions go towards cathode and Cl- ions move towards anode.

At anode: Cl- ions get oxidised to give chlorine gas.

Cathode material	Favoured cathode reaction
Platinum	H+ + e - ------> 1/2 H2(g)
Mercury	Na + e - ------> Na

Na+ ions move towards cathode, but do not get reduced (due to its highly negative electrode potential). Instead, H⁺ ions produced due to the self-ionization of water get reduced at cathode.

Na⁺ ions migrated to cathode, produce Na⁺OH^- with OH^- liberated at cathode.

It is important to note that the products of electrolysis also depend upon the material used in making the electrodes. For example, during the electrolysis of aqueous solution of sodium chloride shown earlier, the products obtained are different when different electrodes are used.

Electrolysis of Molten Lead Bromide

A small quantity of solid lead bromide is taken in a silica crucible and two graphite electrodes (may be obtained from used torch cells) are inserted. A battery consisting of two dry cells is connected to the electrodes through a key and an ammeter.

Fig: 9.11 - Electrolysis of molten lead bromide

When the key is pressed, no current flows through the system. This is because solid lead bromide does not conduct electricity. But, when the crucible containing lead bromide is heated the solid lead bromide melts. Now, on pressing the key, electricity flows through the system and a red brown gas (bromine) evolves at the anode and metallic lead deposits at the cathode.

The following reactions occur at the two electrodes:

Thus, electrolysis of lead bromide using graphite electrodes produces lead metal at the cathode and bromine gas at the anode.

Electrolysis of Water

Electrolysis of water is generally carried out in a specially designed glass apparatus.

Fig: 9.12 - Apparatus for electrolyzing acidified water

Pure water does not conduct electricity because water molecule is a covalent molecule and the concentration of free H⁺ and OH^- ions in pure water is very small ( 10^-7mol L^-1). However, by the addition of a small quantity of an acid (preferably H₂SO₄) or an alkali such as NaOH, water can be made conducting. During the passage of electricity through acidulated water, hydrogen gas is evolved at the cathode and oxygen gas at the anode. The volumes of hydrogen and oxygen gases are found to be in the ratio 2 : 1. The reactions at the two electrodes are:

It must be noted here that although the anion (e.g., SO₄^2-) of the acid used for making the water acidulated carries current through the solution, but does not take part in the electrode reaction at the anode. This is because the oxidation of SOions is extremely difficult and before it could occur, the OH^- ions gets, oxidized to give oxygen gas.

Electrolytic Cell V/s Galvanic Cell

The main points of difference between an electrolytic cell and a galvanic cell (electrochemical cell) are:

Difference between an electrolytic cell and a galvanic cell

Electrochemical cell (Galvanic Cell)	Electrolytic cell
A Galvanic cell converts chemical energy into electrical energy.	An electrolytic cell converts electrical energy into chemical energy.
Here, the redox reaction is spontaneous and is responsible for the production of electrical energy.	The redox reaction is not spontaneous and electrical energy has to be supplied to initiate the reaction.
The two half-cells are set up in different containers, being connected through the salt bridge or porous partition.	Both the electrodes are placed in a same container in the solution of molten electrolyte.
Here the anode is negative and cathode is the positive electrode. The reaction at the anode is oxidation and that at the cathode is reduction.	Here, the anode is positive and cathode is the negative electrode. The reaction at the anode is oxidation and that at the cathode is reduction.
The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit.	The external battery supplies the electrons. They enter through the cathode and come out through the anode.

Problem

18. Chromium metal can be plated out from an acidic solution containing CrO₃ according to the following equation,

Calculate:

• How many grams of chromium will be plated out by 24000 C ?

• How long will it take to plate out 1.5 g of chromium by using 12.5 ampere current (Atomic mass of Cr = 52).

Solution

The given reaction corresponds to,

6 F 1 mol

6 x 96,500 C 52 g

(i) Then, 6 x 96,500 C produce 52 g Cr

(ii) For 52 g of Cr one requires 6 x 96,500 C

Then, 12.5 A x t = 16,701.9 C