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Mechanism and Criteria of Product Formation in Electrolytic Cells

The process of electrolysis can be explained on the basis of the theory of ionisation. When an electrolyte is dissolved in water, it splits up into charged particles called ions. The positively charged ions are called cations while the negatively charged ions are called anions. The ions are free to move about in aqueous solution. When electric current is passed through the solution, the ions respond to the applied potential difference and their movement is directed towards the oppositely charged electrodes. The cations move towards the negatively charged electrode while anions move towards the positively charged electrode. The formation of products at the respective electrodes is due to oxidation (loss of electrons) at the anode and reduction (gain of electrons) at the cathode.

For example, when electricity is passed through the molten sodium chloride [NaCl)], sodium is deposited at the cathode while C~ gas is liberated at the anode. The process can be represented as:

Similarly, electrolysis of molten lead bromide [PbBr2(l)] produces lead at the cathode and Br2 at the anode.

In case there is a possibility of formation of more than one products at the electrodes, or there is a competition between the liberation of ions at the electrodes, then the products formed depends upon the following factors in general:

• Position of ion/species in the electromotive series

• Concentration of ion/species

• Nature of electrodes.

One of the dominant factors which control the criterion of product formation at the electrodes is the values of electrode potentials of the species. Let us understand it as follows:

(a) At the Cathode. Cathode involves reduction process at its surface. Therefore, for the different competing reduction processes, the one with higher reduction potential, will preferably take place. For example, during the electrolysis of aqueous solution of sodium chloride there is possibility of following reactions at the cathode:

The reduction of water will preferably take place at the cathode because E red of water is higher. Hence, the product of electrolysis of aqueous solution of NaCl at the cathode will be H2 gas instead of Na(s).

Similarly, during electrolysis of aqueous solution of Copper (II) tetraoxosulphate (VI) reduction of Cu2+ ions will take place at the cathode in preference to the reduction of 0 molecules because E2 is greater than E2

(b) At the Anode. Anode involves oxidation process at its surface. Therefore, for different competing oxidation processes, the one with higher oxidation potential (or lower reduction potential) will preferably occur. For example, if we carry out electrolysis of aqueous solution of copper(II) tetraoxosulphate(VI), the competing oxidation processes at the anode are as follows:

As oxidation potential of water is higher, the product formed at the anode will be O2 gas instead of S2O8O 2- ion.

Similarly, if the electrolysis of copper sulphate solution is carried out using copper electrodes, then the process occurring at the anode will be oxidation of copper atoms to copper ions instead of oxidation of water because oxidation potential of Cu is higher.

Thus, in such a case copper from anode will go on dissolving into solution as Cu2+ ions while Cu2+ ions from solution will go on depositing at the cathode as copper atoms.

The above discussion leads us to a general conclusion that for different competing reactions at the electrodes:

It may be noted that in some cases the unexpected results are obtained due to overvoltage*. For example, let us compare the oxidation potentials of CI- ion and water

Although oxidation potential of H2O is more than that of CI- ions, yet during the electrolysis of concentrated solution of sodium chloride, the chloride ions oxidise in preference to H2O molecules at the anode giving Cl2 gas as the product.

 

EXAMPLE OF ELECTROLYSIS

In the light of above discussion let us discuss some examples of electrolytic reactions. Before we take up the actual examples, it is quite important to know about inert electrodes and active electrodes.

Inert electrodes are those electrode which do not gain or lose electrons during the process. Two common examples are carbon (graphite) electrode and platinum electrodes.

Active electrodes are those electrodes which themselves participate in the gain or loss of electrons.

1. Electrolysis of molten lead (IT) bromide

Electrodes: Inert electrodes

Electrolyte: Molten lead (II) bromide Molten lead (II) bromide ionises as

Reaction at anode (oxidation of Br-)

2. Electrolysis of concentration solution sodium chloride (Brine solution)

Electrodes: Inert electrodes

Electrolyte: Sodium chloride dissolved in water solvent.

NaCl ionises in aqueous solution as

NaCl (l) à Na + + Cl

Cathode reaction: Reduction of H2O occurs in preference to Na+ ions as Na+ is much higher in electromotive series

2H2O (l) - à H2 (g) + 2OH (aq)

Anode reaction: Oxidation of CI- ions occur in preference to H2O because of higher concentration of CI- ions and overvoltage

2Cl (aq) à Cl2 (g) + 2e

Thus, Cl2 gas is produced at anode and H2 gas is liberated at cathode. The pH of solution increases due to increase in the concentration of OH ions in solution.


3. Electrolysis of very dilute solution of sodium chloride

Electrode: Inert electrodes.

Electrolyte: Very dilute brine (NaCl) solution.

NaCl ionises as

Cathode reaction: Reduction of water occurs in preference to Na+ ions

4H2O (l) + 4e – à 2H2 (g) + 4 OH – (aq)

Here, H2 is liberated at cathode whereas O2 is liberated at the anode. The pH of solution does not change due to equal consumption of H+ and OH ions. However, concentration of NaCl gradually increase due to decomposition of water.

 

4. Electrolysis of aqueous solution of copper (II) tetraoxosulphate (VI) solution: using platinum electrodes.

Electrodes: Inert electrodes (Pt).

Electrolyte: CuSO 4 dissolved in water CuSO 4 ionises in aqueous solution as

Cathode reaction: eu2+ undergoes reduction in preference to H2O because H2O lies above Cu2+ in electrochemical series

Cu2+ (aq) + 2e – à Cu (s)

Thus, copper is deposited at cathode and 0 2 is liberated at anode. The pH of the solution decreases gradually due to increase in concentration of H+ ions. Blue colour of the solution gradually fades due to decrease in concentration of eu2+ ions.

 

5. Electrolysis of aqueous solution of copper(II) tetraoxosulphate(VI) solution using copper electrodes

Electrodes: Active electrodes.

Electrolyte: CuSO4 dissolved in water.

CuSO4 ionises in aqueous solution as:

Cathode reaction: Cu2+ ions undergo reduction in preference ~0 molecules

eu2+ (aq) + 2e- à7 Cu (s)

Anode reaction: Cu atoms from the copper electrode get oxidised in preference to ~0 because Cu lies below Hp in activity series.

Cu (s) à Cu2+ (aq)

Thus, copper dissolves from anode and get deposited at cathode. Mass of cathode increases and that of anode decreases. Blue colour of the solution does not fade because concentration of Cu2+ ions in solution remains unaltered. The pH of the solution also does not change as the electrolysis proceeds.