At the end of the article, you will able to describe – Electrolysis- [ Process, Mechanism, Examples, Uses] of Electrolysis. Let’s start discussing one by one.
The conductors are of two types-
- Metallic Conductors and
- Electrolytic Conductors.
Metallic Conductors – Examples of metallic conductors are copper, silver, aluminum etc. The conduction through them is due to the flow of electrons.
Electrolytic Conductors – Electrolytic Conductors conduct only when they are in a molten or aqueous state. When electrolytic conductors melt to molten or aqueous state there is a formation of ions. As soon as we connect to battery their movement starts. The conduction through the solution is due to the movement of ions. Ions are the carrier of current through solutions. On passing electric current these ions move towards oppositely charged electrodes and thus carry the current. Electrolytic conductance increases with the rise in temperature. Examples are electrolytic solution of acids, bases, and salts.
Must Read: Electrolytes and its Types
|The process of chemical decomposition of an electrolyte (in the dissolved or molten state) by the passage of electric current is known as electrolysis.|
For instance, when an aqueous solution of hydrogen chloride is subjected to electrolysis, hydrogen chloride gets decomposed to form hydrogen and chlorine gases.
The process of electrolysis is carried out in a vessel known as an electrolytic tank or electrolytic cell. The tank which contains either the fused electrolyte or aqueous solution of the electrolyte. Two metallic plates or rods known as electrodes are suspended into the electrolyte. One electrode is connected to a positive terminal and is called anode. The other electrode is connected to the negative terminal and called cathode.
According to Scientist Arrhenius, when an electrolyte is fused or dissolved in water, it dissociates into positively and negatively charged particles called ions.These ions move about here and there in the solution.
Mechanism of Electrolysis
The mechanism starts on passing electric current the ions start moving towards oppositely charged electrodes. The positively charged ions move towards the cathode and are called cations while negatively charged ions move towards the anode and are called anions. The movement of cations and anions is responsible for the electrical conductivity of the solutions of electrolytes.
In addition to electrical conductivity, the movement of the ions also causes transport of matter from one part of the system to another. As the ions reach the respective electrodes, a chemical reaction takes place. Such a reaction that takes place at the electrode between the electrolyte and electrode is called electrochemical reaction. A few electrochemical reactions which may take place at cathode and anode are mentioned below.
(1) Reactions at Cathode. Reaction taking place at cathode is always reduction, which simply involves the gain of electrons by the cation, e.g.,
(2) Reactions at the anode. Reaction taking place at anode is always oxidation which simply involves the loss of electrons by a substance e.g.,
Let us explain the electrolysis by taking examples of molten sodium chloride and aqueous solutions of sodium chloride and copper sulfate as given below.
Electrolysis of Molten Sodium Chloride
Molten sodium chloride contains free moving Na+ and Cl– ions.
NaCl ⇌ Na+ + Cl–
On passing the electric current through the molten sodium chloride, Na+ ions migrate towards the cathode while Cl– ions migrate towards the anode. The following reactions take place as soon as the ions reach their respective electrodes.
- At the cathode: 2Na+ +2e– ⇌ 2Na
- At the anode: 2CI– → 2Cl +2e–
- 2Cl → Cl2
Therefore, the overall reaction is: 2Na+Cl– → 2Na+ Cl2
Thus, sodium metal is deposited at the cathode while chlorine gas is liberated at the anode.
Electrolysis of Aqueous Sodium Chloride Solution
An aqueous solution of sodium chloride contains a large number of sodium ions and chloride ions along with some H+ ions and OH–, ions (which come from the dissociation of water). Thus, there are two types of cations, i.e., Na+ and H+ ions along with two types of anions, i.e., CI– and OH– ions in the aqueous solution of sodium chloride.
On passing an electric current through the aqueous solution of sodium chloride, the following reactions take place:
At the cathode. Both Na+ and H+ ions are attracted by the cathode but neither is actually deposited. On the other hand, water molecules react with the electrons available at the cathode producing hydrogen gas as:
2H2O +2e– → H2+2OH–
It is because the decomposition potential of water is lower than that of Na+ or H+ both.
The discharge potential is defined as the minimum potential that must be applied across the electrodes to bring about the electrolysis and discharge of the ion at the electrode
At the anode. Both CI– and OH– ions are attracted to the anode but it is only Cl– ions which get deposited there as:
2Cl– + 2e → 2Cl2
Therefore, the overall reaction is:
2H2O + 2Cl– → H2 + Cl2 +2OH–
If we include the Na+ ions by adding them to both sides, the overall reaction is:
2Na+2H2O → H2 + Cl2 +2Na+ +2OH–
Since both hydrogen gas and chlorine gas get liberated at their respective electrodes, the residual solution becomes rich in Na+ and OH–, ions. Consequently, the residual solution becomes alkaline. Such a solution on evaporation yields solid sodium hydroxide.
Electrolysis of Copper Sulphate Solution using Copper Electrodes
When copper electrodes are used in electrolysis, the anode is attacked by anions so that it starts dissolving. The process may be represented as follows:
- CuSO4 → Cu2+ + SO42- (Almost complete ionization)
- H2O ⇌ H+ + OH– (Weakly ionized)
At cathode. On passing electric current both Cu2+ and H+ ions move towards the cathode. However, Cu2+ ions having lower discharge potential get liberated at the cathode as:
Cu2+ + 2e – →Cu
At anode. Both OH– and SO42, ions move towards anode but, unlike what happens when platinum electrodes are used. None of the ions get liberated. Rather the copper electrode itself starts dissolving by losing electrons due to its lower discharge potential than that of OH– or SO42, ions. It may be shown as:
Cu → Cu2+ + 2e –
It may, therefore, be noted that in this electrolysis, copper is deposited at cathode from solution and an equivalent amount of copper from the anode dissolves in solution forming Cu2+ ions.
Examples of Electrolysis
|Electrolyte||Electrode||Cathode Reaction||Anode Reaction|
|Silver nitrate||Pt||Ag+ + e– → Ag||2OH– → 1/2 O2 + H2O + 2e–|
|Sodium nitrate||Pt||2H+ + 2e– → H2||2OH– → 1/2 O2 + H2O + 2e–|
|Molten PbBr2||Pt||Pb2+ + 2e– → Pb||2Br– → Br2 + 2e–|
|CuCl2 solution||Pt||Cu2+ + 2e–→ Cu||2Cl– → Cl2 + 2e–|
Uses of Electrolysis
- A large number of chemicals used in industry are obtained by electrolysis. For example, hydrogen and oxygen are manufactured by electrolysis of acidulated water.
- Caustic soda is obtained by the electrolysis of aqueous sodium chloride in suitable cells. Chlorine is obtained as a byproduct.
- Sodium metal is obtained by the electrolysis of either fused caustic soda or fused sodium chloride. Similarly, aluminum is extracted by the electrolysis of bauxite (i.e., Al2O3) in fused cryolite.
- For preservation. Metals like iron are electroplated with tin, nickel or chromium to protect them from rusting.
- For decoration. Cheap jewelry and other fancy articles made of copper are electroplated with gold or silver to enhance their beauty.
- For repairs. Broken parts of machinery are sometimes repaired by electrodeposition of the metal between the broken parts.
This is all about the basics of – Electrolysis- [ Process, Mechanism, Examples, Uses] of Electrolysis.