Electrolysis: Definition of Electrolysis, Mechanism of electrolysis, Electrolysis of molten lead bromide, Electrolysis of using electrode, Practice Problems, and FAQs

Are the process of extraction of metals from earth crust is same for all elements? Well, different metals have different processes of extraction based on their reactivity. Calcination and roasting are not applicable in the case of highly reactive metals such as Na and K as they react vigorously with oxygen. Hence, a different process was invented which is Electrolysis.

It’s very important to understand the process and mechanism of electrolysis as it has a wide range of applications, apart from extraction of active metals in pure state. You must have seen gold plating on many fancy ornaments and utensils.

Let’s understand, what are the different types and techniques of electrolysis along with their examples.

Table of Content

• Definition of Electrolysis
• Types of Electrolysis
• Metallurgy
• Electrolytic Refining of Metals
• Electroplating
• Application of Electrolysis.
• Practice Problems
• Frequently Asked Questions

Definition of Electrolysis

• A compound that is present in the form of freely movable ions upon dissolution or in a molten state is an electrolyte. Electrolysis is the process of decomposing an electrolyte by passing direct current through it in either a molten state or an aqueous solution.
• Ionic compounds must be subjected to a direct electric current while they are fluid in order to disintegrate into their constituent parts.
• The cations are reduced at the cathode, while the anions are oxidised at the anode. The essential elements required to perform electrolysis are an electrolyte, electrodes, and a power source. The use of a salt bridge or an ion-exchange membrane as a partition is optional. These are primarily utilised to prevent product diffusion in close proximity to the opposing electrode.
• The processing unit used to carry out the electrolysis process is called an electrolytic cell. It consists of

1. Vessel in which the electrolyte is in the form of an aqueous solution.
2. Electrodes:

• Anode: A metal rod that is dipped in the solution and connected to the positive end of the battery is called anode.
• Cathode: A metal rod that is dipped in the solution and connected to the negative end of the battery is called cathode.

Types of Electrolysis

There are different types of electrolysis depending on the effect itis used for like,

1. Electrometallurgy
2. Electrolytic refining of metals
3. Electroplating and
4. Electrosynthesis of chemicals.

Electrometallurgy

A process used to extract metals in their pure form from its molten solutions with the help of electricity is referred to as electrometallurgy. Minerals are made up of soil, rocks, limestone, sand, and metal compounds. Commercial metals are easily and cheaply extracted from minerals. Ores are the name for these minerals. Flux is a substance that is added to the furnace charge to remove the gangue (impurities). Purification of metals and the creation of alloys are the subjects of metallurgy. Metallurgy is the collective term for the various procedures used to extract metals from their ores and refine them for use.

The different steps in the metal extraction or metallurgical process are as follows:

• The ore is crushed and ground.
• Ore enrichment, ore concentration, is another name for this process.
• extraction of metal from concentrated ore.
• Metals are extracted from the solutions. Aluminum is for example is extracted from its oxide by mixing it with cryolite and electrolsing

Electrolytic refining of metals

By using the electrolytic process, a metal (most commonly copper) is refined through electrolytic refining. When it comes to the mechanism of the process, electrolysis involves using a large slab or chunk of impure metal as the anode and a thin strip of pure metal as the cathode. Depending on the metal, an electrolyte (metal salt aqueous solution) is frequently used in this setup.

Example:

Copper sulphate and water ionizes as under

$CuS{O}_4 \left(aq\right)\to C{u}^{2+}\left(aq\right)+ S{O_4}^{2-}$ (almost completely ionised)

$H^{2}O \left(l\right) \rightleftharpoons H^{+}\left(aq\right)+ O{H}^{-} \left(aq\right)$ (only weakly ionised)

On passing electricity, Cu²⁺ and H⁺ move towards cathode while OH^- and SO₄^2- ions move towards anode.

At cathode:

$C{u}^{2+}+ 2e^{-} \to Cu \left(s\right)$ (reduction, primary change)

Thus, copper is deposited on the cathode.

At anode:

$Cu \left(s\right) \to C{u}^{2+}+ 2e^{-}$(oxidation, primary change)

Anode itself undergoes oxidation to form Cu2+ions that go into the solution.

Electroplating

Electroplating is essentially the hydrolysis-based plating of one metal onto another, usually to stop corrosion of the metal or for aesthetic reasons. In order to create a lean coherent metal coating on the electrode, the process reduces dissolved metal cations using an electric current. When anions are electrically oxidised on a solid substrate, such as when silver chloride is formed on silver wire to create silver chloride electrodes, electroplating is frequently used.

Example:

• Let's use a gold coating as an example to better grasp the idea. In this case, metallic jewelry is to have a layer of gold electrodeposited on it to improve its appearance.

• Typically, the gold plating is attached to the circuit's anode (the positively charged electrode), while the jewelry is kept at the cathode (-ve charged electrode). Both are maintained submerged in an advanced electrolytic bat (solution). The anode is now given a DC current, which oxidises the gold atoms and dissolves them in the solution.
• At the cathode, the gold dissolved ions are reduced and applied to the jewelry as plating.

Application of Electrolysis.

• Preparation of hydrogen by electrolysis of water.
• Electrolytic extraction of metals like Na, K, Mg, Ca, Al.
• Preparation of chlorine by electrolysis of aqueous NaCl solution.
• Manufacture of heavy water D₂O, persulphates
• Electroplating and electrolytic refining of metals.

Practice Problems

Q 1. Which of the following is responsible for the conduction of electricity in electrolyte?

A. Holes
B. Ions
C. electrons
D. None of the above

Answer: (B)

Solution: A substance that segregates into ions when dissolved in water or aqueous medium (particles with electrical charges)is called an electrolyte. The charge carriers in an electrolyte are ions.

Q 2. Which among the following is not an electrolyte?

A. CH₃COOK
B. Glucose
C. KCl
D. NaOH

Answer: (B)

Solution: Although glucose, also known as sugar, readily dissolves in water, it is regarded as a nonelectrolyte because it does not separate into ions inside the solution.

Q 3. What are the products obtained when molten NaCl undergoes electrolysis?

A. Sodium gas and chlorine metal
B. Only Sodium metal
C. Sodium metal and chlorine gas
D. Only chlorine gas

Answer: (C)

Solution: Sodium metal and chlorine gas are called product of electrolysis ​of molten NaCl.​

During the electrolysis of molten NaCl following half cell reaction occurs

Cathode: $N{a}^{+}\left(aq\right)+e^{-}\to Na \left(aq\right)$

Anode: $2C{l}^{-}\left(aq\right)\to C{l}_2 \left(g\right)+2e^{-}$

Overall reaction

$2N{a}^{+}\left(aq\right)+2C{l}^{-}\left(aq\right)\to 2Na \left(aq\right)+C{l}_2 \left(g\right)$

Q 4. The positive ions move towards the cathode and the negative ions move towards the anode when an electric current is passed through a cell containing an electrolyte. What will happen if the cathode is removed from the solution? ​

A. The ions that are positive and negative will travel to the anode.
B. The ions, both positive and negative will begin to move at random.
C. While the negative ions stop moving, the positive ions begin to move in the direction of the anode.
D. None of the above

Answer: (B)

Solution: The positive ions move towards the cathode and the negative ions move towards the anode when an electric current is passed through a cell containing an electrolyte. The circuit will not be complete if the cathode is removed from the solution, which will cause the ions to move randomly because they lack direction.

Frequently asked questions (FAQs)

Q 1. What theory underlies the electrolysis process?
Answer: Electrolysis may be defined as a process of decomposition of an electrolyte (a compound which on dissolution or in molten state dissociates into ions) in a molten state or in an aqueous solution by the passage of direct current. The mechanism of electrolysis is based on Ionization theory.

Q 2. Why does NaCl only conduct electricity when it is molten state instead of solid state?
Answer: NaCl is an ionic compound. It doesn’t conduct electricity in its solid form because the ions Na⁺ and Cl^- have a fixed position in solid-state but in molten state, these ions become mobile and can conduct electricity.

Q 3. Is there a difference in how metallic and ionic substances conduct electricity?
Answer: In metallic substances, the flow of electricity is due to the flow of electrons while in the case of ionic substances in the aqueous solution or molten state, the flow of electricity is due to the flow of ions.

Q 4. What are examples of metals that can be purified using electrolysis?
Answer: The principle of electrolysis is widely used in electroplating and purification of less reactive metals such as Pb, Cu, Ag, etc. The impure metal is made the anode that dissolves on passing current and pure metal is deposited at the cathode.