Electrolysis

Electrolysis is the process of decomposing an electrolyte by passing a direct electric current (DC) through its aqueous solution or molten state. During this process, electrical energy is converted into chemical energy, driving a non-spontaneous redox reaction (Δ G > 0).

Mechanism of an Electrolytic Cell

An electrolytic cell consists of a single vessel containing an electrolyte and two electrodes connected to an external DC power source (such as a battery).

• The Anode: Connected to the positive terminal of the battery. It possesses a positive charge. Oxidation (loss of electrons) always takes place at the anode.
• The Cathode: Connected to the negative terminal of the battery. It possesses a negative charge. Reduction (gain of electrons) always takes place at the cathode.
• Ionic Movement: When the current is switched on, cations (positively charged ions) migrate toward the negative cathode, while anions (negatively charged ions) migrate toward the positive anode.

Preferential Discharge Theory

When an aqueous solution of an electrolyte contains more than one type of cation or anion, all ions do not discharge at the electrodes simultaneously. Instead, one ion is preferentially discharged based on its position in the electrochemical series.

Competition at the Cathode

The cation with a higher standard reduction potential (lower in the electrochemical/reactivity series) is reduced preferentially because it has a greater tendency to accept electrons.

• Example: In an aqueous solution containing both H^+ and Na^+ ions, H^+ ions are preferentially reduced to hydrogen gas because E°_H^+/H2 (0.00 V) > E°_Na^+/Na (-2.71 V).

Competition at the Anode

The anion with a lower standard reduction potential (higher oxidation potential, or ease of losing electrons) is oxidized preferentially.

• Standard Order of Anion Discharge: I^- > Br^- > Cl^- > OH^- > NO₃^- > SO₄^2-.
• Exception (Overpotential): Concentrated chloride solutions preferentially evolve chlorine gas over oxygen gas, despite oxygen having a lower theoretical discharge potential, due to a kinetic hindrance known as overvoltage.

Quantitative Aspects: Faraday’s Laws of Electrolysis

Michael Faraday formulated two fundamental quantitative laws that govern the mass of substances liberated during electrolysis.

Faraday’s First Law

The mass (m) of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity (Q) passed through the electrolyte.

Since Q = I · t (where I is current in Amperes and t is time in seconds): Where z is the Electrochemical Equivalent (ECE) of the substance, defined as the mass of the substance deposited by a current of 1 Ampere flowing for 1 second.

Faraday’s Second Law

When the same quantity of electricity is passed through several electrolytic solutions connected in series, the masses (m₁, m₂) of the substances liberated at the electrodes are directly proportional to their chemical equivalent weights (E₁, E₂).

The chemical equivalent weight is calculated as: Equivalent Weight (E) = Atomic Mass/Valency.

Core Industrial Applications of Electrolysis

Electrolysis is the backbone of several heavy metallurgical and chemical manufacturing industries globally.

Process / Application	Electrolyte Used	Products Obtained
Production of Green Hydrogen	Water acidified with dilute H2SO4 or alkaline water.	Hydrogen gas at the Cathode; Oxygen gas at the Anode.
Down’s Process	Molten Sodium Chloride (NaCl) mixed with CaCl2.	Sodium metal at the Cathode; Chlorine gas at the Anode.
Hall-Héroult Process	Molten Alumina (Al2O3) dissolved in molten Cryolite (Na3AlF6).	Pure Aluminium metal at the carbon-lined Cathode.
Electrorefining of Copper	Aqueous Copper Sulphate (CuSO4) acidified with H2SO4.	Pure Copper deposits on the Cathode; impurities drop as “Anode Mud”.
Chlor-Alkali Industry	Concentrated aqueous Sodium Chloride (Brine solution).	Hydrogen gas (Cathode), Chlorine gas (Anode), Sodium Hydroxide (NaOH).

Electroplating and Electrorefining

Electroplating

Electroplating is the process of coating a superior metal onto an inferior metal surface using an electric current to prevent corrosion or for decorative purposes.

• The article to be plated is always made the Cathode.
• The pure coating metal (e.g., Gold, Silver, Chromium) is made the Anode.
• The electrolyte must be a soluble salt solution of the coating metal.

Electrorefining

This process purifies crude metals. A thick block of impure metal acts as the anode, while a thin strip of pure metal acts as the cathode. As current passes, metal ions from the anode dissolve into the electrolyte and deposit in pure form on the cathode. Valuable unreactive impurities (like Gold and Platinum) do not dissolve and settle below the anode as anode mud.

High-Yield Trivia for Civil Services Prelims

• What is 1 Faraday? One Faraday (F) is the total electrical charge carried by one mole of electrons. It is approximately equal to 96,487 Coulombs (rounded to 96,500 C for calculations).
• The Role of Cryolite: In the extraction of aluminium, pure alumina (Al₂O₃) has an extremely high melting point (≈ 2050°C) and is a poor electrical conductor. Adding Cryolite (Na₃AlF₆) and Fluorspar (CaF₂) lowers the melting point to about 950°C and enhances electrical conductivity, saving massive amounts of industrial energy.
• Anode Mud Composition: In copper electrorefining, the anode mud often contains precious metals like Silver, Gold, Selenium, and Platinum. The recovery of these byproducts frequently covers the entire operational cost of the electrolysis plant.

Last Modified: May 26, 2026