Q3Utilization Of Electrical Power

Basic Principle of Electrolysis

Electrolysis is the process by which an ionic compound (electrolyte), when dissolved in a suitable solvent (usually water) or in the molten state, is decomposed into its constituent elements or ions by passing a direct current through it via two electrodes immersed in the electrolyte and connected to an external DC source. In the electrolyte, the ionic compound dissociates into positively charged ions (cations) and negatively charged ions (anions); when the external DC field is applied, cations migrate toward the negative electrode (cathode) where they gain electrons and are reduced (deposited or liberated as neutral atoms/molecules), and anions migrate toward the positive electrode (anode) where they lose electrons and are oxidized. The overall current flow through the cell is thus sustained by this directed migration of ions to the electrodes, and the quantity of chemical change (mass of substance liberated or deposited at each electrode) is governed quantitatively by Faraday's laws of electrolysis.

Faraday's first law states that the mass m of a substance liberated or deposited at an electrode is directly proportional to the quantity of electric charge (I times t, current multiplied by time) passed through the electrolyte, the constant of proportionality Z being called the electrochemical equivalent of the substance - the mass deposited per unit charge (typically expressed in grams per ampere-hour or kilograms per coulomb). Faraday's second law states that when the same quantity of charge is passed through different electrolytes connected in series, the masses of different substances liberated at their respective electrodes are directly proportional to their chemical equivalent weights, reflecting the fact that a fixed amount of charge (one Faraday, approximately 96500 coulombs) corresponds to exactly one gram-equivalent of any substance undergoing a single-electron-transfer electrode reaction.

Applications of Electrolysis

• Electroplating: depositing a thin, adherent, uniform coating of a more valuable, corrosion-resistant, or decorative metal (such as chromium, nickel, silver, gold, or zinc) onto a base metal object by making the object the cathode in a bath containing a solution of the coating metal's salt.
• Electro-refining: purifying an impure metal (most commonly copper) by making the impure metal the anode and depositing pure metal onto a thin starter cathode, the impurities either dissolving into the electrolyte or falling to the bottom of the cell as anode mud (from which valuable impurities like silver and gold are separately recovered).
• Electrolytic extraction (electrowinning): extracting a metal directly from its ore or a purified compound solution by electrolysis, used industrially for aluminium (Hall-Heroult process, electrolysis of alumina dissolved in molten cryolite), and for zinc, copper, and other metals extracted from leached ore solutions.
• Electro-forming: producing or reproducing metal articles of intricate shape by electrodepositing metal onto a shaped mandrel or mould (mother form), which is subsequently removed, leaving a precise metallic replica; used for producing gramophone record masters, printing plates, and precision metal mesh and foil components.
• Anodizing: forming a controlled, hard, corrosion-resistant, and often dyeable oxide layer on the surface of a metal (commonly aluminium) by making the metal object the anode in a suitable electrolyte, the oxide layer growing by electrochemical oxidation at the anode surface.
• Production of industrial chemicals: electrolysis of brine (sodium chloride solution) is used industrially to produce chlorine gas, hydrogen gas, and sodium hydroxide (the chlor-alkali process), fundamental to the chemical manufacturing industry.

Terms Used in Electrolytic Processes

• Current efficiency: the ratio of the actual mass of substance deposited or liberated at an electrode to the theoretical mass that should be deposited according to Faraday's law for the same quantity of charge passed, expressed as a percentage; current efficiency is less than 100 percent when some of the current is consumed by competing (parasitic) electrode reactions, such as hydrogen evolution during metal electroplating.
• Energy efficiency: the ratio of the theoretical minimum electrical energy required to bring about a given electrochemical change to the actual electrical energy consumed in the process, accounting for both current efficiency losses and the additional voltage (overvoltage and IR drop in the electrolyte and connections) beyond the theoretical decomposition voltage that must be applied to drive the reaction at a practical rate.
• Throwing power: the ability of an electrolytic bath to deposit a metal coating of reasonably uniform thickness over an object of irregular or complex shape, including recessed areas that are geometrically further from the anode and hence would otherwise receive a thinner deposit due to the non-uniform current density distribution; a bath with good throwing power compensates for this geometric effect through its specific electrolyte composition and conductivity characteristics.
• Electrochemical equivalent: the mass of a substance liberated or deposited at an electrode per unit quantity of electric charge (commonly expressed in grams per ampere-hour), a characteristic constant for each substance and each specific electrode reaction, used directly in Faraday's first law to calculate deposited mass from the current and time of electrolysis.

The industrial success and economic viability of electrolytic processes such as electroplating and electro-refining depend critically on maintaining high current efficiency and adequate throwing power, since low current efficiency directly wastes electrical energy on unwanted side reactions, while poor throwing power leads to uneven, unreliable coatings on components of complex geometry, requiring careful selection of electrolyte composition, additives, current density, and electrode/component geometry in practical bath design.

The decomposition voltage of an electrolytic cell, the minimum applied voltage below which no sustained electrolysis occurs, is also a key practical parameter distinct from the terms defined above: it is determined by the thermodynamic (reversible) potential required to drive the specific electrode reactions in the opposite direction to their spontaneous tendency, together with any additional overvoltage (activation and concentration polarization effects) at each electrode; industrial electrolytic cells are always operated at an applied voltage somewhat above this theoretical decomposition voltage, both to overcome overvoltage effects and to drive the reaction at an economically practical rate, and the gap between the theoretical decomposition voltage and the actual operating voltage is precisely what the energy efficiency figure defined above quantifies.

This closes the requested explanation of the basic principle of electrolysis, its applications, and the definitions of current efficiency, energy efficiency, throwing power, and electrochemical equivalent as used in electrolytic processes.