About Electrolysis
Electrolysis is the process of driving a non-spontaneous chemical reaction using electrical energy. When current passes through an electrolyte solution, positive ions (cations) migrate to the negatively charged cathode and are reduced, while negative ions (anions) migrate to the anode and are oxidized. Faraday's laws of electrolysis quantitatively link the amount of substance deposited or dissolved to the charge passed.
In the electrolysis of water, the cathode reaction produces hydrogen gas (2H₂O + 2e⁻ → H₂ + 2OH⁻) and the anode reaction produces oxygen gas (2H₂O → O₂ + 4H⁺ + 4e⁻). Electrolysis of brine (sodium chloride solution) instead produces chlorine gas at the anode and sodium hydroxide solution, forming the industrial basis of the chlor-alkali process. The required applied voltage must exceed the decomposition voltage plus overpotential losses.
This simulator visualizes ion migration, bubble formation at electrodes, and the progressive change in electrolyte composition over time. It demonstrates Faraday's first law (mass ∝ charge) and second law (mass ∝ molar mass / valence), providing intuition for industrial electrochemical processes including electroplating, aluminum smelting, and green hydrogen production.
Frequently Asked Questions
What is Faraday's first law of electrolysis?
Faraday's first law states that the mass of substance deposited or liberated at an electrode is directly proportional to the quantity of electric charge passed. Mathematically, m = (Q × M) / (n × F), where Q is charge in coulombs, M is molar mass, n is the number of electrons transferred per ion, and F is Faraday's constant (96,485 C/mol). Passing more current for longer produces more product.
Why does water electrolysis require more than 1.23 V?
The thermodynamic decomposition voltage for water is 1.23 V, derived from the Gibbs free energy of the reaction. In practice, overpotentials at both electrodes (especially the oxygen evolution overpotential), ohmic losses in the electrolyte, and contact resistances add up to require 1.8–2.0 V or more. Catalysts like platinum or iridium oxide reduce overpotentials, improving efficiency.
What determines whether a substance is reduced at the cathode?
The electrode reaction that occurs is determined by standard reduction potentials. Species with the most positive reduction potential are reduced preferentially. In a brine solution, although Na⁺ is present, water is reduced to H₂ instead because H₂O/H₂ has a higher reduction potential than Na⁺/Na under typical conditions. Temperature, concentration, and electrode material all influence the actual reaction.
How is electrolysis used in industry?
Electrolysis underpins several major industries. The chlor-alkali process electrolyzes brine to produce chlorine and sodium hydroxide, essential for plastics and paper production. Hall-Héroult electrolysis of alumina in molten cryolite produces nearly all the world's aluminum. Electroplating deposits thin metal coatings onto objects for protection or aesthetics. Proton exchange membrane electrolyzers are increasingly used to produce green hydrogen from renewable electricity.
What is the difference between electrolytic and galvanic cells?
A galvanic (voltaic) cell converts chemical energy to electrical energy spontaneously—the reaction is thermodynamically favorable and drives current through an external circuit. An electrolytic cell does the opposite: external electrical energy forces a non-spontaneous reaction to occur. Batteries discharge as galvanic cells and recharge as electrolytic cells using the same electrochemical principles in reverse.