half-cell

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A half cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally-occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a potential difference between the electrode and the electrolyte. The typical reaction involves a metal atom in the electrode being dissolved and transported as a positive ion across the double layer, causing the electrolyte to acquire a net positive charge while the electrode acquires a net negative charge. The growing potential difference creates an intense electric field within the double layer, and the potential rises in value until the field halts the net charge-pumping reactions.

A standard half cell, used in electrochemistry, consists of a metal electrode in a 1 molar (1 mol/L) aqueous solution of the metal's salt, at 298 kelvin (25 °C). The electrochemical series, which consists of standard electrode potentials and is closely related to the reactivity series, was generated by measuring the difference in potential between the metal half cell in a circuit with a standard hydrogen half cell, connected by a salt bridge.

The half cell of a Daniell cell:

Original equation

Zn + Cu²⁺ → Zn²⁺ + Cu

Half cell of Zn

Zn → Zn²⁺ + 2e⁻

Half cell of Cu

Cu²⁺ + 2e⁻ → Cu

v • d • e Galvanic cells Non-rechargeable: primary cells Alkaline battery | Aluminium battery | Bunsen cell | Chromic acid cell | Clark cell | Daniell cell | Dry cell | Grove cell | Leclanché cell | Lithium battery | Mercury battery | Nickel oxyhydroxide battery | Silver-oxide battery | Weston cell | Zamboni pile | Zinc-air battery | Zinc-carbon battery Rechargeable: secondary cells Air-fueled lithium-ion battery | Lead-acid battery | Lithium-ion battery | Lithium-ion polymer battery | Lithium iron phosphate battery | Lithium sulfur battery | Lithium-titanate battery | Nickel-cadmium battery | Nickel hydrogen battery | Nickel-iron battery | Nickel-metal hydride battery | Nickel-zinc battery | Rechargeable alkaline battery | Sodium-sulfur battery | Vanadium redox battery | Zinc-bromine battery Kinds of cells Battery | Concentration cell | Flow battery | Fuel cell | Trough battery | Voltaic pile Parts of cells Anode | Catalyst | Cathode | Electrolyte | Half cell | Ions | Salt bridge | Semipermeable membrane

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