Half-cell

{{Short description|Structure in electrochemistry}} {{Use American English|date = April 2019}} [[File:Standard electrode potential of zinc.PNG|thumb|Diagram of the standard hydrogen electrode (SHE); Galvanic cell made of two half-cells, with zinc cathode (left) and platinum anode (right)]] In electrochemistry, 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 anode 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. This self-limiting action occurs almost instantly in an isolated half-cell; in applications two dissimilar half-cells are appropriately connected to constitute a Galvanic cell.

A standard half-cell consists of a metal electrode in an aqueous solution where the concentration of the metal ions is 1 molar (1&nbsp;mol/L) at 298 kelvins (25&nbsp;°C).<ref>{{Cite web |title=An Introduction to Redox Equilibria and Electrode Potentials |url=https://www.chemguide.co.uk/physical/redoxeqia/introduction.html |access-date=2024-02-11 |website=www.chemguide.co.uk|date=May 2013|last=Clark|first=Jim}}</ref> In the case of the standard hydrogen electrode (SHE), a platinum electrode is used and is immersed in an acidic solution where the concentration of hydrogen ions is 1M, with hydrogen gas at 1atm being bubbled through solution.<ref>{{Cite web |title=R07 Table showing several standard reduction potentials |url=https://old.iupac.org/didac/Didac%20Eng/Didac03/Content/R07.htm |access-date=2024-02-11 |website=old.iupac.org}}</ref> 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 standard hydrogen half-cell: :2H⁺(aq) + 2e⁻ → H₂(g)

The half-cells of a Daniell cell: :Original equation ::Zn + Cu²⁺ → Zn²⁺ + Cu :Half-cell (anode) of Zn ::Zn → Zn²⁺ + 2e⁻ :Half-cell (cathode) of Cu ::Cu²⁺ + 2e⁻ → Cu

== See also == * Standard electrode potential (data page)

==References== {{Reflist}}

{{Electrolysis}} {{Galvanic cells}}

Category:Electrochemistry Category:Electrochemical cells

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