View source for Half-cell

{{Short description|Structure in electrochemistry}}
{{Use American English|date = April 2019}}
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 charge]]s 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 [[concentration|molar]] (1&nbsp;mol/L) at 298 [[kelvin]]s (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}}</ref> In the case of the [[Standard hydrogen electrode|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 1[[Standard atmosphere (unit)|atm]] being bubbled through solution.<ref>{{Cite web |title=Untitled Document |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 reduction potential|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<sup>+</sup>(aq) + 2e<sup>−</sup> → H<sub>2</sub>(g)
The half-cells of a [[Daniell cell]]:
:Original equation
::Zn + Cu<sup>2+</sup> → Zn<sup>2+</sup> + Cu
:Half-cell ([[anode]]) of Zn
::Zn → Zn<sup>2+</sup> + 2e<sup>−</sup>
:Half-cell ([[cathode]]) of Cu
::Cu<sup>2+</sup> + 2e<sup>−</sup> → Cu
== See also ==
* [[Standard electrode potential (data page)]]
==References==
{{Reflist}}
{{Electrolysis}}
{{Galvanic cells}}
[[Category:Electrochemistry]]
[[Category:Electrochemical cells]]
{{electrochem-stub}}