Half-cell

In <a href="/facts/Electrochemistry/C0RUWpBH">electrochemistry</a>, a half-cell is a structure that contains a conductive <a href="/facts/Electrode/251PaKX8">electrode</a> and a surrounding conductive <a href="/facts/Electrolyte/4YE8bIVa">electrolyte</a> separated by a naturally occurring <a href="/facts/Helmholtz_double_layer/QhTTkyVs">Helmholtz double layer</a>. Chemical reactions within this layer momentarily pump <a href="/facts/Electric_charge/xxsBfmlB">electric charges</a> between the electrode and the electrolyte, resulting in a <a href="/facts/Potential_difference/EN2po6N2">potential difference</a> 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 <a href="/facts/Electric_field/tt4MooBs">electric field</a> 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 <a href="/facts/Galvanic_cell/a4V7CQC8">Galvanic cell</a>.
A standard half-cell consists of a metal electrode in an <a href="/facts/Aqueous_solution/eRNr3MH2">aqueous solution</a> where the concentration of the metal ions is 1 <a href="/facts/Concentration/wqBmz5od">molar</a> (1 mol/L) at 298 <a href="/facts/Kelvin/E3trzC4C">kelvins</a> (25 °C). In the case of the <a href="/facts/Standard_hydrogen_electrode/c6mksrr1">standard hydrogen electrode (SHE)</a>, 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<a href="/facts/Standard_atmosphere_(unit)/aTbYALII">atm</a> being bubbled through solution. The <a href="/facts/Electrochemical_series/SNnf8sw7">electrochemical series</a>, which consists of <a href="/facts/Standard_reduction_potential/PqpNUgSl">standard electrode potentials</a> and is closely related to the <a href="/facts/Reactivity_series/kbv5QP1Y">reactivity series</a>, 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 <a href="/facts/Salt_bridge/z6qzSx5h">salt bridge</a>.
The standard hydrogen half-cell:

2H+(aq) + 2e− → H2(g)
The half-cells of a <a href="/facts/Daniell_cell/VuHXBmBT">Daniell cell</a>:

Original equation
Zn + Cu2+ → Zn2+ + Cu
Half-cell (<a href="/facts/Anode/TjYt95zG">anode</a>) of Zn
Zn → Zn2+ + 2e−
Half-cell (<a href="/facts/Cathode/tqdlGayU">cathode</a>) of Cu
Cu2+ + 2e− → Cu

Half-cell open-in-new

Half-cell