Half cell

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 kelvins (25 °C). The electrochemical series, which consists of standard electrode potentials and is closely related to the reactivity series, was generated by measuring the 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: Zn + Cu+2 &rarr; Zn+2 + Cu   :Original equation Zn &rarr; Zn+2 + 2e−    :Half cell of Zn          Cu+² + 2e− &rarr; Cu     :Half cell of Cu

Halbzelle