electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...

, a half-cell is a structure that contains a conductive

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

and a surrounding conductive

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

separated by a naturally occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

s between the electrode and the electrolyte, resulting in a

potential difference Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge ...

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 An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

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 galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous oxidation–reduction reactions. An example of a ...

. A standard half-cell consists of a metal electrode in an

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...

where the concentration of the metal ions is 1 molar (1 mol/L) at 298

kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

s (25 °C). 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 1 atm being bubbled through solution. The electrochemical series, which consists of standard electrode potentials and is closely related to the

reactivity series In chemistry, a reactivity series (or reactivity series of elements) is an empirical, calculated, and structurally analytical progression of a series of metals, arranged by their "reactivity" from highest to lowest. It is used to summarize inform ...

, 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 In electrochemistry, a salt bridge or ion bridge is an essential laboratory device discovered over 100 years ago. It contains an electrolyte solution, typically an inert solution, used to connect the Redox, oxidation and reduction Half cell, ...

. The standard hydrogen half-cell: :2H⁺(aq) + 2e⁻ → H₂(g) The half-cells of a

Daniell cell The Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenw ...

: :Original equation ::Zn + Cu²⁺ → Zn²⁺ + Cu :Half-cell (

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

) of Zn ::Zn → Zn²⁺ + 2e⁻ :Half-cell (

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

) of Cu ::Cu²⁺ + 2e⁻ → Cu

References

Electrochemistry Electrochemical cells {{electrochem-stub

See also

References