physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mech ...

, the Faraday constant (symbol , sometimes stylized as ℱ) is a

physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally. It is distinct from a mathematical constant, which has a ...

defined as the quotient of the total

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 ...

() by the amount () of elementary charge carriers in any given sample of matter: it is expressed in

units Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

s per mole (C/mol). As such, it represents the " molar

elementary charge The elementary charge, usually denoted by , is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single electron, ...

", that is, the electric charge of one mole of elementary carriers (e.g., protons). It is named after the English

scientist A scientist is a person who Scientific method, researches to advance knowledge in an Branches of science, area of the natural sciences. In classical antiquity, there was no real ancient analog of a modern scientist. Instead, philosophers engag ...

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...

. Since the

2019 revision of the SI In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Effective 20 May 2019, the 144th ...

, the Faraday constant has an exactly defined value, the product of the elementary charge (, in coulombs) and the

Avogadro constant The Avogadro constant, commonly denoted or , is an SI defining constant with an exact value of when expressed in reciprocal moles. It defines the ratio of the number of constituent particles to the amount of substance in a sample, where th ...

(, in reciprocal moles): :

Derivation

The Faraday constant can be thought of as the proportionality factor between the charge in coulombs (used in physics and in practical electrical measurements) and the

amount of substance In chemistry, the amount of substance (symbol ) in a given sample of matter is defined as a ratio () between the particle number, number of elementary entities () and the Avogadro constant (). The unit of amount of substance in the International ...

in moles (used in chemistry), and is therefore of particular use in

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 ...

, particularly in

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

calculations. Because the elementary charge is exactly , and there are exactly entities per mole, the Faraday constant is given by the product of these two quantities: : : : The value of was first determined in the 1800s by weighing the amount of

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

deposited in an electrochemical reaction, in which a measured current was passed for a measured time, and using Faraday's law of electrolysis. Until about 1970, the most reliable value of the Faraday constant was determined by a related method of electro-dissolving silver metal in

perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. It is an oxoacid of chlorine. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxid ...

Other common units

* 96.485 kJ per volt–gram-equivalent * 23.061 kcal per volt–gram-equivalent * 26.801 A·h/mol

Faraday – a unit of charge

Related to the Faraday constant is the "faraday", a unit of

electrical charge Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

. Its use is much less common than of the

, but is sometimes used in electrochemistry.''Foundations of Physics, Volume 2'', by R. S. Gambhir, 1993, p. 51
/ref> One faraday of charge is the charge of one mole of

s (or of negative one mole of electrons), that is, : 1 faraday = ''F'' × 1 mol = = . Conversely, the Faraday constant ''F'' equals 1 faraday per mole. The faraday is not to be confused with the

farad The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units, International System of Units (SI), equivalent to 1 coulomb per volt (C/V). It is named afte ...

, an unrelated unit of

capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...

().

References

{{Mole concepts Electrochemical concepts Physical constants Michael Faraday Units of electrical charge Molar quantities Quotients

Derivation

Other common units

Faraday – a unit of charge

See also

References