View source for Faraday constant

{{Short description|Physical constant: Electric charge of one mole of electrons}}
{{distinguish|farad}}
{{Infobox physical quantity
| name = Faraday constant
| othernames =
| width =
| background =
| image = Michael_Faraday_(1791-1867).jpg
| caption = [[Michael Faraday]], the constant's namesake
| unit = coulomb per mole (C/mol)
| otherunits =
| symbols = {{math|''F''}}
| baseunits = s⋅A⋅mol<sup>−1</sup>
| dimension = wikidata
| extensive =
| intensive =
| conserved =
| transformsas =
| derivations = {{math|1=''F'' = ''eN''<sub>A</sub>}}
| value = {{val|96,485.3321233100184 | u=C/mol}}
}}
In [[physical chemistry]], the '''Faraday constant''' (symbol {{math|''F''}}, sometimes stylized as ℱ) is a [[physical constant]] defined as the quotient of the total [[electric charge]] ({{math|''q''}}) by the [[amount of substance|amount]] ({{math|''n''}}) of [[elementary charge carrier]]s in any given sample of matter: {{nowrap|{{math|''F''}} {{=}} {{math|''q''/''n''}};}} it is expressed in [[unit of measurement|units]] of [[coulomb]]s per [[mole (unit)|mole]] (C/mol). 
As such, it represents the "[[molar quantity|molar]] [[elementary charge]]",<ref name="SI2019" /> that is, the electric charge of one mole of elementary carriers (e.g., protons).  It is named after the English [[scientist]] [[Michael Faraday]]. Since the [[2019 revision of the SI]],<ref name="SI2019">{{Cite book |last=Newell |first=David B. |url=https://www.nist.gov/si-redefinition/meet-constants |title=The International System of Units (SI) |last2=Tiesinga |first2=Eite |publisher=National Institute of Standards and Technology |year=2019 |series=NIST Special Publication 330 |location=Gaithersburg, Maryland |doi=10.6028/nist.sp.330-2019 |doi-access=free |s2cid=242934226}}</ref> the Faraday constant has an exactly defined value, the product of the elementary charge ({{math|''e''}}, in coulombs) and the [[Avogadro constant]] ({{math|''N''<sub>A</sub>}}, in reciprocal moles):
: {{math|1=''F'' = ''e'' × ''N''<sub>A</sub> = {{val|9.64853321233100184|e=4|u=C/mol}}.}}

== Derivation ==
The Faraday constant can be thought of as the [[Proportionality (mathematics)|proportionality factor]] between the charge in [[coulomb|coulombs]] (used in physics and in practical electrical measurements) and the [[amount of substance]] in [[Mole (unit)|moles]] (used in chemistry), and is therefore of particular use in [[electrochemistry]], particularly in [[electrolysis]] calculations. Because the elementary charge is exactly {{Physical constants|e|symbol=no|ref=no}},<ref name="SI2019" /> and there are exactly {{Physical constants|NA|symbol=yes|unit=no|ref=no}} entities per mole,<ref name="SI2019" /> the Faraday constant is given by the product of these two quantities:
: {{math|1=''F'' = ''e'' × ''N''<sub>A</sub>}}
: {{math|1=&ensp; = {{val|1.602176634|e=-19|u=C}} × {{val|6.02214076|e=23|u=mol-1}}}}
: {{math|1=&ensp; = {{val|9.64853321233100184|e=4|u=C/mol}}.}}

The value of {{math|''F''}} was first determined in the 1800s by weighing the amount of [[silver]] deposited in an electrochemical reaction, in which a measured [[Current (electricity)|current]] was passed for a measured time, and using [[Faraday's law of electrolysis]].<ref>[http://physics.nist.gov/cuu/Constants/historical1.html NIST Introduction to physical constants]</ref> 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]].<ref name="IUPAC">{{Cite journal |last=IUPAC |author-link=International Union of Pure and Applied Chemistry |year=1976 |title=Status of the Faraday constant as an analytical standard |journal=Pure and Applied Chemistry |volume=45 |issue=2 |pages=125–130 |doi=10.1351/pac197645020125 |doi-access=free}}</ref>

== 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]]. Its use is much less common than of the [[coulomb]], but is sometimes used in electrochemistry.<ref>[https://books.google.com/books?id=r-Qpy0KQayIC&pg=PA51 ''Foundations of Physics, Volume 2'', by R. S. Gambhir, 1993, p. 51]</ref> One faraday of charge is the charge of one [[Mole (unit)|mole]] of [[elementary charge]]s (or of negative one mole of electrons), that is,
: 1&nbsp;faraday&nbsp;=&nbsp;''F''&nbsp;×&nbsp;1&nbsp;mol&nbsp;=&nbsp;{{Val|9.64853321233100184E4|u=C}} =&nbsp;''N''<sub>0</sub>&nbsp;×&nbsp;''e''&nbsp;=&nbsp;{{Val|6.02214076E23|u=''e''}}.

Where ''N''<sub>0</sub> is [[Avogadro's number]], the unitless counterpart to ''N''<sub>A</sub>. Conversely, the Faraday constant ''F'' equals 1 faraday per mole.

The faraday is not to be confused with the [[farad]], an unrelated unit of [[capacitance]] ({{nowrap|1=1 farad = 1 coulomb / 1 volt}}).

== See also ==
* [[Farad]], the unit of [[electrical capacitance]]
* [[Faraday efficiency]]
* [[Faraday's laws of electrolysis]]
* [[Faraday cup]]

== References ==
{{reflist}}

{{Scientists whose names are used in physical constants}}
{{Michael Faraday}}
{{Mole concepts}}
[[Category:Electrochemical concepts]]
[[Category:Physical constants]]
[[Category:Michael Faraday]]
[[Category:Units of electrical charge]]
[[Category:Molar quantities]]
[[Category:Quotients]]