Faraday constant

In physical chemistry, the Faraday constant, denoted by the symbol F and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of SI base units,[2] which took effect on 20 May 2019, the Faraday constant has the exactly defined value given by the product of the elementary charge e and Avogadro constant NA:

F = e × NA
  = 1.602176634×10−19 C × 6.02214076×1023 mol−1
  = 9.64853321233100184×104 C⋅mol−1.
Not to be confused with farad.
Faraday constant
Michael Faraday, the constant's namesake
DefinitionThe electric charge of one mole of elementary charges
SymbolF
Value9.648533212...×104 C⋅mol−1[1]

Derivation

The Faraday constant can be thought of as the conversion factor between the mole (used in chemistry) and the coulomb (used in physics and in practical electrical measurements), and is therefore of particular use in electrochemistry. Because 1 mole contains exactly 6.02214076×1023 entities,[2] and 1 coulomb contains exactly 1/e = 1019/1.602176634 elementary charges,[2] the Faraday constant is given by the quotient of these two quantities:

F = NA/1/e = 9.64853321233100184×104 C⋅mol−1.

One common use of the Faraday constant is in electrolysis calculations. One can divide the amount of charge (the current integrated over time) by the Faraday constant in order to find the chemical amount of a substance (in moles) that has been electrolyzed.

The value of F was first determined by weighing the amount of silver deposited in an electrochemical reaction in which a measured current was passed for a measured time, and using Faraday's law of electrolysis.[3]

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. It is much less common than the coulomb, but is sometimes used in electrochemistry.[4] One faraday of charge is the magnitude of the charge of one mole of electrons, i.e.,

1 faraday = F × 1 mol = 9.648533212...×104 C.

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 (1 farad = 1 coulomb / 1 volt).

The Simpsons episode "Dark Knight Court" has Mr. Burns asking Comic Book Guy how much he wants for his entire comic book inventory. He says "the speed of light expressed as dollars" and Mr. Burns tells Smithers to "just give him Faraday's Constant". The check is written for $96,485.34.

See also

References
  1. "2018 CODATA Value: Faraday constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  2. Newell, David B.; Tiesinga, Eite (2019). The International System of Units (SI). NIST Special Publication 330. Gaithersburg, Maryland: National Institute of Standards and Technology. doi:10.6028/nist.sp.330-2019. S2CID 242934226.
  3. NIST Introduction to physical constants
  4. Foundations of Physics, Volume 2, by R. S. Gambhir, 1993, p. 51
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