Faraday constant

The Faraday constant, denoted by the symbol F and sometimes stylized as ℱ, is named after Michael Faraday. In chemistry and physics, this constant represents the magnitude of electric charge per mole of electrons.[1] It has the currently accepted value

F = 96485.33212... C mol−1.[2]
Not to be confused with farad.

Since 1 mol electrons = 6.022 140 76 x 1023 electrons (Avogadro's number),[3] the Faraday constant is equal to the elementary charge e, the magnitude of the charge of an electron:[4]

F = 96 485.3... C/(1 mol) = 96 485.3... C /(6.022...x 1023) = 1.602 176 634 10-19 C = e

One common use of the Faraday constant is in electrolysis calculations. One can divide the amount of charge in coulombs by the Faraday constant in order to find the chemical amount (in moles) of the element that has been oxidized.

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.[5]

2019 redefinition

Since the 2019 redefinition of SI base units, which introduced exactly defined values for the elementary charge and the mole, the Faraday constant is exactly

e x (1 mol) mol-1 = 1.602176634×10−19 C × (6.02214076×1023 ) mol−1 = 96485.3321233100184 C mol−1.

Other common units
  • 96 485 J (96.485 kJ) per volt gram equivalent
  • 23.061 kcal per volt gram equivalent
  • 26.801 A·h/mol

Faraday unit of charge

Related to Faraday's constant is the "faraday", a unit of electrical charge. It is much less common than the coulomb, but sometimes used in electrochemistry.[6] One faraday of charge is the magnitude of the charge of one mole of electrons, i.e. 96485.33212... C.[2]

Expressed in faradays, the Faraday constant F equals "1 faraday of charge per mole".

This faraday unit 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. The term "magnitude" is used in the sense of "absolute value": The charge of an electron is negative, but F is always defined to be positive.
  2. "2018 CODATA Value: Faraday constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  3. Brown, L.; Holme, T. (2011) Chemistry for Engineering Students, Brooks/Cole.
  4. Schmidt-Rohr, K. (2020). "Analysis of Two Definitions of the Mole That Are in Simultaneous Use, and Their Surprising Consequences” J. Chem. Educ. 97: 597-602. http://dx.doi.org/10.1021/acs.jchemed.9b00467
  5. NIST Introduction to physical constants
  6. Foundations Of Physics, Volume 2, by R. S. Gambhir, 1993, p. 51
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