Faraday constant

Not to be confused with farad.

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

96485.33289(59) C mol−1.[2]

The constant F has a simple relation to two other physical constants:

F\,=\,eN_{A}

where

e ≈ 1.6021766×10−19 C;[3]
NA ≈ 6.022141×1023 mol−1.[4]

NA is the Avogadro constant (the ratio of the number of particles, N, which is unitless, to the amount of substance, n, in units of moles), and e is the elementary charge or the magnitude of the charge of an electron. This relation is true because the amount of charge of a mole of electrons is equal to the amount of charge in one electron multiplied by the number of electrons in a mole.

One common use of the faraday constant is electrolysis. One can divide the amount of charge in coulombs by the Faraday constant in order to find the 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] Research is continuing into more accurate ways of determining the interrelated constants F, NA, and e.

Other common units of Faraday's constant

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. 96485.33289(59) 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).

Popular media

The Simpsons episode "Dark Knight Court" (RABF10) 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 says to Smithers, "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. 1 2 "CODATA Value: Faraday constant". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 2015-09-25.
  3. ↑ "CODATA Value: elementary charge". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 2015-09-22.
  4. ↑ "CODATA Value: Avogadro constant". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 2015-09-25.
  5. ↑ NIST Introduction to physical constants
  6. ↑ Foundations Of Physics, Volume 2, by R. S. Gambhir, p51
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