Faraday's laws of electrolysis

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Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833.[1][2] [3]

First law[edit]

Michael Faraday reported that the mass () of elements deposited at an electrode is directly proportional to the charge ( in ampere seconds or coulombs).[3]

Here, the constant of proportionality is called the electro-chemical equivalent (e.c.e) of the substance. Thus, the e.c.e. can be defined as the mass of the substance deposited/liberated per unit charge.

Second law[edit]

Faraday discovered that when the same amount of electric current is passed through different electrolytes/elements connected in series, the mass of the substance liberated/deposited at the electrodes in g is directly proportional to their chemical equivalent/equivalent weight ().[3] This turns out to be the molar mass () divided by the valence ()

(From 1st Law)

Derivation[edit]

A monovalent ion requires 1 electron for discharge, a divalent ion requires 2 electrons for discharge and so on. Thus, if electrons flow, atoms are discharged.

So the mass discharged

where is the Avogadro constant, Q = xe, and is the Faraday constant.

Mathematical form[edit]

Faraday's laws can be summarized by

where is the molar mass of the substance (in grams per mol) and is the valency of the ions .

For Faraday's first law, , , and are constants, so that the larger the value of the larger m will be.

For Faraday's second law, , , and are constants, so that the larger the value of (equivalent weight) the larger m will be.

In the simple case of constant-current electrolysis, leading to

and then to

where:

  • n is the amount of substance ("number of moles") liberated: n = m/M
  • t is the total time the constant current was applied.

For the case of an alloy whose constituents have different valencies, we have

where wi represents the mass fraction of the i-th element.

In the more complicated case of a variable electric current, the total charge Q is the electric current I() integrated over time :

Here t is the total electrolysis time.[4]

See also[edit]

References[edit]

  1. ^ Faraday, Michael (1834). "On Electrical Decomposition". Philosophical Transactions of the Royal Society. 124: 77–122. doi:10.1098/rstl.1834.0008. S2CID 116224057.
  2. ^ Ehl, Rosemary Gene; Ihde, Aaron (1954). "Faraday's Electrochemical Laws and the Determination of Equivalent Weights". Journal of Chemical Education. 31 (May): 226–232. Bibcode:1954JChEd..31..226E. doi:10.1021/ed031p226.
  3. ^ a b c "Faraday's laws of electrolysis | chemistry". Encyclopedia Britannica. Retrieved 2020-09-01.
  4. ^ For a similar treatment, see Strong, F. C. (1961). "Faraday's Laws in One Equation". Journal of Chemical Education. 38 (2): 98. Bibcode:1961JChEd..38...98S. doi:10.1021/ed038p98.

Further reading[edit]