Faraday's law of electrolysis

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Faraday's law of electrolysis predicts the mass of material that will be deposited at an electrode during electrolysis. It was originally stated as two separate laws by Michael Faraday in 1834.

Contents

[edit] Original form

Faraday's 1st Law of Electrolysis
The mass of a substance produced at an electrode during electrolysis is proportional to the number of moles of electrons (the quantity of electricity) transferred at that electrode.
Faraday's 2nd Law of Electrolysis
The number of Faradays of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion.

[edit] Modern form

In modern form, Faraday's law states:

m \ = \ { Q \over q \ n } \cdot { M \over N_A } \ = \ { 1 \over q \ N_A } \cdot { Q M \over n } \ = \ { 1 \over F } \cdot { Q M \over n } \ = \ { 1 \over 96,485 \ \mathrm{C} \cdot \mathrm{mol^{-1}} } \cdot { Q M \over n }

where

m is the mass of the substance produced at the electrode (in grams),
Q is the total electric charge that passed through the solution (in coulombs),
q is the electron charge = 1.602 x 10-19 coulombs per electron,
n is the valence number of the substance as an ion in solution (electrons per ion),
F = qN_A = 96,485 \ \mathrm{C} \cdot \mathrm{mol^{-1}} is Faraday's constant,
M is the molar mass of the substance (in grams per mole), and
NA is Avogadro's number = 6.022 x 1023 ions per mole.

In practice, the total charge Q is calculated by integrating the electric current I(t) over time t:

Q = \int_0^T I(t) \cdot dt

where T is the total amount of time of the electrolysis.

In the simple case of constant current electrolysis:

Q = I \cdot t

[edit] References

  • Serway, Moses, and Moyer, Modern Physics, third edition (2005).

[edit] See also


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