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Concept
Faraday's laws of electrolysis
Summary
Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833.
Michael Faraday reported that the mass (m) of elements deposited at an electrode is directly proportional to the charge (Q; SI units are ampere seconds or coulombs).
Here, the constant of proportionality, Z, is called the electro-chemical equivalent (ECE) of the substance. Thus, the ECE can be defined as the mass of the substance deposited/liberated per unit charge.
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 is directly proportional to their chemical equivalent/equivalent weight (E). This turns out to be the molar mass (M) divided by the valence (v)
A monovalent ion requires 1 electron for discharge, a divalent ion requires 2 electrons for discharge and so on. Thus, if x electrons flow, atoms are discharged.
So the mass m discharged is
where
N_A is the Avogadro constant;
Q = xe is the total charge, equal to the number of electrons (x) times the elementary charge e;
F is the Faraday constant.
Faraday's laws can be summarized by
where M is the molar mass of the substance (usually given in SI units of grams per mole) and v is the valency of the ions .
For Faraday's first law, M, F, v are constants; thus, the larger the value of Q, the larger m will be.
For Faraday's second law, Q, F, v are constants; thus, the larger the value of (equivalent weight), the larger m will be.
In the simple case of constant-current electrolysis, Q = It, leading to
and then to
where:
n is the amount of substance ("number of moles") liberated:
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(τ) integrated over time τ:
Here t is the total electrolysis time.
Official source
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