Faraday's laws of electrolysis are quantitative relationships based on the
electrochemical
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...
research published by
Michael Faraday in 1833.
First law
Michael Faraday reported that the mass (
) of elements deposited at an electrode is directly proportional to the charge (
;
SI units are
ampere seconds or
coulombs).
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
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 (
).
This turns out to be the molar mass (
) divided by the valence (
)
:
:
:
:
(From 1st Law)
:
Derivation
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
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
.
Mathematical form
Faraday's laws can be summarized by
:
where
is the
molar mass
In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, ...
of the substance (usually given in SI units of grams per mole) and
is the
valency of the
ions .
For Faraday's first law,
,
, and
are constants, so that the larger the value of
the larger
will be.
For Faraday's second law,
,
, and
are constants, so that the larger the value of
(equivalent weight) the larger
will be.
In the simple case of constant-
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
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 ''w
i'' 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.
[For a similar treatment, see ]
See also
*
Electrolysis
*
Faraday's law of induction
*
Tafel equation
References
Further reading
* Serway, Moses, and Moyer, ''Modern Physics'', third edition (2005), principles of physics.
Experiment with Faraday's laws
{{DEFAULTSORT:Faraday'S lawS Of electrolySiS
Electrochemistry
Electrolysis
Electrochemical equations
Scientific laws