Faraday's laws of electrolysis

Michael Faraday.

Faraday's laws of electrolysis are quantitative relationships based on the electrochemical researches published by Michael Faraday in 1834.^[1] In the same years, in Italy, Carlo Matteucci discovered the laws of electrolysis by a method totally independent Faraday's methods. The laws of electrolysis can also be called Faraday-Matteucci's laws.

Mathematical form

Faraday's laws can be summarized by

$m \ = \ \left({ Q \over F }\right)\left({ M \over z }\right)$

where:

• m is the mass of the substance liberated at an electrode in grams
• Q is the total electric charge passed through the substance
• F = 96485 C mol⁻¹ is the Faraday constant
• M is the molar mass of the substance
• z is the valency number of ions of the substance (electrons transferred per ion).

Note that M/z is the same as the equivalent weight of the substance altered.

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

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

In the simple case of constant-current electrolysis, $Q = I t$ leading to

$m \ = \ \left({ I t\over F }\right)\left({ M \over z }\right)$

and then to

$n \ = \ \left({ I t\over F }\right)\left({ 1 \over z }\right)$

where:

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

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

$Q = \int_0^t I(\tau) \ d \tau$

Here t is the total electrolysis time.^[2]

See also

• Electrolysis
• Faraday's law of induction

References

1. 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.
2. 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

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