Faraday's laws of electrolysis

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Faraday's laws of electrolysis are quantitative relationships based on the electrochemical researches published by Michael Faraday in 1834.^[1]

Contents 1 Statements of the laws 2 Mathematical form 3 References 4 Further reading 5 See also

[edit] Statements of the laws

Several versions of the laws can be found in textbooks and the scientific literature. The most-common statements resemble the following:

Faraday's 1^st Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to electrical charge, typically measured in coulombs, and not to electrical current.

Faraday's 2^nd Law of Electrolysis - For a given quantity of electricity (electric charge), the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight. The equivalent weight of a substance is its molar mass divided by an integer that depends on the reaction undergone by the material.

[edit] Mathematical form

Faraday's laws can be summarised by

where

m is the mass of the substance altered at an electrode

Q is the total electric charge passed through the substance

F = 96 485 C mol^-1 is the Faraday constant

M is the molar mass of the substance

z is the valence 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 = It leading to

and then to

where

n is the amount of substance ("number of moles") altered: n = m / M

t is the total time the constant current was applied.

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

Here t is the total electrolysis time. Note that the parentheses in I(τ) do not indicate multiplication, but rather that the current is a function of time, τ.^[2]

[edit] 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. 
2. ^ For a similar treatment, see Strong, F. C. (1961). "Faraday's Laws in One Equation". Journal of Chemical Education 38: 98. 

[edit] Further reading

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

[edit] See also

• Electrolysis
• Michael Faraday
• Faraday constant
• Faraday's law of Electromagnetic induction

v • d • e Articles related to electrolysis Principles of electrolysis Electrochemical cell • Electrolytic process • Faraday's laws of electrolysis • Half cell • High-temperature electrolysis • Standard electrode potential Electrolytic processes Betts electrolytic process • Castner process • Castner-Kellner process • Chloralkali process • Downs cell • Electrolysis of water • Electrowinning • Hall-Héroult process • Hofmann voltameter • Kolbe electrolysis Materials produced by electrolysis Aluminium • Calcium metal • Chlorine • Copper • Electrolyzed water • Fluorine • Hydrogen • Lithium metal • Magnesium • Potassium metal • Sodium metal • Sodium hydroxide • Zinc See also Electrochemistry • Standard electrode potential (data page)