Tuesday, February 15, 2011


Faraday's laws of electrolysis:

Michael Faraday, a pioneer in the properties of electric currents, formulated two basic laws of electrolysis:

FARADAY'S FIRST LAW may be stated as follows:

Faraday's First Law of Electrolysis
"The amount of any substance deposited, evolved, or dissolved at an electrode is directly proportional to the amount of electrical charge passing through the circuit."

The amount of electricity passing through the circuit in a given time is the number of moles of electrons passing through the circuit in that time, and the charge Q is related to the current I by

Q =  It

The charge on the electron is 1.602 x 10-19 C, and Avogadro's number is 6.023 x 1023. It follows that one mole of electrons has a charge of 9.65 x 104 C. This quantity is known as the FARADAY or FARADAY'S CONSTANT (F).

FARADAY'S SECOND LAW may be stated as follows:

Faraday's Second Law of Electrolysis
"The mass of different substances produced by the same quantity of electricity are directly proportional to the molar masses of the substances concerned, and inversely proportional to the number of electrons in the relevant half-reaction."

This means that z moles of electrons are needed to discharge an ion Xz+ or Xz-.

In the apparatus below,

1 Faraday will discharge 9 g Al (1/3 mole), 20 g Ca (1/2 mole) and 23 g Na (1 mole).

The relevant half reactions are:

Al3+ + 3e- Al
Ca2+ +2e- Ca
Na+ + e- Na

Faraday's second law, illustrated

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