Electrostatics - Faraday Electrolysis Law

Faraday Electrolysis Law

Faraday, form his experimental study arrived at the two laws of electrolysis which are given below first law. The mass of the substance liberated or deposited at on electrode during electrolysis is directly proportional to the quantity of charge pads through the electrolyte.

If m is the mass of a substance deposited or liberated at an electrode during electrolysis when a charge q passes through electrolyte then according to faraday first law of electrolysis, m ∝q or m = zq

Where z is a constant of proportionality and is called electrochemical equivalent (E.C.E) of the substance.

If an electric current flows for a time t to pass the charge q through the electrolyte them q = it

From (4) m = zlt

When q = 1 form (4) m = z x 1 = z 
 
Hence electrochemical equivalent (E.C.E) of a substance is defined as the mass of the substance liberated or deposited on an electrode during electrolysis when one coulomb of charge (or 1 ampere current for 1 second) is passed through the electrolyte.

Generally the E.C.E of a substance is expressed in gram/coulomb (gc^-1). The value of E.C.E of copper and hydrogen are 0.0003294 gC^-1 and 0.0000105 gC ^{– 1} respectively 

If m₁ m₂ = masses of the substance liberated or deposited on various electrodes when same current is passed of the same time through their electrolytes.

E₁ E₂ = chemical equivalents* of the substances liberated or deposited.

Then according to faraday’s second law of electrolysis

(m₁ / m₂) /(E₁ / E₂)

Another form of faraday’s second law of electrolysis (relation between ECE and CE)

Consider during electrolysis one mole of a substance of atomic mass M is liberated on an electrode. If N us the Avogadro’s number then the number of atoms involved and e is the charge of an electron then,

Charge flowing from deposition of one atom = per

∴ Total charge flowing for deposition of N atoms = Npe

Now for one mole of substance,

M = m and q = Npe

Form faraday’s first law of electrolysis m = zq

We get M = z N pe

Or z = 1 / Ne M / P

Or z   ∝ M / P (∴N and e are constant)

Or z ∝ E 

The quantity M/p = E chemical equivalent of the substance and the fundamental constant Ne is called faraday constant (F).

Thus faraday’s second law of electrolysis also states that the electrochemical equivalent of a substance is directly proportional to its chemical equivalent.

If E₁ E₂ = chemical equivalent of two substances.

z₁, z₂ = E.C.E of those two substances.

Then from faraday’s second law of electrolysis

z₁ / z₂ = E₁ / E₂

Faraday constant 

From faraday’s second law of electrolysis

Z ∝E or E ∝ z 

Or E = Fz or E/z =F = a constant,

Called faraday’s constant

Thus faraday constant,

F = E / z = E / m/q = qE / m (∴m = zq)

If m = E , then F = q

Hence faraday constant is equal to the amount of charge required to liberate the mass of a substance at an electrode during electrolysis equal to its chemical equivalent (in gram)

For example E.C.E of copper = 0.0003294 gC^-1

Chemical equivalent of copper = 31.75 g 

∴ one faraday = 31.75/0.0003294 = 96387 . 4 ≈ 96500 C / gram equivalent or coulomb / mole 

It means 96500 C of charge is required to liberate one gram equivalent of a substance at an electrode during electrolysis.

Faraday constant F = Ne = 3.025 x 10²³ x 1.602 x 10^-19 = 96488.7 = 96500 C/gram equivalent or C / mole

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