Faraday's First and Second Laws of Electrolysis

Contents

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Key learnings:

Faraday’s Laws of Electrolysis Defined: Faraday’s law is a set of two scientific laws that quantify how chemical deposition during electrolysis is proportional to the electricity used.
First Law Explained: The first law states that the mass of a substance deposited at an electrode during electrolysis is directly proportional to the amount of electricity that flows through the electrolyte.
Second Law Elucidation: According to the second law, when electricity passes through different electrolytes, the substance mass deposited is proportional to their chemical equivalents.
Electrolysis Process: Electrolysis involves ions moving to electrodes, where they gain or lose electrons, resulting in chemical deposition.
Historical Insight: Michael Faraday, who formulated these laws in 1834, significantly advanced our understanding of electrolysis and its practical applications.

Faraday’s Laws of Electrolysis

Faraday’s laws of electrolysis say how much substance is deposited when charge passes through an electrolyte. The first law says mass is proportional to charge. The second says mass is proportional to chemical equivalent. Those statements come from the electrolysis of a metal sulfate.

When a metal sulfate dissolves in water, its molecules split into positive and negative ions. The metal cations move to the electrode on the negative battery terminal, take electrons and deposit as a metal atom.

The sulfate ions move to the electrode on the positive battery terminal, give up electrons and become an SO₄ radical. Neutral SO₄ cannot stand alone, so it attacks the metal anode and forms a sulfate that dissolves again.

Faraday’s laws of electrolysis are the two quantitative relations for that ion transfer.

Faraday’s First and Second Laws of Electrolysis

Faraday’s First Law of Electrolysis

The current in the battery circuit equals the electrons transferred from the cathode to the metal cations. A cation of valency two, such as Cu⁺⁺, takes two electrons. Each electron carries – 1.602 × 10^-19 coulomb, written – e. Depositing one Cu atom therefore transfers – 2.e from the cathode to that cation.

If n copper atoms deposit in time t, the transferred charge is – 2.n.e coulomb. Mass m of that copper depends on how many atoms arrive. The deposited mass is therefore proportional to the charge Q that passes through the electrolyte: m ∝ Q.

Faraday’s First Law of Electrolysis states that the mass deposited by current through an electrolyte is proportional to the charge (coulombs) that passes.

Mass of chemical deposition:

Z is the constant of proportionality, the electrochemical equivalent of the substance.

If Q = 1 coulomb then Z = m. The electrochemical equivalent is the mass deposited by 1 coulomb, usually given in milligrams per coulomb or kilograms per coulomb.

Faraday’s Second Law of Electrolysis

The first law says only that deposited mass is proportional to the charge that passes.

The deposited mass also depends on atomic weight, so different substances have different masses for the same number of atoms.

The number of atoms deposited also depends on valency. A higher valency means fewer atoms for the same charge. A lower valency means more atoms for that charge.

For the same charge through different electrolytes, deposited mass is proportional to atomic weight and inversely proportional to valency.

Faraday’s second law of electrolysis states that when the same charge passes through several electrolytes, the deposited masses are proportional to their chemical equivalents, or equivalent weights.

Chemical Equivalent or Equivalent Weight

The chemical equivalent used in Faraday’s laws is the weight of a substance that combines with or replaces a unit weight of hydrogen.

Hydrogen therefore has chemical equivalent 1. Valency is the number of hydrogen atoms a substance can replace or combine with, so chemical equivalent is atomic weight divided by valency.