Faraday’s Laws of Electrolysis

Faraday’s Law of Electrolysis proposed by Michael Faraday deals with the quantitative analysis of Electrolysis. It relates the mass deposited at any electrode and the amount of charge passed through them. Electrolysis is a process in which an electric current stimulates a non-spontaneous chemical reaction. In this process, metal electrodes are dipped in an electrolytic solution and a flow of ions is created through the passage of current by connecting wires to the electrodes. This flow of ions causes a chemical reaction that results in mass deposition at one electrode and the dissolution of the other. The mass deposited at the electrode depends on the amount of charge passed through the solution. Faraday’s law of electrolysis provides a basis for mathematical analysis of the mass deposited at electrodes and the amount of charge passed through them.

In this article, we will discuss two laws of electrolysis given by Faraday, their application, and limitations, solve numerical problems based on them, and FAQs related to them.

What is Electrolysis?

Electrolysis is the process of breaking a compound into its constituent ions with the use of current. The word electrolysis is composed of two words Electro and lysis. Electro stands for electricity and lysis means meaning. Hence, in this process, when current is passed to the ionic solution, the ions get separated and deposited on the electrodes. The electrochemical cell in which the process of electrolysis occurs is called the electrolytic cell.

Faraday’s Law of Electrolysis

Faraday’s Law of Electrolysis was given by an English scientist named Michael Faraday in 1834. His main contributions were in the field of electrochemistry and electromagnetism. It deals with the amount of ions produced and the charge passed through the cell. Let’s discuss the two laws of electrolysis given by him one by one as follows:

Faraday’s First Law of Electrolysis

Faraday’s First Law of Electrolysis states that the mass of a substance (m) deposited or liberated at an electrode is directly proportional to the amount of charge (Q) passed through the electrolytic solution. This can be mathematically represented as follows,

m ∝ Q

The amount of charge passed is calculated as the product of current (I) and the time for which it is passed through the solution (t), i.e. Q = I×t.

The proportionality relation, m ∝ Q can be converted to an equation by introducing a proportionality constant Z as follows,

m = Z×Q

Z is the proportionality constant and can be defined as the mass deposited or liberated at any electrode due to the passage of one unit of charge through the solution. The units of Z depend on the units taken for mass (m) and charge (Q). For instance, if mass is taken in grams and charged in coulombs, then units for the proportionality constant Z would be grams/coulombs. Through various experiments, it has been established that,

Z = E/F

where,

• E = equivalent weight of the substance
• F = 1 Faraday, i.e. 96500 coulombs

Faraday’s first law of electrolysis can thus be expressed as follows,

m = (E × I × t)/F

where,

• m = mass of the substance deposited or liberated
• E = equivalent weight/chemical equivalent of the substance
• I = current passed through the electrolytic solution
• t = time for which the current is passed
• F = 1 Faraday, i.e. 96500 coulombs

Faraday’s Second Law of Electrolysis

Faraday’s Second Law of Electrolysis states that when the same amount of charge is passed through different electrolytes, the masses of the substances deposited or liberated are directly proportional to their respective chemical equivalent or equivalent weight. It can be mathematically represented as follows,

m ∝ E

where,

• m = mass of the substance deposited or liberated
• E = equivalent weight/chemical equivalent of the substance

For two solutions connected to the same source of current and current being passed through them for equal time, then Faraday’s 2nd law for them can be written as follows,

w₁/w₂ = E₁/E₂

where,

• w₁ and w₂ are mass of substance
• E₁ and E₂ are equivalent weight of substance

The equivalent weight of an element is defined as the ratio of atomic weight and valency of the element.

What is 1 Faraday?

Consider the following equation

Na⁺ + e- → Na

In this reaction, to convert one mole of Na+ to Na, 1 mole of electrons is required.

One electron has a charge equivalent to 1.6 × 10^-19 C

Therefore, 1 mole of the electron has a charge equivalent to 1.6 × 10^-19 C × 6.023 × 10²³ = 96500 C = 1 Faraday

Hence, if 1 Faraday of charge is passed then 1 gram of equivalent weight of substance gets deposited.

Applications of Faraday’s Law of Electrolysis

Faraday’s law of electrolysis is used for the following purposes:

• It is used to estimate the equivalent mass of various elements
• It is used in electroplating and refining to calculate the amount of deposition or refinement
• It is applicable in metallurgy to extract metals
• It is used in the manufacturing of fuel cells and galvanic cells
• It is used in reactions that involve oxidation and reduction

Limitations of Faraday’s Law of Electrolysis

Faraday’s law of electrolysis helps in calculating the amount of ion deposition and the amount of current passed through the solution. Although, it has several benefits it has got some limitations. These limitations are mentioned below:

• This law is only valid for ideal conditions. It assumes the conditions of electrolysis to be ideal such as uniform surface, homogeneous electrolyte, etc.
• It applies to reactions in which only one substance is deposited. It fails in the case of complex electrochemical reactions.
• It does not count the additional voltage to carry on the reaction.
• It is not applicable in the case of non-electrolytes.
• It assumes that a constant current is passing throughout the reaction

Solved Examples on Faraday’s Law of Electrolysis

Example 1: Electrolysis of dilute aqueous NaCl solution is carried out by passing 20 mA current. What will be the time required to liberate 2 grams of H₂ gas at the cathode?

Solution:

Here we have,

Mass of substance, i.e. H₂ (m) = 2 g

Equivalent weight of H₂ (E) = 1 g

Current (I) = 20 mA = 20 × 10^-3 A

According to first law of electrolysis, we have,

m = E × I × t/F

⇒ t = (m × F)/(E × I)

On putting the known values in above equation, we get,

⇒ t = (2 × 96500)/(1 × 20 × 10^-3) = 9.65 × 10⁶ seconds

Thus, we obtain that after 9.65 × 10⁶ seconds, 2 grams of hydrogen would be liberated at cathode using Faraday’s first law of electrolysis.

Example 2: A current of 2.5 A is passed through a solution of silver nitrate for 2 hours. Calculate the mass of silver deposited. (Given: Atomic mass of silver = 107.87 g/mol, Faraday’s constant = 96500 C/mol).

Solution:

Here we have,

Atomic mass of substance, i.e. Silver (Ag) = 107.87 g/mol

Equivalent weight of Ag (E) = (Atomic mass)/Valency = 107.87/1 = 107.87 g/mol

Current (I) = 2.5 A

Time (t) = 2 × 3600 = 7200 secs

According to first law of electrolysis, we have,

m = E × I × t/F

⇒ m = 107.87 × 2.5 × 7200/96500 = 20.12 grams

Thus, we obtain that 20.12 grams of silver would be deposited under given set of conditions.

Faraday’s Law of Electrolysis FAQs

What is Electrolysis?

Electrolysis is a chemical process in which an electric current is used to stimulate a non-spontaneous chemical reaction which typically results in deposition or liberation of substances at the electrodes.

What is meant by 1 Faraday?

One Faraday or Faraday’s constant is the amount of charge required to deposit or liberate one equivalent weight of a substance during an electrolysis process. Its value is approximately taken as 96500 coulombs.

What is meant by the term ‘equivalent weight’ in Faraday’s second law of electrolysis?

Equivalent weight of a substance implies the weight of a substance that liberates one mole of electrons during a chemical reaction in electrolysis.

What are the applications of Faraday’s law of electrolysis?

Faraday’s law of electrolysis are used in various practical applications where electrolysis is carried out for some commercial purpose such as electroplating, electrolytic refining and electrochemical machining, etc.

What is an Electrolytic Cell?

Electrolytic cell is an electrochemical cell in undergoes reaction when electric current is passed through it.

What is an Electrode?

An electrode is a conducting to that conducts electric current to the solutions. All the oxidation and reduction occur at electrodes.