Quantitative Aspects of Electrolysis: Definition, Formula, Factors

Quantitative Aspects of Electrolysis was first described by Michael Faraday. In simple terms, electrolysis is a process where an electric current drives a chemical reaction across the electrodes. The medium in which electrolysis takes place is known as electrolyte which may consist of an ionic solution or molten mass.

Key Terms: Electrolysis, Anode, Cathode, Laws of Electrolysis, Weight, Factors, Current, Reaction, Conductivity, Mass, Electrolyte, Electrode, Oxidation, Reduction, Electron

---

Introduction to Electrolysis

[Click Here for Sample Questions]

Electrolysis is defined as a process by which electric current is passed through a substance to effect a chemical change. The chemical change is one in which the substance either gains or loses an electron which is known as oxidation or reduction. The cations are so reduced at cathode and anions are oxidized at the anode. Also during the process an ion exchange membrane is introduced so that products do not diffuse on opposite electrodes. The product of electrolysis generally depends on the nature of material being electrolysed and the type of electrodes being used.

Also Read:

---

Faraday’s Laws of Electrolysis

[Click Here for Sample Questions]

Michale Faraday explained the process of electrolysis by proposing two laws of electrolysis as mentioned below:

First Law of Electrolysis

First Law of Electrolysis states that, “the amount of chemical reaction or change so produced at any electrode during electrolysis by a current is proportional to the quantity of electric charge passing through an electrolytic cell”. Mathematically, it can be expressed as:

m ∝ Q

where, ‘m’ is the mass of a substance (in grams) deposited at electrode and ‘Q’ is the amount of charge (in coulombs) passed through it

On removing proportionality constant, the equation becomes:

m = ZQ

where, ‘Z’ is the electrochemical equivalent or proportionality constant or it can also be defined as the mass of a substance deposited on an electrode on passing 1 coulomb of charge during electrolysis. The unit of Z is grams per coulomb (g/C).

Second Law of Electrolysis

Second Law of Electrolysis can be stated as the amount of different substances released by the same quantity of electricity passing through the solution is proportional to their chemical equivalent weights. Mathematically, it can be expressed as

w ∝ E

where, ‘w’ is the mass of the substance and ‘E’ is the equivalent weight of the substance. Or, it can also be expressed as

w₁/w₂ = E₁/E₂

---

Equivalent Weight

[Click Here for Sample Questions]

The equivalent weight of a substance can be defined by Faraday’s Second Law of Electrolysis as the ratio of its atomic weight and valency. Mathematically, it can be expressed as

Equivalent Weight = Atomic Weight/ Valency

Let us take a solution of electrolytes: AgNO₃, CuSO₄ and Al (NO₃)₃ subsequently and pass some potential of voltage through them for the same time then, it will be clearly visible that metals Ag, Cu and Al get collected at cathode. Their masses are proportionate to their equivalent masses.

Corresponding Mass = Atomic Mass of Element / Valency of Element

According to Faraday, if 1 Faraday or 96,500 Coulombs is passed through them then we get, Ag = 108/1 = 108g; Cu = 63.5/2 = 31.75g; Al = 27/3 = 9g which are the equivalent masses of Ag, Cu and Al respectively.

---

Examples

[Click Here for Sample Questions]

Example 1: The resistance of a conductivity cell filled with 0.1 mol L^-1 KCl solution is 100 ohm. If the resistance of the same cell when filled with 0.02 mol L^-1KCl solution is 520 ohm, calculate the conductivity and molar conductivity of 0.02 mol L^-1KCl solution. The conductivity of 0.1 mol L^-1KCl solutions is 1.29 S/m.

Solution: The cell constant is given as: cell constant = G *= conductivity × resistance = 1.29 S/m × 100 ohm = 129m^-1 = 1.29 cm^-1.

Conductivity of 0.02 mol L^-1 KCl solution = cell constant/resistance = G*/R = 129m^-1/520 ohm = 0.248 S/m.

Concentration= 0.02 mol L^-1 = 1000 × 0.02 mol m^-3 = 20 mol m^-3

Molar conductivity = Am = K/C

= 248 × 10^-3 S/m/20 mol m^-3

= 124 × 10^-4 S m²/mol.

Example 2: A solution of CuSO₄ is electrolyzed for 10 minutes with a current of 1.5 A. What is the concentration of Cu accumulated at Cathode?

Solution: Given, t = 600 seconds

Charge = current × time= 1.5A × 600 = 900 C.

As per the reaction: Cu²⁺ (aq) + 2e^- = Cu(s).

Now we require 2F or 2 × 96487 C to accumulate 1 mol or 63 g of Cu.

For 900 C, the mass of Cu accumulated = (63g/mol × 900C) / (2 × 96487C/mol) = 0.2938 g.

Example 3: If you do electrolysis of dilute aq. NaCl solution and carry out passing 10 milliampere current then the time required releasing 0.01 mol of H₂ gas at the cathode is 19.3 × 10⁴ seconds.

Solution: Let’s calculate:

According to Faraday’s first law of electrolysis:

m = EIt/F

Where, m = mass of substance

E = Equivalent weight of substance

I = current in ampere

T = Time

F = 96500 C

Therefore, t = (0.01 × 2) × 96500 / 1 × 10 × 10^-3

= 19.3 × 10⁴ seconds.

Where again, m = mass of H₂ = 0.01mol x 2 g/mol

Equivalent wt. of H₂ = E = Atomic wt/valence = 1g/mol/1 = 1g/mol

Current = I = 10 milliampere = 10 × 10^-3 Amperes.

---

Factors affecting Electrolysis

[Click Here for Sample Questions]

The factors that may affect the electrolysis are:

1. nature of the electrode
2. nature and state of the electrolyte
3. nature and electrode budding of ions present in the electrolyte and
4. over voltage at the electrodes

---

Things to Remember

• Electrolysis is a simple process where an electric current drives a chemical reaction across the electrodes. It was first discovered by Michael Faraday Law in 1833-34.
• The main parts of an electrolytic cell are an electrolyte, DC current and two electrodes.
• Formula for electrolysis: m = E × I × t / 96,485
• where, m is the mass of the substance in grams, I is current in ampere (A), t is time
• In seconds, 96,485 is Faraday's constant.
• First Law of Electrolysis gives relation between mass of chemical deposition (m) and amount of charge or electricity passed through it (Q).
• Second Law of Electrolysis gives relation between mass of substance and its equivalent weight.

Also Read: