Faraday's Laws of Electrolysis

Faraday's Laws of Electrolysis

Electrolysis

1. Electrolytes:

• These are the substances which allow electricity to pass through them in their molten states or in the form of their aqueous solution and undergo chemical decomposition.

Examples – acids, bases & salts.

2. Strong Electrolytes:

• The electrolytes, which are almost completely dissociated into ions in solution, are called strong electrolytes.

Example – NaCl, KCl, HCl, NaOH, etc.

3. Weak Electrolytes:

• The electrolytes which do not ionise completely in solution are called weak electrolytes.

Example – CH₃COOH, H₂CO₃, HCN, ZnCl₂, NH₄OH, etc.

4. Electrolysis:

• The process of chemical decomposition of an electrolyte by passage of electric current through its molten state or its solution is called electrolysis.

5. Electrodes:

• To pass the current through an electrolyte in a molten state or in an aqueous solution, two rods or plates are needed to connect with the terminals of a battery. These rods or plates are called electrodes.

• Anode:

  • The electrode which is attached to the positive terminal of the battery is called the anode.

  • Oxidation occurs at the anode.

• Cathode:

  • The electrode which is attached to the negative terminal of the battery is called the cathode.

  • Reduction occurs at the cathode.

Examples – Electrolysis of Molten NaCl:

• At the anode:

  • Cl⁻ → Cl + e

  • Cl + Cl → Cl₂

• At Cathode:

  • Na⁺ + e → Na

• Cl₂ gas occurs at the anode, and Na occurs at the cathode.

• Molten NaCl is a good conductor of electricity due to the presence of free ions.

• When electricity is passed through brine (aqueous NaCl), H₂ appears at the cathode. Cl₂appeared at the anode. The solution becomes alkaline due to the presence of NaOH.

Faraday’s Laws of Electrolysis

The relation between the quantity of charge passed through an electrolyte and the amount of substance deposited at the electrode is obtained from Faraday’s Laws of Electrolysis.

Faraday’s First Law of Electrolysis

The amount of the substance (w) deposited or liberated at any electrode is directly proportional to the quantity of charge (Q) passed.

• w ∝ Q (Q = charge in coulomb)

• w ∝ I·t (Q = current (I) × time (t))

• w = Z·t
  Where Z is a proportionality constant known as the electrochemical equivalent.

The charge carried in one mole of electrons is normally taken as 96500 C mol⁻¹, which is called Faraday’s constant (F).

• 1 F = 96500 C mol⁻¹

Faraday’s Second Law of Electrolysis

When some quantity of charge is passed through the solutions of two different electrolytes connected in series, the masses of substances deposited at respective electrodes are directly proportional to their equivalent masses.

Conductance (C)

Conductance is the ease with which the current flows through the electrolyte. It is the reciprocal of the resistance (R).

• i.e., C = 1 / R

• The unit of conductance is ohm⁻¹ or Siemens (S).

Specific Conductance (κ)

The conductance of the solution of an electrolyte enclosed in a cell with two electrodes of unit area of cross-section separated by one centimetre is called specific conductance.

• The unit of specific conductance is ohm⁻¹ cm⁻¹ or S cm⁻¹.

Equivalent Conductance (Λₑ)

The conductance of all the ions produced by one gram equivalent of electrolyte in solution is known as equivalent conductance.

• The unit of equivalent conductance is ohm⁻¹ cm² (g-equiv)⁻¹ or S cm² (g-equiv)⁻¹.

Corrosion

• When a metal is attacked by substances such as moisture, acid, etc., it is said to corrode, and this process is called corrosion. The black coating on silver and the green coating on copper are examples of corrosion.

• Corrosion slowly coats the surface of metallic objects with oxides or salts of the metal. Iron articles are shiny when new, but they get coated with a reddish-brown powder when exposed to moisture. This process is commonly known as rusting of iron. Rust is hydrated ferric oxide (Fe₂O₃·3H₂O).

• Corrosion causes damage to car bodies, bridges, iron railings, ships, and to all objects made of metals, especially those of iron.

Final Thoughts

Electrolysis is the process of chemical decomposition of an electrolyte by passing an electric current through its molten state or aqueous solution. Electrolytes can be strong (like NaCl) or weak (like CH₃COOH), depending on how fully they dissociate into ions. During electrolysis, oxidation occurs at the anode, and reduction takes place at the cathode.

Faraday’s Laws of Electrolysis explain the relationship between charge passed and the substance deposited. Conductance measures how easily current flows through an electrolyte, with specific conductance and equivalent conductance being key properties.

Additionally, corrosion is a natural process where metals, especially iron, degrade when exposed to moisture, causing rust formation.