Electrolysis

• It is the breakdown of an electrolyte- ionic compound, molten or aqueous solution- by passing an electric current
• This is possible due to the presence of mobile electrons

Electrolyte

• Aqueous solution of ionic substance or molten ionic salt
• Conducts electricity due to the presence of mobile ions

 

Principle

• Reduction of positive cations happens at the cathode
• Oxidation of negative anions happens at the anode

 

For example:

• At the anode: 2Cl^– → Cl₂ + 2e^–
• At the cathode: 2H⁺ + 2e^– → H₂

Examples of Electrolysis

Voltaic Cell

• Used to produce electrical energy from chemical energy
• The electrodes are made from metals with different reactivity

1.Negative electrode: More reactive metal, ex. Zinc

• Positive electrode: Less reactive metal, ex. Iron

• Electrolyte is a strong acid, ex. sulfuric acid
• The negative electrode loses electrons; these flow through the simple circuit to the positive electrode to produce voltage. This is measured on the attached voltmeter.

Note: The greater the difference in reactivity, the greater voltage produced

Electroplating

The process of coating the surface of a metal (more reactive) with another metal (less reactive) using electrolysis

• Components:
  • Anode: pure metal being used to electroplate the object
  • Cathode: object being electroplated
  • Electrolyte: aqueous solution of the soluble salt of pure metal (same as anode)
• Used to:
  • Prevent corrosion
  • Enhance appearance

Electrolytic purification of metals

• Cathode: thin strip of pure metal
• Anode: impure metal
• Electrolyte: Aqueous Salt Solution of metal

 

Purification of copper

Impure copper as the anode and pure copper as the cathode; the aqueous copper (II) sulfate helps the copper ions move from the anode to the cathode. Here the ions gain electrons and become copper atoms, making the pure copper cathode thicker.

• Reaction at Anode: Cu – 2e Cu²⁺ (mass decreases)
• Reaction at Cathode: Cu²⁺ + 2e Cu (mass increases)

Electrolytic purification of copper

• Electrolyte: aq copper(ii)sulphate
• Cathode: thin strip of pure copper
• Anode: impure metal
• Copper ions from the solution lose thier charge and copper is deposited at the cathode, mass increases
• Copper atoms lose their valency electrons and go into the solution as ions at the anode, mass decreases

Electroplating
Electroplating is done to coat a metal with another metal to improve its appearance or to improve its resistance to corrosion. The metal used for plating is used as the anode and the object to be electroplated acts as the cathode. The electrolyte used is the salt solution of the metal used for
plating.

Copper metal acts as the anode as it is used to plate the object. The electrolyte used is a salt solution of its salt (copper(II) sulfate solution) and the object to be plated acts as the cathode.

At the anode, the copper atoms are oxidised into Cu2+ ions, which enter the electrolyte. At the cathode, Cu2+ ions are discharged and deposited on the object, plating it with
copper metal.

 

Extraction of Aluminum

• The main ore of aluminum is bauxite – high m.p.
• Aluminum (III) oxide (alumina) is dissolved in molten cryolite (Na₃AlF₆) – this mixture has a lower m.p. (industrially preferred)

• During electrolysis aluminum ( Al³⁺ + 3e-  → Al ) is produced at the carbon cathode and oxygen ( 2O^2- – 4e-   → O₂ ) at the carbon anode.
• Due to the high temp. the oxygen reacts with the carbon in the graphite anode to form CO₂ and so anode has to be periodically replaced

 

Electrolysis of Brine

• Brine is concentrated aqueous NaCl solution
• Ions present: Na⁺, H⁺, Cl^– and OH^–