PRELIMS

Electrolysis and Electrochemistry

Electrolysis is the process of chemical decomposition caused by passing an electric current through an electrolyte.

Electrolytes

• Electrolytes: Substances that conduct electricity in molten or aqueous states and undergo chemical decomposition (e.g., acids, bases, salts).
• Strong Electrolytes: Almost completely dissociated into ions ( ).
• Weak Electrolytes: Partially ionised in solution ( ).

Electrodes and Cell Processes

To pass current, two rods/plates (electrodes) are connected to a battery:

1. Anode: Attached to the positive terminal. Oxidation occurs here.
2. Cathode: Attached to the negative terminal. Reduction occurs here.

Galvanic (Voltaic) Cell vs. Electrolytic Cell

• Galvanic/Voltaic Cell: Converts chemical energy into electrical energy via a spontaneous redox reaction (e.g., Daniell cell, dry cell, Li-ion battery in discharge mode). Anode is negative, Cathode is positive.
• Electrolytic Cell: Converts electrical energy into chemical energy; requires an external power source to drive a non-spontaneous reaction (e.g., electrolysis, electroplating, battery charging). Anode is positive, Cathode is negative.
• Common Rule (both cells): Oxidation always occurs at the anode; reduction always occurs at the cathode — only the polarity of the electrodes flips between the two cell types.

Faraday's Laws of Electrolysis

1. First Law: The mass of substance deposited/liberated at an electrode is directly proportional to the quantity of electric charge (current × time) passed through the electrolyte. (where ).
2. Second Law: When the same quantity of electricity is passed through different electrolytes connected in series, the masses of substances deposited are directly proportional to their chemical equivalent weights.
3. Faraday Constant: Charge on 1 mole of electrons = 96,500 coulombs.

Example: Electrolysis of Molten NaCl

• At Anode (Oxidation): ; then .
• At Cathode (Reduction): .
• Result: Chlorine gas is liberated at the anode, while Sodium metal is deposited at the cathode.

UPSC Relevance

Electrolysis is the basis for several industrial and modern technologies.

• Electroplating: Covering a metal with another for protection or aesthetics (e.g., gold plating).
• Metal Extraction: Metals like Sodium, Aluminum, and Magnesium are extracted via electrolysis of their molten ores.
• Battery Technology: Understanding oxidation/reduction at electrodes is essential for Li-ion and Lead-acid battery concepts.