Question

what is colbe method of preparation of Ethylene

Answer 1

The electrolysis of salts of Carboxylic acids which finally produces alkanes is called Kolbe de-Carboxylation reaction.
During this reaction Carbon dioxide is released at anode along with formation of alkanes.
2R-COONa (on Electrolysis) → R - R + 2CO2 + H2 + NaOH
Example: Sodium acetate on electrolysis produces ethane.
2CH3COONa → CH3 - CH3 + 2CO2 + H2 + NaOH