Answer. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180°C. In first step, ethyl hydrogensulphate is formed which decomposes on heating to produce ethene, which is collected over water.
180 °C
CH3-CH2OH + H2S04 ——> CH3CH2-OSO3H + H20
heat
CH3-CH2OSO3H ———> H2C=CH2 + H2SO4
On heating ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen from adjacent carbon atoms forms double bond. This method is called dehydrohalogenation, of alkyl halides.
H3C-CH2-Br + KOH (alcoholic) ———> H2OCH2 + KBr + H2O