What is hydrogenation? How hydrogenation of ethene occurs? Give one use of hydrogenation.

Hydrogenation is the addition of hydrogen to an unsaturated hydrocarbon such as ethene, across the double bond.
Hydrogenation of ethene occurs by heating it in hydrogen gas in the presence of a catalyst (nickel or palladium).

$\underset{\mathrm{Ethene}\left(\mathrm{unsaturated}\right)}{{\mathrm{CH}}_2={\mathrm{CH}}_2}+{\mathrm{H}}_2\stackrel{\mathrm{Ni}/\mathrm{Heat}}{\to }\underset{\mathrm{Ethane}\left(\mathrm{saturated}\right)}{{\mathrm{CH}}_3-{\mathrm{CH}}_3}$

Hydrogenation is used to convert vegetable oils (containing unsaturated hydrocarbons) into vegetable ghee or vanaspati ghee (containing saturated hydrocarbons). This gives a longer shelf life to these vegetable oils and reduces the risk of oxidation.