Answer. Reactions in which two or more substances combine to form a big molecule are called addition reactions. Addition of hydrogen in alkenes and alkynes is called addition reaction. Alkenes and alkynes are unsaturated compound, so they have the capacity to add up atoms in them. This reaction is carried out in the presence of nickel catalyst at 250 °C to 300 °C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature,
Ni
H2C=CH2 + H2 250-300 C H3C-CH3