CBSE Class 10 Science 2017 · Paper 31/1 · Question 19

Hydrocarbons: compounds made up of only carbon and hydrogen are called hydrocarbons. General formulae: alkanes ${\text{C}}_n{\text{H}}_{2n+2}$; alkenes ${\text{C}}_n{\text{H}}_{2n}$; alkynes ${\text{C}}_n{\text{H}}_{2n-2}$. First members: methane, ${\text{CH}}_4$, a carbon atom with single bonds to four hydrogen atoms; ethene, ${\text{C}}_2{\text{H}}_4$, ${\text{H}}_2\text{C}={\text{CH}}_2$; ethyne, ${\text{C}}_2{\text{H}}_2$, $\text{H}-\text{C}\equiv \text{C}-\text{H}$ (structures in the figure). Reaction: hydrogenation, an addition reaction, which needs a catalyst such as nickel (with heating) or palladium: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$.