Question

Why are certain compounds called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur.

Answer 1

The compounds entirely consisting of carbons and hydrogens are known as Hydrocarbons. There are different categories in which hydrocarbons are divided out of which the two are:

Saturated Hydrocarbons: The compounds of carbon having only single bonds between the carbon atoms are called saturated compounds. This includes alkanes, having a general formula C_nH_2n+2.The first member of homologous series of alkanes is methane (CH₄).
Structure of methane is as follows:

Unsaturated Hydrocarbons: The compounds of carbon having double and triple bonds between the carbon atoms are called unsaturated compounds. This includes alkenes and alkynes having general formula C_nH_2n  and C_nH_2n-2, respectively. The first member of homologous series of alkenes is Ethene (C₂H₄).
The structural formula of ethene is as follows: H₂C=CH_2. 
The first member of homologous series of alkynes is Ethyne (C₂H₂) having structural formula: HC=CH.

The reaction which converts unsaturated hydrocarbons to saturated hydrocarbons, i.e. alkenes to alkanes is know as Hydrogenation Reaction. 
It is used to obtain ghee from oil. Vegetable oil contains unsaturated fatty acids, and is liquid at room temperature. When they are hydrogenated, they become saturated and change to a solid state.

The reaction is as follows:
Hydrogen can be added to unsaturated hydrocarbons in the presence of nickel to yield saturated hydrocarbons. In this reaction, nickel acts as a catalyst.