Question

General formula of alkene is
a.) ${{\text{C}}_{\text{n}}}{{\text{H}}_{\text{2n+2}}}$
b.) ${{\text{C}}_{\text{n}}}{{\text{H}}_{2n}}$
c.) ${{\text{C}}_{\text{n}}}{{\text{H}}_{\text{2n-2}}}$
d.) ${{\text{C}}_{\text{n}}}{{\text{H}}_{\text{2n+1}}}$

Answer 1

Hint: Ethene is an alkene with molecular formula: ${{\text{C}}_{2}}{{\text{H}}_{4}}$. To find out the general formula, put ‘2’ in place of ’’n’ in all the options and find out the correct formula. The correct option would be the one which will match with the formula of ${{\text{C}}_{2}}{{\text{H}}_{4}}$.

Complete Solution :
Alkenes are unsaturated hydrocarbons containing at least one double bond between two carbon atoms. The hydrocarbons of this class are also called olefins. (olefiant = oil forming).
In the laboratory, alkenes are generally prepared either from haloalkanes (alkyl halides) or alcohols.
The general formula for alkene is given by: ${{\text{C}}_{\text{n}}}{{\text{H}}_{2n}}$

Some of the physical properties of alkanes are as follows-
Physical state- Unbranched alkenes containing up to four carbon atoms are gases and containing five to sixteen carbon atoms are liquids while those with more than 16 carbon atoms are solids.
Boiling point- The boiling points of alkene increase with molecular mass. The increase in boiling point can be attributed to the Van der Waals forces which increase with the number of carbon atoms of the alkene. The branched chain alkenes have lower boiling points than those of straight chain isomers.
Melting point- In alkenes, there is an increase in the melting point with the increase in molecular mass.
So, the correct answer is “Option B”.

Note: We should know about usage of important alkenes. Ethene is used in making mustard gas, which is a poisonous gas used in warfare. It is also used for artificial ripening of fruits, as a general anaesthetic and for producing other useful materials such as polythene, ethylene glycol (antifreeze), ethylene oxide (fumigant) etc.