CnH2n is the general formula for alkene group. The first member of this group is C2H4 and the second one is C3H6.

Homologous Series

The homologous series is a group of organic compounds that have similar chemical properties and a similar structural formula, differing from each other by a -CH2- unit. These compounds have a general formula and exhibit a gradual change in physical properties as the number of carbon atoms in the molecule increases.

General Formula CnH2n

The general formula CnH2n represents the second member of the homologous series, which is an alkene. Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. These compounds are characterized by the presence of sp2 hybridized carbon atoms.

Name and Formula of the 2nd Member

The second member of the homologous series with the general formula CnH2n is called ethene. It is also known as ethylene. Ethene consists of two carbon atoms and four hydrogen atoms, and its molecular formula is C2H4.

Structure and Properties of Ethene

Ethene has a linear structure with a carbon-carbon double bond. Each carbon atom in the molecule is bonded to two hydrogen atoms. The double bond in ethene consists of one sigma bond and one pi bond. The carbon atoms are sp2 hybridized, resulting in a trigonal planar geometry around each carbon atom.

Physical Properties
- Ethene is a colorless gas at room temperature and pressure.
- It has a slightly sweet odor.
- Ethene is lighter than air, with a density of 0.97 kg/m³.
- It is highly flammable and can form explosive mixtures with air.

Chemical Properties
- Ethene is highly reactive due to the presence of the carbon-carbon double bond.
- It readily undergoes addition reactions, where atoms or groups of atoms are added to the double bond.
- Ethene can undergo polymerization to form polyethylene, which is a widely used plastic.
- It can also participate in combustion reactions, where it reacts with oxygen to produce carbon dioxide and water.

Applications
- Ethene is used in the production of various chemicals, including ethanol, ethylene oxide, and polyethylene.
- It is an important raw material in the plastics industry.
- Ethene is also used as a plant hormone to promote fruit ripening.

In conclusion, the second member of the homologous series with the general formula CnH2n is ethene. Ethene is an unsaturated hydrocarbon with a molecular formula of C2H4. It is a colorless gas with unique physical and chemical properties, making it useful in various industrial applications.