NCERT Solution for Class Tenth Science Chapter 4
Carbon and its Compounds
CBSE NCERT Book Session-2022-2023
Intext Questions
Page No-21
Questions No-6
What is an homologous series? Explain with an example.
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NCERT Solution for Class Tenth Science Chapter 4
Carbon and its Compounds
CBSE NCERT Book Session-2022-2023
Intext Questions
Page No-21
Questions No-6
A homologous series is a series of carbon compounds that have different numbers of
carbon atoms but contain the same functional group.
For example, methane, ethane, propane, butane, etc. are all part of the alkane
homologous series. The general formula of this series is CnH2n+2.
Methane CH₄
Ethane CH₃CH₃
Propane CH₃CH₂CH₃
Butane CH₃CH₂CH2CH₃
It can be noticed that there is a difference of -CH₂ unit between each successive
compound.
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A homologous series is a group of organic compounds having
similar structures and similar chemical properties in which the successive compounds differ by -CH2 group.
For example, general formula of the homologous series of alkanes is CnH2n+2, in which ‘n’ denotes number of carbon atoms in one molecule of alkane. Following are the first five members of the homologous series of alkanes (general formula CnH2n+2).
The best-known example of an homologous series is the alkanes. Alkanes are a group of saturated hydrocarbons (meaning they contain only carbon-carbon and carbon-hydrogen single bonds) with the general molecular formula CnH2n+2, where “n” represents the number of carbon atoms in the molecule. Here’s an example of an alkane homologous series:
1. Methane (CH4)
2. Ethane (C2H6)
3. Propane (C3H8)
4. Butane (C4H10)
5. Pentane (C5H12)
6. Hexane (C6H14)
7. Heptane (C7H16)
8. Octane (C8H18)
9. Nonane (C9H20)
10. Decane (C10H22)
In this series, each compound differs from the previous one by a “CH2” unit. As you move from methane to ethane, for example, you add one carbon atom and two hydrogen atoms. This pattern continues throughout the series. The systematic increase in the number of carbon and hydrogen atoms results in changes in physical properties such as boiling point, melting point, and density, as well as changes in chemical reactivity.
Another example of an homologous series is the alkenes, which are unsaturated hydrocarbons with double bonds between carbon atoms. Alkenes have the general formula CnH2n and include compounds like ethene (C2H4), propene (C3H6), and butene (C4H8). In this series, the number of carbon atoms increases by one, and the number of hydrogen atoms increases by two as you move from one member to the next.
Homologous series are essential in organic chemistry because they allow for the prediction of properties and behaviors of related compounds within the series, which simplifies the study of a wide range of organic molecules.