(a) What is a homologous series of compounds ? List any two of its characteristics. (b) What is the next higher homologue of $C_3H_7OH$ ? What is its formula and what is it called ?

Q: (a) What is a homologous series of compounds ? List any two of its characteristics. (b) What is the next higher homologue of $C_3H_7OH$ ? What is its formula and what is it called ?

(a) A **homologous series** is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, resulting in similar chemical properties. Two characteristics are: 1. Any two adjacent members of the series differ by a $-CH_2-$ unit in their molecular formula. 2. The difference in molecular mass between two successive members is $14u$. (b) The next higher homologue of $C_3H_7OH$ (Propanol) is **Butanol**. Its formula is $C_4H_9OH$.

Carbon and its Compounds

Homologous Series

Official Answer

(a) A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, resulting in similar chemical properties.

Two characteristics are:

Any two adjacent members of the series differ by a $-CH_2-$ unit in their molecular formula.
The difference in molecular mass between two successive members is $14u$ .

(b) The next higher homologue of $C_3H_7OH$ (Propanol) is Butanol. Its formula is $C_4H_9OH$ .

homologous seriesfunctional group-CH2-14uButanolC4H9OH

Marking Scheme

1Definition of homologous series: 1 mark
2Two characteristics: 1 mark (0.5 each)
3Formula and name of next homologue: 1 mark

Hint

Add a $-CH_2-$ group to the given formula to find the next member.

Quick Oral Answer

A homologous series is a family of compounds with the same functional group where each member differs from the next by a -CH2 group. For example, the next member after propanol is butanol.

Analysis & Explanation

A homologous series (like the alkanes shown in Table 4.2) shows a regular gradation in physical properties while maintaining similar chemical reactivity due to the same functional group. For alcohols, the series starts with Methanol ( $CH_3OH$ ), Ethane ( $C_2H_5OH$ ), etc. Adding a $-CH_2-$ unit to Propanol ( $C_3H_7OH$ ) gives Butanol ( $C_4H_9OH$ ).

Common Mistakes

1Adding only a Carbon atom without adjusting Hydrogen atoms.
2Confusing the mass difference (stating 12u instead of 14u).

Interesting Facts

In a homologous series, as the molecular mass increases, the melting and boiling points also increase due to stronger intermolecular forces.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 3 marks in the CBSE Class 10 Science 2011 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Homologous Series" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.