Class 10th Science Chapter 4 – Carbon and Its Compounds | Homologous Series

Carbon and Its Compounds | Homologous Series

Class 10 Science | Chapter 4 | CBSE + Foundation + Competitive Level

Carbon and Its Compounds | Homologous Series

1. What is a Homologous Series? | समजातीय श्रेणी क्या है?

English:
A homologous series is a group or family of organic compounds having:

• The same functional group.
• The same general formula.
• Similar chemical properties.
• A gradual change in physical properties.
• Successive members differing by a –CH₂– unit.

Hindi:
समजातीय श्रेणी (Homologous Series) कार्बनिक यौगिकों का ऐसा समूह है जिसमें:

• समान functional group होता है।
• समान general formula होता है।
• रासायनिक गुण लगभग समान होते हैं।
• भौतिक गुणों में क्रमिक परिवर्तन होता है।
• क्रमिक सदस्यों में –CH₂– का अंतर होता है।

2. Definition | परिभाषा

English:
A homologous series is a family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a –CH₂– group.

Hindi:
समजातीय श्रेणी समान functional group वाले कार्बनिक यौगिकों का समूह है, जिसमें क्रमिक सदस्यों के molecular formula में –CH₂– का अंतर होता है।

3. Difference of –CH₂– Between Successive Members

Consider the alkane series:

Compound	Formula	Carbon Atoms
Methane	CH₄	1
Ethane	C₂H₆	2
Propane	C₃H₈	3
Butane	C₄H₁₀	4
Pentane	C₅H₁₂	5

C₂H₆ − CH₄ = CH₂
C₃H₈ − C₂H₆ = CH₂
C₄H₁₀ − C₃H₈ = CH₂

Exam Point:
Successive members of a homologous series differ by one –CH₂– unit, whose relative molecular mass is 14 u.
Hindi: प्रत्येक अगले सदस्य में molecular mass सामान्यतः 14 u बढ़ता है।

4. General Formula of Homologous Series

Different homologous series have different general formulae.

Series	General Formula	Example
Alkanes	CₙH₂ₙ₊₂	CH₄, C₂H₆, C₃H₈
Alkenes	CₙH₂ₙ	C₂H₄, C₃H₆, C₄H₈
Alkynes	CₙH₂ₙ₋₂	C₂H₂, C₃H₄, C₄H₆
Alcohols	CₙH₂ₙ₊₁OH	CH₃OH, C₂H₅OH
Carboxylic Acids	CₙH₂ₙ₊₁COOH	HCOOH, CH₃COOH

5. Homologous Series of Alkanes | एल्केन की समजातीय श्रेणी

Name	Formula	Molecular Mass
Methane	CH₄	16 u
Ethane	C₂H₆	30 u
Propane	C₃H₈	44 u
Butane	C₄H₁₀	58 u
Pentane	C₅H₁₂	72 u
Hexane	C₆H₁₄	86 u

Alkanes → CₙH₂ₙ₊₂

6. Homologous Series of Alkenes | एल्कीन

Alkenes contain at least one C=C double bond. General formula:

CₙH₂ₙ

Compound	Formula
Ethene	C₂H₄
Propene	C₃H₆
Butene	C₄H₈
Pentene	C₅H₁₀

7. Homologous Series of Alkynes | एल्काइन

Alkynes contain at least one C≡C triple bond. General formula:

CₙH₂ₙ₋₂

Compound	Formula
Ethyne	C₂H₂
Propyne	C₃H₄
Butyne	C₄H₆
Pentyne	C₅H₈

8. Homologous Series of Alcohols | अल्कोहॉल

Alcohols contain the functional group –OH. General formula:

CₙH₂ₙ₊₁OH

Alcohol	Formula
Methanol	CH₃OH
Ethanol	C₂H₅OH
Propanol	C₃H₇OH
Butanol	C₄H₉OH

9. Homologous Series of Carboxylic Acids

Carboxylic acids contain the functional group: –COOH Examples:

Compound	Formula
Methanoic acid	HCOOH
Ethanoic acid	CH₃COOH
Propanoic acid	C₂H₅COOH
Butanoic acid	C₃H₇COOH

10. Characteristics of a Homologous Series

1. Same functional group
Members have the same functional group.

2. Same general formula
All members follow the same general formula.

3. Similar chemical properties
Because the functional group is the same, their chemical reactions are similar.

4. Difference of –CH₂–
Two consecutive members differ by –CH₂–.

5. Molecular mass increases by 14 u
Since CH₂ = 12 + 2 = 14 u.

6. Gradual change in physical properties
Boiling point, melting point, density and solubility may change gradually as molecular mass increases.

11. Why Do Members Have Similar Chemical Properties?

English:
The chemical properties of organic compounds mainly depend on their functional groups. Since all members of a homologous series contain the same functional group, they show similar chemical reactions.

Hindi:
कार्बनिक यौगिकों के रासायनिक गुण मुख्य रूप से उनके functional group पर निर्भर करते हैं। क्योंकि homologous series के सभी सदस्यों में functional group समान होता है, इसलिए उनके रासायनिक गुण समान होते हैं।

12. Variation in Physical Properties

As molecular size increases:

• Boiling point generally increases.
• Melting point generally changes gradually.
• Density generally increases gradually.
• Solubility and physical state may change.

Example: Methane → gas
Ethane → gas
Propane → gas
Butane → gas
Higher alkanes → liquids/solids depending on molecular size.

13. Why is the Difference 14 u?

CH₂ = 12 + (2 × 1)

CH₂ = 14 u

Therefore, when one carbon and two hydrogen atoms are added to the next member, molecular mass increases by 14 u.

14. Example: Finding the Next Member

Suppose the compound is: C₃H₈ The next member will contain one additional –CH₂– group.

C₃H₈ + CH₂ = C₄H₁₀

Therefore: Propane → Butane

Similarly:

C₄H₁₀ + CH₂ = C₅H₁₂

Butane → Pentane

15. Importance of Homologous Series

Homologous series helps us to:

1. Classify organic compounds.
2. Predict the formula of the next member.
3. Predict molecular mass.
4. Understand similarities in chemical reactions.
5. Study gradual changes in physical properties.
6. Understand organic chemistry systematically.

16. 30 MCQs | बहुविकल्पीय प्रश्न

Q1. What is the characteristic difference between two successive members of a homologous series?

A. –CH₃

B. –CH₂–

C. –OH

D. –COOH

Answer: B — Successive members differ by –CH₂–.

Q2. What is the relative molecular mass of –CH₂–?

A. 12 u

B. 13 u

C. 14 u

D. 15 u

Answer: C — 12 + 2(1) = 14 u.

Q3. Which of the following represents the general formula of alkanes?

A. CₙH₂ₙ₊₂

B. CₙH₂ₙ

C. CₙH₂ₙ₋₂

D. CₙHₙ

Answer: A

Q4. Which functional group is present in alcohols?

A. –COOH

B. –CHO

C. –OH

D. –CO–

Answer: C

Q5. Which pair belongs to the same homologous series?

A. CH₄ and C₂H₆

B. CH₄ and C₂H₄

C. C₂H₆ and C₂H₄

D. C₂H₂ and C₃H₈

Answer: A

Q6. What is the molecular formula of propane?

A. C₃H₆

B. C₃H₈

C. C₃H₄

D. C₂H₆

Answer: B

Q7. Which compound is the next homologous member after ethane?

A. Methane

B. Ethene

C. Propane

D. Butane

Answer: C

Q8. The general formula of alkenes is:

A. CₙH₂ₙ₊₂

B. CₙH₂ₙ

C. CₙH₂ₙ₋₂

D. CₙHₙ₊₂

Answer: B

Q9. Which bond is characteristic of alkenes?

A. C–C single bond only

B. C=C double bond

C. C≡C triple bond

D. C–H ionic bond

Answer: B

Q10. Which of the following is an alkyne?

A. C₂H₆

B. C₂H₄

C. C₂H₂

D. CH₄

Answer: C

Q11. The members of a homologous series have:

A. Different functional groups

B. Same functional group

C. Different general formulae

D. No chemical similarity

Answer: B

Q12. Which property changes gradually in a homologous series?

A. Chemical formula only

B. Atomic number

C. Physical properties

D. Functional group

Answer: C

Q13. Which is the molecular formula of butane?

A. C₄H₈

B. C₄H₁₀

C. C₄H₆

D. C₃H₈

Answer: B

Q14. Which compound follows the formula CₙH₂ₙ?

A. Alkane

B. Alkene

C. Alkyne

D. Alcohol

Answer: B

Q15. What is the molecular formula of pentane?

A. C₅H₁₀

B. C₅H₈

C. C₅H₁₂

D. C₄H₁₀

Answer: C

Q16. Which one is NOT a characteristic of a homologous series?

A. Same functional group

B. Same general formula

C. Successive members differ by CH₂

D. Completely different chemical properties

Answer: D

Q17. Methanol and ethanol belong to:

A. Alkane series

B. Alkene series

C. Alcohol homologous series

D. Alkyne series

Answer: C

Q18. Which functional group is found in carboxylic acids?

A. –OH

B. –COOH

C. –CHO

D. –O–

Answer: B

Q19. Which is the next member after C₃H₈?

A. C₄H₁₀

B. C₄H₈

C. C₃H₆

D. C₅H₁₂

Answer: A

Q20. Why do members of the same homologous series show similar chemical properties?

A. Same colour

B. Same molecular mass

C. Same functional group

D. Same number of atoms

Answer: C

Q21. What is the molecular mass of ethane?

A. 16 u

B. 28 u

C. 30 u

D. 44 u

Answer: C

Q22. Which of the following is a member of alkane homologous series?

A. C₃H₆

B. C₄H₁₀

C. C₂H₂

D. C₅H₁₀

Answer: B

Q23. What is the general formula of alkynes?

A. CₙH₂ₙ₊₂

B. CₙH₂ₙ

C. CₙH₂ₙ₋₂

D. CₙHₙ

Answer: C

Q24. Which compound has the formula C₄H₈?

A. Butane

B. Butene

C. Butyne

D. Propane

Answer: B

Q25. The boiling point generally increases with:

A. Decreasing molecular size

B. Increasing molecular size

C. Decreasing carbon atoms

D. Removal of functional group

Answer: B

Q26. Which pair differs by –CH₂–?

A. CH₄ and C₂H₆

B. CH₄ and C₂H₄

C. C₂H₆ and C₃H₆

D. C₂H₄ and C₃H₄

Answer: A

Q27. Which statement is correct?

A. Homologous members have different functional groups.

B. Successive members differ by CH₂.

C. They have completely different chemical properties.

D. Their general formula is always different.

Answer: B

Q28. Ethanoic acid belongs to which homologous series?

A. Alcohols

B. Alkenes

C. Carboxylic acids

D. Alkynes

Answer: C

Q29. If one member has molecular mass 44 u, the next member in the same series will generally have molecular mass:

A. 54 u

B. 58 u

C. 60 u

D. 72 u

Answer: B — 44 + 14 = 58 u.

Q30. Which factor mainly determines the chemical properties of a homologous series?

A. Functional group

B. Colour

C. Physical state

D. Molecular volume only

Answer: A

17. 30 Subjective Questions | वर्णनात्मक प्रश्न

1 Mark
Q1. What is a homologous series?
समजातीय श्रेणी क्या है?

Answer: A homologous series is a family of organic compounds having the same functional group and successive members differing by –CH₂–.
Hindi: समान functional group वाले कार्बनिक यौगिकों की श्रेणी जिसमें क्रमिक सदस्य –CH₂– से भिन्न हों।

1 Mark
Q2. What is the molecular mass of –CH₂–?

Answer: 14 u.

1 Mark
Q3. Name the functional group of alcohols.

Answer: Hydroxyl group, –OH.

1 Mark
Q4. Write the general formula of alkanes.

Answer: CₙH₂ₙ₊₂.

1 Mark
Q5. Give one example of a homologous pair.

Answer: Methane (CH₄) and ethane (C₂H₆).

2 Marks
Q6. State two characteristics of a homologous series.

Answer:

1. Members have the same functional group.
2. Successive members differ by –CH₂–.

2 Marks
Q7. Why do members of a homologous series have similar chemical properties?

Answer: Because members of a homologous series contain the same functional group, which determines their characteristic chemical reactions.

2 Marks
Q8. Why does molecular mass increase by 14 u in a homologous series?

Solution:
Each successive member contains one additional CH₂ group.

CH₂ = 12 + 2 = 14 u Therefore molecular mass increases by 14 u.

2 Marks
Q9. Write the first four members of the alkane series.

Answer:
Methane – CH₄
Ethane – C₂H₆
Propane – C₃H₈
Butane – C₄H₁₀

2 Marks
Q10. Write two differences between alkanes and alkenes.

Answer:

Alkanes	Alkenes
Only single C–C bonds	At least one C=C bond
CₙH₂ₙ₊₂	CₙH₂ₙ

3 Marks
Q11. Explain the term homologous series with an example.

Solution:
A homologous series is a family of organic compounds having the same functional group and general formula. Consecutive members differ by –CH₂–. Example: CH₄ → C₂H₆ → C₃H₈ → C₄H₁₀ Each successive member differs by CH₂.

3 Marks
Q12. Write three characteristics of a homologous series.

Answer:

1. Same functional group.
2. Same general formula.
3. Successive members differ by –CH₂–.

3 Marks
Q13. Write the first five members of the alkane homologous series.

Answer:
CH₄, C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂.

3 Marks
Q14. Explain the variation of physical properties in a homologous series.

Answer:
As molecular size increases, physical properties such as boiling point, melting point and density generally change gradually. The change occurs because molecular mass and intermolecular forces increase.

3 Marks
Q15. Find the next two members after C₂H₆ in the alkane series.

Solution:
C₂H₆ + CH₂ = C₃H₈
C₃H₈ + CH₂ = C₄H₁₀

Answer: C₃H₈ and C₄H₁₀.

4 Marks
Q16. Explain four important characteristics of a homologous series.

Answer:

1. Same functional group.
2. Same general formula.
3. Successive members differ by CH₂.
4. Members show similar chemical properties but gradual changes in physical properties.

4 Marks
Q17. Compare alkanes, alkenes and alkynes.

Series	Bond	General Formula	Example
Alkanes	Single	CₙH₂ₙ₊₂	C₂H₆
Alkenes	Double	CₙH₂ₙ	C₂H₄
Alkynes	Triple	CₙH₂ₙ₋₂	C₂H₂

4 Marks
Q18. Explain why chemical properties are similar but physical properties change gradually in a homologous series.

Solution:
Chemical properties depend mainly on the functional group, which remains the same. Hence chemical properties are similar. Physical properties depend considerably on molecular mass and size. Since molecular mass increases gradually by 14 u, physical properties also change gradually.

4 Marks
Q19. Write the first four members of alcohol homologous series.

Answer:
Methanol – CH₃OH
Ethanol – C₂H₅OH
Propanol – C₃H₇OH
Butanol – C₄H₉OH

4 Marks
Q20. Explain with examples that consecutive members of a homologous series differ by CH₂.

Solution:
CH₄ → C₂H₆ Difference = CH₂

C₂H₆ → C₃H₈ Difference = CH₂

C₃H₈ → C₄H₁₀ Difference = CH₂

Thus consecutive members differ by one CH₂ unit.

5 Marks
Q21. Explain homologous series in detail with an example.

Solution:
A homologous series is a family of organic compounds with the same functional group and general formula. Important characteristics:

1. Same functional group.
2. Same general formula.
3. Similar chemical properties.
4. Successive members differ by CH₂.
5. Molecular mass increases by 14 u.
6. Physical properties change gradually.

Example of alkanes: CH₄, C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂.

5 Marks
Q22. Explain the importance of homologous series in organic chemistry.

Answer:

1. Helps classify organic compounds.
2. Helps predict molecular formulae.
3. Helps predict the next member.
4. Helps understand similar chemical reactions.
5. Helps study changes in physical properties.

5 Marks
Q23. Describe the homologous series of alkanes with molecular formulae of the first six members.

Member	Formula
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₄H₁₀
Pentane	C₅H₁₂
Hexane	C₆H₁₄

All consecutive members differ by CH₂.

5 Marks
Q24. Explain the relationship between molecular mass and physical properties in a homologous series.

Solution:
The molecular size increases as CH₂ units are added. Consequently, intermolecular attractions generally increase. Therefore properties such as boiling point, melting point and density show a gradual trend with increasing molecular mass.

5 Marks
Q25. Differentiate between homologous series and isomers.

Homologous Series	Isomers
Same functional group and general formula.	Same molecular formula.
Successive members differ by CH₂.	Different structural arrangements.
Example: methane, ethane, propane.	Example: n-butane and isobutane.

6 Marks
Q26. Explain all major characteristics of a homologous series with suitable examples.

Solution: A homologous series has the following major characteristics:

1. Same functional group: Members contain the same functional group.
2. Same general formula: Members follow the same general formula.
3. Similar chemical properties: Same functional group produces similar chemical behaviour.
4. Difference of CH₂: Consecutive members differ by CH₂.
5. 14 u increase: Molecular mass increases by 14 u.
6. Gradual physical change: Boiling point, melting point and density change gradually.

Example: CH₄ → C₂H₆ → C₃H₈ → C₄H₁₀

6 Marks
Q27. Explain the homologous series of alkanes, alkenes and alkynes with their general formulae.

Series	General Formula	Examples
Alkanes	CₙH₂ₙ₊₂	CH₄, C₂H₆
Alkenes	CₙH₂ₙ	C₂H₄, C₃H₆
Alkynes	CₙH₂ₙ₋₂	C₂H₂, C₃H₄

All members within each series have similar chemical properties and consecutive members differ by CH₂.

6 Marks
Q28. How can you identify whether given compounds belong to the same homologous series?

Solution: Check the following:

1. Whether they have the same functional group.
2. Whether they follow the same general formula.
3. Whether their chemical properties are similar.
4. Whether successive members differ by CH₂.

Example: C₂H₆, C₃H₈ and C₄H₁₀ belong to the same alkane homologous series.

6 Marks
Q29. A compound has molecular formula C₄H₁₀. Find the next two members of its alkane homologous series and calculate their molecular masses.

Solution: C₄H₁₀ + CH₂ = C₅H₁₂
C₅H₁₂ + CH₂ = C₆H₁₄

Molecular mass of C₅H₁₂: 5 × 12 + 12 × 1 = 72 u

Molecular mass of C₆H₁₄: 6 × 12 + 14 × 1 = 86 u

Answer: C₅H₁₂ and C₆H₁₄; masses = 72 u and 86 u.

6 Marks
Q30. Explain why homologous series are useful for predicting properties of organic compounds.

Solution: A homologous series follows a definite pattern. Because consecutive members differ by CH₂ and have the same functional group:

• The molecular formula of the next member can be predicted.
• The molecular mass can be calculated.
• Similar chemical reactions can be predicted.
• Changes in physical properties can be estimated.

Therefore homologous series provides a systematic way to study large numbers of organic compounds.

18. Quick Revision | त्वरित पुनरावृत्ति

⭐ Homologous Series = Same Functional Group + Same General Formula + CH₂ Difference

Important Points:

• Successive members differ by –CH₂–.
• CH₂ has molecular mass 14 u.
• Chemical properties are similar.
• Physical properties change gradually.
• Alkanes → CₙH₂ₙ₊₂
• Alkenes → CₙH₂ₙ
• Alkynes → CₙH₂ₙ₋₂
• Alcohols contain –OH.
• Carboxylic acids contain –COOH.

19. Exam Booster | परीक्षा के लिए महत्वपूर्ण

Remember:

1. Homologous = Same functional group
2. Consecutive members = CH₂ difference
3. CH₂ = 14 u
4. Same series = Similar chemical properties
5. Increasing molecular mass = Gradual change in physical properties