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Science Grade XI Chemistry

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Chapter 9: Hydrocarbons

Grade 11 Science | Chapter 9

Hydrocarbons

Hydrocarbons contain only carbon and hydrogen. This chapter classifies them as alkanes, alkenes, alkynes and aromatics, and contrasts their saturation, formulae and reactions.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Carbon and Hydrogen Only

Classifying Hydrocarbons

Alkanes

Alkenes and Alkynes

Reactions: Substitution and Addition

Aromatic Hydrocarbons

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Carbon and Hydrogen Only

A hydrocarbon is a compound of carbon and hydrogen only, and hydrocarbons are the basis of fuels and of much of organic chemistry. This chapter classifies them by the kind of carbon to carbon bonds they contain, gives their general formulae, and contrasts the characteristic reactions of saturated and unsaturated hydrocarbons.

Core idea

Hydrocarbons are saturated (only single bonds, alkanes) or unsaturated (double or triple bonds, alkenes and alkynes). Saturated hydrocarbons react by substitution, unsaturated ones by addition.

2. Classifying Hydrocarbons

Hydrocarbons fall into families by their bonding. Alkanes have only single carbon to carbon bonds and are saturated. Alkenes have at least one double bond and alkynes at least one triple bond, both unsaturated. Aromatic hydrocarbons contain a special stable ring, such as benzene. The bond type sets the family’s chemistry.

Diagram 1 – Single, Double and Triple Bonds

Ethane with a single bond, ethene with a double bond and ethyne with a triple bond

Fig 1. Alkanes have single bonds (saturated); alkenes have a double bond and alkynes a triple bond (unsaturated).

3. Alkanes

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2, such as methane CH₄ and ethane C₂H₆. Their single bonds make them relatively unreactive, though they burn readily as fuels. Because they have no double or triple bonds, they cannot add atoms across a bond.

4. Alkenes and Alkynes

Alkenes contain a carbon to carbon double bond and have the general formula C_nH_2n, such as ethene C₂H₄. Alkynes contain a triple bond and have the formula C_nH_2n-2, such as ethyne C₂H₂. The extra bonds make them unsaturated and far more reactive than alkanes.

5. Reactions: Substitution and Addition

The reactions of a hydrocarbon follow from its bonds. Saturated alkanes react by substitution, where one atom replaces another, since there is no double bond to react across. Unsaturated alkenes and alkynes react by addition, where atoms add across the multiple bond, turning it into a single bond. All hydrocarbons also undergo combustion, burning in oxygen to give carbon dioxide and water.

6. Aromatic Hydrocarbons

Aromatic hydrocarbons contain a benzene ring, six carbon atoms in a ring with electrons shared, or delocalised, around it. This delocalisation makes the ring unusually stable, so benzene tends to react by substitution rather than addition, despite appearing unsaturated. Aromatic compounds are widespread in dyes, medicines and plastics.

Diagram 2 – The Benzene Ring

A benzene ring of six carbons with a circle showing delocalised electrons

Fig 2. Benzene is a ring of six carbons with delocalised electrons, which makes it unusually stable.

7. Key Reasoning (Principles)

Principle 1: Bond type sets saturation

Only single bonds make a hydrocarbon saturated (alkane); a double or triple bond makes it unsaturated (alkene or alkyne), with a different general formula.

Principle 2: Saturated substitutes, unsaturated adds

Alkanes react by substitution because they have no multiple bond, while alkenes and alkynes react by addition across their multiple bond.

Principle 3: Delocalisation stabilises aromatics

In benzene the shared, delocalised electrons make the ring especially stable, so it reacts by substitution rather than addition.

8. Worked Examples

Example 1

Q: Write the general formula of an alkene.

▶ Show Solution

Alkenes have one double bond and the formula C_nH_2n.

Answer: C_nH_2n.

Example 2

Q: Write the general formula of an alkyne.

▶ Show Solution

Alkynes have one triple bond and the formula C_nH_2n-2.

Answer: C_nH_2n-2.

Example 3

Q: Find the formula of the alkene with 3 carbons.

▶ Show Solution

C_nH_2n with n = 3 gives 6 hydrogens.

C₃H₆.

Answer: C₃H₆.

Example 4

Q: Find the formula of the alkyne with 2 carbons.

▶ Show Solution

C_nH_2n-2 with n = 2 gives 2 hydrogens.

C₂H₂.

Answer: C₂H₂.

Example 5

Q: Which family is saturated?

▶ Show Solution

Only single bonds means saturated.

The alkanes.

Answer: Alkanes.

Example 6

Q: How do alkenes typically react?

▶ Show Solution

Their double bond allows atoms to add across it.

By addition.

Answer: By addition.

Example 7

Q: How do alkanes typically react?

▶ Show Solution

With no double bond, one atom replaces another.

By substitution.

Answer: By substitution.

Example 8

Q: Find the formula of the alkane with 4 carbons.

▶ Show Solution

C_nH_2n+2 with n = 4 gives 10 hydrogens.

C₄H₁₀.

Answer: C₄H₁₀.

Example 9

Q: What makes benzene unusually stable?

▶ Show Solution

Its delocalised, shared ring electrons.

Answer: Delocalised electrons.

Example 10

Q: What do all hydrocarbons give on complete combustion?

▶ Show Solution

Burning in oxygen gives carbon dioxide and water.

Answer: Carbon dioxide and water.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. A hydrocarbon contains only: (a) C and O (b) C and H (c) H and O (d) C and N

2. Alkanes have the general formula: (a) CnH2n (b) CnH(2n+2) (c) CnH(2n-2) (d) CnHn

3. A double bond is found in: (a) alkanes (b) alkenes (c) noble gases (d) salts

4. Alkanes react mainly by: (a) addition (b) substitution (c) neutralisation (d) precipitation

5. Benzene is an example of a hydrocarbon that is: (a) ionic (b) aromatic (c) saturated chain (d) metallic

▶ Reveal Answers

1. (b) C and H.

2. (b) CnH(2n+2).

3. (b) alkenes.

4. (b) substitution.

5. (b) aromatic.

Set B – Short Answer (Understanding)

1. Define a hydrocarbon.

2. State the difference between saturated and unsaturated hydrocarbons.

3. Write the general formulae of alkanes, alkenes and alkynes.

4. How do alkenes and alkynes react, and why?

5. Why is benzene unusually stable?

▶ Reveal Answers

1. A compound of carbon and hydrogen only.

2. Saturated hydrocarbons have only single bonds; unsaturated ones have double or triple bonds.

3. Alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n-2.

4. By addition across their multiple bond, since the extra bond can open up.

5. Because its ring electrons are delocalised, which gives extra stability.

Set C – Application and Reasoning

1. Find the alkene formula for 4 carbons.

2. Find the alkyne formula for 3 carbons.

3. Which is more reactive, ethane or ethene, and why?

4. Find the alkane formula for 5 carbons.

5. Why can an alkane not undergo addition?

▶ Reveal Answers

1. C₄H₈.

2. C₃H₄.

3. Ethene, because its double bond lets atoms add across it.

4. C₅H₁₂.

5. Because it has no double or triple bond for atoms to add across.

Set D – Higher Order (Challenge)

1. Balance the combustion of methane: CH₄ plus O₂ gives CO₂ plus H₂O.

2. Explain why an alkene decolourises bromine water but an alkane does not.

3. Find the molecular formula of the alkene with 6 carbons.

4. Explain why benzene reacts by substitution despite appearing unsaturated.

5. Compare the number of hydrogens in the 4-carbon alkane, alkene and alkyne.

▶ Reveal Answers

1. CH₄ + 2O₂ gives CO₂ + 2H₂O.

2. The alkene adds bromine across its double bond, removing the colour; the alkane has no double bond to react.

3. C_nH_2n with n = 6 gives C₆H₁₂.

4. Its delocalised electrons make the ring so stable that substitution, which keeps the ring intact, is preferred over addition.

5. Alkane C₄H₁₀ (10 H), alkene C₄H₈ (8 H), alkyne C₄H₆ (6 H); each extra bond removes two hydrogens.

Chapter Summary

Hydrocarbon

A compound of carbon and hydrogen only.

Saturated

Alkanes, only single bonds, C_nH_2n+2.

Unsaturated

Alkenes (double, C_nH_2n) and alkynes (triple, C_nH_2n-2).

Reactions

Alkanes substitute; alkenes and alkynes add; all combust.

Aromatic

Benzene ring with delocalised, stabilising electrons.

Combustion

All burn in oxygen to give carbon dioxide and water.

Quantity

Unit

Symbol

Alkane

CnH(2n+2)

saturated

Alkene

CnH2n

double

Alkyne

CnH(2n-2)

triple

8-Point Exam Quick-Check

A hydrocarbon contains only carbon and hydrogen.

Alkanes are saturated (single bonds), formula CnH(2n+2).

Alkenes have a double bond, formula CnH2n.

Alkynes have a triple bond, formula CnH(2n-2).

Alkanes react by substitution; alkenes and alkynes by addition.

All hydrocarbons burn to give carbon dioxide and water.

Benzene is aromatic with delocalised, stabilising electrons.

Each extra carbon-carbon bond removes two hydrogens.

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Class 11 Chemistry Chapter 9: Hydrocarbons, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Chemistry syllabus and develops hydrocarbons, covering their classification, saturated alkanes and unsaturated alkenes and alkynes with their general formulae, the substitution and addition reactions, combustion, and aromatic hydrocarbons such as benzene, with two diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.