Hydrocarbons — Class 11 Chemistry NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Chemistry chapter "Hydrocarbons" — 6 important questions with detailed answers for CBSE board exam preparation.
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TL;DR: Free step-by-step NCERT solutions for Class 11 Chemistry chapter "Hydrocarbons" — 6 important questions with detailed answers for CBSE board exam prep…
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Key Questions Covered:
- What are hydrocarbons? Classify them with examples and properties.
- Explain cracking of hydrocarbons. What is the difference between thermal and …
- What are aromatic hydrocarbons? Explain structure and aromaticity of benzene …
- Write reactions of benzene showing substitution patterns. Explain ortho-, met…
- Compare alkanes, alkenes, and alkynes. Give combustion reactions and bond ene…
- Explain addition reactions of alkenes. Give mechanism of HBr addition and Mar…
Solutions Summary:
| Question | Status |
|---|---|
| What are hydrocarbons? Classify them with examples and pr… | ✓ Solved |
| Explain cracking of hydrocarbons. What is the difference … | ✓ Solved |
| What are aromatic hydrocarbons? Explain structure and aro… | ✓ Solved |
| Write reactions of benzene showing substitution patterns.… | ✓ Solved |
| Compare alkanes, alkenes, and alkynes. Give combustion re… | ✓ Solved |
| Explain addition reactions of alkenes. Give mechanism of … | ✓ Solved |
Showing 6 of 6 questions
Q1: What are hydrocarbons? Classify them with examples and properties.
Hydrocarbons: Organic compounds containing only carbon and hydrogen atoms.
Formula: CnHm
Classification 1: Based on Saturation
1. Saturated Hydrocarbons (Alkanes)
- General formula: CnH(2n+2)
- Only single bonds between carbons
- Non-polar, hydrophobic
- Low reactivity, combustible
- Example: CH3CH2CH3 (Propane)
2. Unsaturated Hydrocarbons
a) Alkenes (C=C double bond)
Formula: CnH(2n)
Reactive, undergo addition reactions
Example: CH2=CHCH3 (Propene)
...
Q2: Explain cracking of hydrocarbons. What is the difference between thermal and catalytic cracking?
Cracking: Process of breaking C-C bonds in long-chain hydrocarbons to produce shorter-chain hydrocarbons.
Conditions:
- Temperature: 400-900°C
- Purpose: Convert heavy fractions into lighter, more useful products
- Products: Alkanes (LPG, petrol) + Alkenes (ethene, propene)
Thermal Cracking:
- Temperature: 800-900°C
- Pressure: 10-20 atm
- No catalyst
- Mechanism: Free radical chain reaction
1. Initiation: C-C bond breaks into radicals
2. Propagation: Radical attack creates new radicals
...
Q3: What are aromatic hydrocarbons? Explain structure and aromaticity of benzene (C6H6).
Aromatic Hydrocarbons: Cyclic, unsaturated hydrocarbons with special stability and properties.
Benzene (C6H6): Most common aromatic hydrocarbon
Structure:
- 6 carbon atoms in hexagonal ring
- Each C bonded to 1 H atom
- Each C undergoes sp2 hybridization
- Three unhybridized p-orbitals on each C
- These 6 p-orbitals overlap to form π electron cloud
Resonance Structures (Kekulé structures):
H C-C H
| || |
H-C C C-H
|| ||
H-C C-C-H
H
Actual Structure: Resonance hybrid
...
Q4: Write reactions of benzene showing substitution patterns. Explain ortho-, meta-, and para-directors.
Benzene Reactions: Substitution (electrophilic aromatic substitution), not addition
1. Halogenation (Bromination)
C6H6 + Br2 ---FeBr3--> C6H5Br + HBr
catalyst
Bromobenzene (monobromination)
With excess Br2:
C6H5Br + Br2 ---FeBr3--> C6H4Br2 + HBr (o-, p-, m-isomers possible)
2. Nitration
C6H6 + HNO3 ---H2SO4--> C6H5NO2 + H2O
conc., 60°C
Nitrobenzene
3. Friedel-Crafts Alkylation
C6H6 + RCl ---AlCl3--> C6H5R + HCl
Alkylbenzene (toluene if R = CH3)
4. Friedel...
Q5: Compare alkanes, alkenes, and alkynes. Give combustion reactions and bond energies.
Comparison of Alkanes, Alkenes, and Alkynes
1. Structure and Formula:
Alkanes (CnH(2n+2)):
- Only single C-C bonds
- Fully saturated
- Example: C2H6 (Ethane) CH3-CH3
Alkenes (CnH(2n)):
- One or more C=C double bonds
- Unsaturated (one degree of unsaturation per double bond)
- Example: C2H4 (Ethene) CH2=CH2
Alkynes (CnH(2n-2)):
- One or more C≡C triple bonds
- Highly unsaturated (two degrees of unsaturation per triple bond)
- Example: C2H2 (Ethyne) HC≡CH
2. Combustion Reactions:
Ethane Comb...
Q6: Explain addition reactions of alkenes. Give mechanism of HBr addition and Markovnikov's rule.
Addition Reactions of Alkenes: Reactions where two atoms or groups add across the C=C double bond.
General Pattern:
R2C=CR2 + A-B -> R2C(A)-C(B)R2
Types of Addition Reactions:
1. Hydrogenation (Addition of H2)
R2C=CR2 + H2 --catalyst(Ni,Pd,Pt)--> R2CH-CHR2
Example: CH2=CH2 + H2 --Ni, 150°C--> CH3-CH3
Requires catalyst (Ni, Pd, Pt, Rh)
2. Hydrohalogenation (Addition of HX where X = F, Cl, Br, I)
R2C=CR2 + HX -> R2C(X)-CHR2
Example: CH3-CH=CH2 + HBr -> CH3-CHBr-CH...
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