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Vidaara.orgClass 11 · Chemistry
CodeVID-C11-13-T1-01
Assignment — Alkanes
Chapter: Hydrocarbons
Topic: Alkanes
Maximum Marks: 30
Time: 60 minutes
Name: ____________________ Roll No.: __________ Date: ____________

General Instructions

  • All questions are compulsory.
  • Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
  • Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions 5 × 1 = 5 marks
1.
The number of structural isomers of C4H10 is:
  • A.1
  • B.2
  • C.3
  • D.4
2.
Kolbe's electrolysis of sodium acetate gives:
  • A.methane
  • B.ethane
  • C.ethene
  • D.acetylene
3.
The first step in the chlorination of methane is called:
  • A.propagation
  • B.termination
  • C.initiation
  • D.substitution
4.
Branching in alkanes generally causes the boiling point to:
  • A.increase
  • B.decrease
  • C.stay the same
  • D.double
5.
Conversion of n-hexane to benzene over Cr2O3 at high T is called:
  • A.isomerisation
  • B.aromatisation
  • C.pyrolysis
  • D.combustion
Section B — Short Answer (2 marks) 3 × 2 = 6 marks
6.
Why are alkanes called paraffins?
7.
Write the equation for the preparation of ethane by the reduction of bromoethane.
8.
Distinguish between the staggered and eclipsed conformations of ethane.
Section C — Short Answer (3 marks) 2 × 3 = 6 marks
9.
Describe the free-radical mechanism for the chlorination of methane, naming each stage.
10.
Give one preparation each of (i) a symmetrical alkane and (ii) a lower alkane with one carbon fewer than the starting acid.
Section D — Long Answer (5 marks) 1 × 5 = 5 marks
11.
Summarise five chemical reactions of alkanes with one equation/example each.

Answer Key

Section A — Multiple Choice Questions
  1. (B) 2
  2. (B) ethane
  3. (C) initiation
  4. (B) decrease
  5. (B) aromatisation
Section B — Short Answer (2 marks)
  1. Paraffin means 'little affinity'; alkanes are saturated with strong, non-polar C–C and C–H bonds, so they are chemically rather unreactive towards common reagents at room temperature.
  2. CH3CH2Br + Zn/H+ → CH3CH3 + (Zn salt) + HBr; the C–Br bond is replaced by C–H.
  3. In staggered ethane the front and back H atoms are 60° apart (minimum repulsion, most stable); in eclipsed ethane they are aligned (maximum torsional strain, least stable, ~12.5 kJ mol-1 higher).
Section C — Short Answer (3 marks)
  1. Initiation: Cl2 ⟶[hν] 2 Cl·. Propagation: Cl· + CH4 → CH3· + HCl, then CH3· + Cl2 → CH3Cl + Cl·. Termination: radicals combine, e.g. CH3· + Cl· → CH3Cl.
  2. (i) Wurtz reaction: 2 CH3CH2Br + 2 Na → CH3CH2CH2CH3 (n-butane). (ii) Soda-lime decarboxylation: CH3COONa + NaOH ⟶[CaO] CH4 + Na2CO3.
Section D — Long Answer (5 marks)
  1. 1) Halogenation: CH4 + Cl2 ⟶[hν] CH3Cl + HCl. 2) Combustion: CH4 + 2O2 → CO2 + 2H2O + heat. 3) Controlled oxidation: 2CH4 + O2 ⟶[Cu] 2CH3OH. 4) Isomerisation: n-butane ⟶[AlCl3/HCl] isobutane. 5) Aromatisation: n-hexane ⟶[Cr2O3] benzene + 4H2. Pyrolysis (cracking) is a further example: large alkanes break into smaller alkanes and alkenes.
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