Topic 14: Hydrocarbons

Question 1

Definitions I:
(a) Cracking
(b) Free-radical substitution
(c) Electrophilic addition

Answer 1

Definitions I:
(a) Breaking long-chain hydrocarbons into shorter, useful molecules.
(b) Reaction where radicals replace atoms, e.g., Cl in alkanes.
(c) Addition to C=C where electrophile attacks π bond first.

Question 2

Write equations for producing ethane:
(a) From ethene [1 Mark]
(b) From decane cracking [1 Mark]

Answer 2

(a) CH₂=CH₂ + H₂ → CH₃CH₃ (Pt, heat) (1 mark)
(b) C₁₀H₂₂ → C₂H₆ + C₈H₁₆ (Al₂O₃, heat) (1 mark)

Question 3

Describe the complete and incomplete combustion of methane in a car engine. [3 Marks]

Answer 3

Complete: CH₄ + 2O₂ → CO₂ + 2H₂O (excess O₂) (1 mark)
Incomplete: CH₄ + 1½O₂ → CO + 2H₂O (limited O₂) (1 mark)
Car engine: often incomplete, CO, NOx (1 mark)

Question 4

Show the mechanism of CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl with UV light. [3 Marks]

Answer 4

Initiation: Cl₂ → 2Cl* (UV) (1 mark)
Propagation: Cl* + CH₃CH₃ → CH₃CH₂* + HCl, CH₃CH₂* + Cl₂ → CH₃CH₂Cl + Cl* (1 mark)
Termination: 2Cl* → Cl₂ (1 mark)

Question 5

Why are alkanes unreactive with NaOH despite being hydrocarbons? [2 Marks]

Answer 5

Strong C-H bonds, non-polar (1 mark)
No attraction to polar NaOH (1 mark)

Question 6

In a lab, propene reacts with Br₂. Write the equation and test for C=C. [2 Marks]

Answer 6

CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br (1 mark)
Aq. Br₂ decolourises, confirms C=C (1 mark)

Question 7

Write equations for producing propene:
(a) From 2-bromopropane [1 Mark]
(b) From propan-2-ol [1 Mark]

Answer 7

(a) CH₃CHBrCH₃ + NaOH → CH₃CH=CH₂ + NaBr + H₂O (ethanolic, heat) (1 mark)
(b) CH₃CH(OH)CH₃ → CH₃CH=CH₂ + H₂O (Al₂O₃, heat) (1 mark)

Question 8

What products form when but-2-ene reacts with hot, conc. KMnO₄? [2 Marks]

Answer 8

CH₃CH=CHCH₃ + [O] → 2CH₃CHO (hot, conc. KMnO₄) (1 mark)
C=C ruptures, forms ethanal (1 mark)

Question 9

In industry, how does cracking of C₁₀H₂₂ yield useful hydrocarbons? [2 Marks]

Answer 9

C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀ (e.g.) (1 mark)
Lighter alkanes, alkenes for fuels, plastics (1 mark)

Question 10

Explain why CH₃CH=CH₂ + HBr forms CH₃CHBrCH₃ (Markovnikov’s rule). [2 Marks]

Answer 10

H⁺ adds to CH₂, forms CH₃CH⁺CH₃ (stable 2° cation) (1 mark)
Br⁻ adds to C⁺, Markovnikov addition (1 mark)

Question 11

What pollutants arise from alkane combustion in engines, and how are they removed? [2 Marks]

Answer 11

CO, NOx, unburnt hydrocarbons (1 mark)
Catalytic converter: CO + NO → CO₂ + N₂ (1 mark)

Question 12

Write the polymerisation equation for propene forming polypropene. [2 Marks]

Answer 12

n CH₃CH=CH₂ → [-CH(CH₃)CH₂-]ₙ (1 mark)
Addition polymerisation, polypropene forms (1 mark)