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AS · Practice questions · Alkanes

Burn it cleanly.

Six original Cambridge-style questions on the alkanes.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Strong, non-polar bonds; good fuels.

01
Explain
[2 marks]

Explain why alkanes are unreactive towards most reagents.

  • C–C and C–H bonds are strong, so much energy is needed to break them. ✓
  • The bonds are almost non-polar, so there are no δ+ or δ− sites to attract nucleophiles or electrophiles. ✓
02
Recall
[2 marks]

Write equations for the complete and incomplete combustion of methane (forming CO).

  • Complete: CH4 + 2O2 → CO2 + 2H2O. ✓
  • Incomplete: 2CH4 + 3O2 → 2CO + 4H2O. ✓
03
Recall
[2 marks]

Write a balanced equation for the complete combustion of octane, C8H18.

  • C8H18 + 12½O2 → 8CO2 + 9H2O, ✓
  • or 2C8H18 + 25O2 → 16CO2 + 18H2O. ✓
04
Analysis
[2 marks]

Decane, C10H22, is cracked to give octane and one other product. Write the equation and state why cracking is useful.

  • C10H22 → C8H18 + C2H4. ✓
  • It converts less useful long chains into petrol-range alkanes and alkenes for making polymers. ✓
05
Calculation
[2 marks]

Calculate the mass of CO2 produced by the complete combustion of 1.00 kg of octane (Mr 114.0).

  • n(octane) = 1000 ÷ 114.0 = 8.77 mol; each gives 8 mol CO2 = 70.2 mol. ✓
  • Mass = 70.2 × 44.0 = 3090 g = 3.09 kg. ✓
06
Explain
[3 marks]

Name three pollutants from a petrol engine and describe how a catalytic converter reduces two of them.

  • CO, NOx and unburnt hydrocarbons (also CO2). ✓
  • CO and NO react on the Pt/Rh surface: 2CO + 2NO → 2CO2 + N2. ✓
  • Unburnt hydrocarbons are oxidised to CO2 and H2O. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.