Answers to questions on Fossil fuels

1. (a) RON (Research octane number) refers to the auto-ignition tendency for a fuel. The lower the octane number the more likely the fuel is to auto-ignite under compression. Aromatic compounds such as methylbenzene have a higher octane rating than 100 as they are less likely to auto-ignite under compression than 2,2,4-trimethylpentane, i.e. they are less likely to cause ‘knocking’.

(b) Octane number = (87/100 x 100) + (8/100 x 23) + (5/100 x 121) = 95

2. Mass of ‘octane’ used = 55 x 0.703 x 1000 x 52 = 2010580 g = 2.0106 x 10⁶ g
Amount of ‘octane’ used  = 2.0106 x 10⁶ / 114.26 = 17597 mol
C₈H₁₈ + 12½O₂  →  8CO₂  +  9H₂O 
Amount of carbon dioxide produced = 8 x 17597 = 140776 mol (assuming complete combustion)
Mass of carbon dioxide released into the atmosphere = 140772 x 44.01 = 6.20 x 10⁶ g
Addition to carbon footprint = 6.20 tonnes per year.

3. C₁₀H₂₂ → C₈H₁₈ + C₂H₄

To make addition polymers e.g. poly(ethene).

4. (a) C₇H₁₆ → CH₃C(CH₃)₂CH₂CH₂CH₃

C₇H₁₆ → C₆H₅CH₃ + 4H₂

(b) Branched alkanes and aromatic compounds are less prone to auto-ignition (have a higher octane rating) than the corresponding straight chain alkanes. The hydrogen produced during aromatization can be used in the Haber process and in hydrogenation reactions.

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