| Date | November 2017 | Marks available | 1 | Reference code | 17N.3.sl.TZ0.12 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Comment | Question number | 12 | Adapted from | N/A |
Question
One method of comparing fuels is by considering their specific energies.
Calculate the specific energy of octane, C8H18, in kJ kg–1 using sections 1, 6 and 13 of the data booklet.
A typical wood has a specific energy of 17 × 103 kJ kg–1. Comment on the usefulness of octane and wood for powering a moving vehicle, using your answer to (a).
If you did not work out an answer for (a), use 45 × 103 kJ kg–1 but this is not the correct answer.
State the name of one renewable source of energy other than wood.
Markscheme
Mr (C8H18) = 114.26 AND ΔH\(_c^\theta \)= –5470 «kJ mol–1»
«specific energy = \(\frac{{5470\,{\text{kJ}}}}{{0.11426\,{\text{kg}}}}\) =» 4.79 x 104/47873/47900 «kJ kg–1»
Award [2] for correct final answer.
Accept “48 x 103 «kJ kg–1»” OR “47.9 x 103 «kJ kg–1»”.
wood is less useful because it requires «about three times» more mass for same energy
Accept “octane is more useful because it has higher specific energy”.
Any one of:
wind
tidal/wave
hydro-electric
solar
thermal/geothermal
plant oil
Accept “biofuel/biodiesel/«bio»ethanol” but not just “water” or “fuel cells”.
[Max 1 Mark]
