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Date	May 2019	Marks available	1	Reference code	19M.3.sl.TZ1.13
Level	SL	Paper	3	Time zone	TZ1
Command term	Suggest	Question number	13	Adapted from	N/A

Question

E10 is composed of 10 % ethanol and 90 % normal unleaded fuel.

Ethanol has a Research Octane Number (RON) of 108.6.

Outline how higher octane fuels affect engine performance.

[1]

Show that, for combustion of equal masses of fuel, ethanol (M_r = 46 g mol⁻¹) has a lower carbon footprint than octane (M_r = 114 g mol⁻¹).

[3]

Biodiesel containing ethanol can be made from renewable resources.

Suggest one environmental disadvantage of producing biodiesel from renewable resources.

[1]

Markscheme

increased AND fuels can be compressed more «before ignition» [✔]

Note: Accept “engines can be designed with higher compression ratio” OR “less chance of pre-ignition/autoignition/
knocking occurring”.

Alternative 1
C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l) / 1 mol ethanol produces 2 mol CO₂
OR
C₈H18 (l) + 12.5O₂ (g) → 8CO₂ (g) + 9H₂O (l) / 1 mol octane produces 8 mol CO₂ [✔]

For 1 g of fuel:
« $\frac{{{\text{1g}}}}{{{\text{46 g mo}}{{\text{l}}^{ - 1}}}}$ × 2 mol CO₂ (g) =» 0.04 «mol CO₂ (g)» from ethanol [✔]

« $\frac{{{\text{1g}}}}{{{\text{114 g mo}}{{\text{l}}^{ - 1}}}}$ × 8 mol CO₂ (g) =» 0.07 «mol CO₂ (g)» from octane [✔]

Alternative 2
ratio of C in ethanol:octane is 2:8, so ratio in carbon dioxide produced per mole will be 1:4 [✔]

ratio amount of fuel in 1 g = $\frac{1}{{46}}:\frac{1}{{114}}$ = 2.5:1 [✔]

4 > 2.5 so octane produces more carbon dioxide
OR
ratio of amount of carbon dioxide = 2.5:4 = 1:1.61 so octane produces more «for combustion of same mass» [✔]

use of «farm» land «for production»
OR
deforestation «for crop production for fuel»
OR
can release more NO_x «than normal fuel on combustion» [✔]

Note: Ignore any reference to cost.

Examiners report

Over half the students could relate a high octane number to a reduced tendency for auto-ignition, though this was usually referred to as “knocking”.

A question that gave the opportunity for a variety of different approaches. This challenge was beyond all but the best students, though there were a number of well argued responses.

Many students did not take into account “production from renewable resources” and answered in terms of the combustion of biodiesel, though about a third correctly identified the area of land biofuel crops require.

Syllabus sections

Options » C: Energy » C.4 Solar energy

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Strength:

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