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Date	May 2019	Marks available	2	Reference code	19M.3.sl.TZ2.10
Level	SL	Paper	3	Time zone	TZ2
Command term	Describe	Question number	10	Adapted from	N/A

Question

This question is about fuel for engines.

Crude oil can be converted into fuels by fractional distillation and cracking.

Contrast these two processes.

[2]

Determine the specific energy, in kJ g⁻¹, and energy density, in kJ cm⁻³, of hexane, C₆H₁₄. Give both answers to three significant figures.

Hexane: M_r = 86.2; ΔH_c = −4163 kJ mol⁻¹; density = 0.660 g cm⁻³

Specific energy:

Energy density:

[2]

Hydrocarbons need treatment to increase their octane number to prevent pre-ignition (knocking) before they can be used in internal combustion engines.

Describe how this is carried out and the molecular changes that take place.

[2]

Markscheme

Note: Award [1] max for any two correct answers from one column OR one from each column.

Award [2] for any two correct from each column; eg: fractional distillation – any two correct award [1 max] AND cracking – any two correct award [1 max].

specific energy = « $\frac{{4163{\text{ kJ mo}}{{\text{l}}^{ - 1}}}}{{86.2{\text{ g mo}}{{\text{l}}^{ - 1}}}}$ =» 48.3 «kJ g^–1» [✔]

energy density =«48.3 kJ g^–1 × 0.660 g cm^–3 =» 31.9 «kJ cm^–3» [✔]

Note: Award [1 max] if either or both answers not expressed to three significant figures.

Any two of:
«hydrocarbons are heated with» catalyst [✔]

long chains break and reform
OR
branching/aromatization occurs
OR
isomerisation/reforming/platforming/cracking [✔]

zeolite separates branched from non-branched
OR
products are distilled
OR
«distillation» separates reformed and cracked products [✔]

Note: Accept a specific catalysis name or formula for M1 such as Pt/Re/Rh/Pd/Ir.

Examiners report

This question was not well answered. Many candidates didn’t answer the question as asked. Candidates needed two correct statements, either about fractional distillation or cracking as a process for 1 mark.

This question was very well answered by most students and many answered with the correct number of significant figures as specified by the question.

Students responded well to at least one mark of this question. There were several different ways to earn the 2 marks possible. The most common way students earned marks were by identifying the use of a catalyst and then the idea of the compound reforming into a smaller or branched compound. Very few students discussed the idea of purification or separation into individual compounds which is another important part of this process.

Syllabus sections

Options » C: Energy » C.2 Fossil fuels

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