Fuels Exam Questions

(a) A compound containing only hydrogen and carbon / made up of hydrogen and carbon atoms only [1]

(b) It contains two or more substances (hydrocarbons) that are not chemically combined / can be separated by physical methods [1]

(c) The crude oil is heated and the vapours rise up the fractionating column [1]. The column is cooler at the top than the bottom, so hydrocarbons with different boiling points condense at different heights and are collected separately [1]

Examiner note: “Hydrocarbon” must state ONLY carbon and hydrogen — any other element means it is not a hydrocarbon. The fractionating column has a temperature gradient: hottest at the bottom, coolest at the top. Small molecules with low boiling points rise to the top before condensing.

(a) Boiling point increases as the number of carbon atoms increases [1]

(b) Bitumen [1]

(c) Petrol [1]

(d) Shorter-chain hydrocarbons are more volatile / have lower boiling points, so they vaporise more easily and ignite more readily [1]

Examiner note: All four properties (boiling point, viscosity, colour, flammability) are linked to chain length. As chain length increases: boiling point increases, viscosity increases, colour darkens, and flammability decreases. These trends are tested regularly.

(a) 2C8H18 + 25O2 → 16CO2 + 18H2O [2] (1 mark for correct products, 1 mark for correct balancing)

(b) Carbon dioxide and water [1]

Examiner note: Complete combustion requires excess/sufficient oxygen. Balance carbon first, then hydrogen, then oxygen. With 8 carbons and 18 hydrogens in C8H18: 8 CO2 and 9 H2O for one molecule, but the oxygen count gives 25/2 = 12.5, so double everything to get whole numbers. Candidates who write CO instead of CO2 are describing incomplete combustion.

Complete combustion:

• Occurs when there is sufficient/excess oxygen [1]
• Products are carbon dioxide and water [1]

Incomplete combustion:

• Occurs when there is insufficient/limited oxygen [1]
• Products are carbon monoxide and/or carbon/soot and water [1]

Examiner note: Incomplete combustion produces less energy than complete combustion. Carbon monoxide is a toxic, colourless, odourless gas. Soot (carbon particles) causes respiratory problems and contributes to global dimming. A yellow/orange flame indicates incomplete combustion; a blue flame indicates complete combustion.

(a) There is greater demand for shorter-chain hydrocarbons (such as petrol) than the crude oil provides [1]. Cracking converts long-chain, less useful hydrocarbons into shorter-chain, more useful ones [1]

(b) High temperature (approximately 500 degrees C) and a catalyst (zeolite/aluminium oxide/broken pottery) [1]

(c) A mixture of shorter-chain alkanes and alkenes [1]

Examiner note: Cracking always produces at least one alkene (unsaturated molecule). This is important because alkenes are used to make polymers and other chemicals. The word equation format: long-chain alkane → shorter-chain alkane + alkene. For example: C10H22 → C8H18 + C2H4.

(a) C10H22 → C8H18 + C2H4 [2] (1 mark for correct formulae, 1 mark for balanced equation)

(b) Ethene (C2H4) — used to make poly(ethene) / polyethylene / used to make ethanol / used as a feedstock for making plastics [1]

Examiner note: Check that your equation balances: carbon atoms (10 = 8 + 2) and hydrogen atoms (22 = 18 + 4). Cracking always produces at least one alkene. Ethene is the most common small alkene product and is extremely valuable industrially.

Any three from:

• Carbon dioxide emissions contribute to global warming / climate change / enhanced greenhouse effect [1]
• Sulfur dioxide emissions cause acid rain, which damages buildings, vegetation, and aquatic ecosystems [1]
• Carbon monoxide from incomplete combustion is toxic to humans [1]
• Particulates/soot cause respiratory problems and global dimming [1]
• Nitrogen oxides cause acid rain and photochemical smog [1]

Examiner note: Name the specific pollutant and its specific effect. “Fossil fuels cause pollution” is worth 0 marks. Each point needs a named gas linked to a named problem.

(a) 2H2 + O2 → 2H2O [1]

(b) Advantage: the only product is water, so no carbon dioxide or pollutant gases are produced / a clean fuel [1] Disadvantage: any one from — hydrogen is explosive/difficult to store safely, hydrogen must be produced using energy (often from fossil fuels), lack of infrastructure for distribution, hydrogen has a low density so large tanks are needed [1]

Examiner note: Hydrogen fuel is only truly “clean” if the energy used to produce it (by electrolysis of water) comes from renewable sources. If electricity from fossil fuels is used for electrolysis, the overall process still produces CO2 indirectly.