Questions on alkanes: fractional distillation, cracking and combustion

Question 1

Explain the two trends in the boiling points of the alkanes given in the table: along the unbranched series, and among the three isomers of C₈H₁₈.

Accepted Answer

Indicative content

along the unbranched series the boiling point rises steadily as the chain gets longer, from −42 °C for propane to 174 °C for decane
the only forces between alkane molecules are van der Waals forces between induced dipoles, and no covalent bonds break when an alkane boils
a longer chain has more electrons, so larger induced dipoles form, and it has a larger surface in contact with neighbouring molecules
the van der Waals forces between longer molecules are therefore stronger, and more energy is needed to overcome them, so the boiling point is higher
the three C₈H₁₈ isomers have the same number of electrons, yet the boiling point falls from 126 °C to 118 °C to 99 °C as the branching increases
a more branched molecule is more compact, cannot lie as closely alongside its neighbours and has less surface contact, so its van der Waals forces are weaker

Levels

Level 3 (5–6 marks) both trends are described with values from the table and both are explained: the rise along the series through more electrons and more surface contact giving stronger van der Waals forces, and the fall with branching through compact molecules touching less; the answer says that only intermolecular forces, not covalent bonds, are overcome
Level 2 (3–4 marks) the rise along the series is put down to stronger van der Waals forces from more electrons and branching is said to lower the boiling point; why branching does so is not explained through surface contact, or the table's values go unused
Level 1 (1–2 marks) the boiling point is said to rise with chain length or fall with branching, typically because longer chains have more covalent bonds to break

Question 2

Explain, using the general formula of the alkanes, what is meant by describing an alkane as saturated, and why cracking a long-chain alkane always produces at least one alkene.

Accepted Answer

B1 the general formula of the alkanes is CₙH₂ₙ₊₂ B1 saturated means the molecule has only single bonds between carbon atoms, so no more atoms can be added to it B1 two alkanes sharing n carbon atoms would need 2n + 4 hydrogen atoms between them, but the alkane CₙH₂ₙ₊₂ has only 2n + 2 B1 so at least one product must have fewer hydrogens, CₙH₂ₙ, and contain a C=C double bond; allow a correct example equation that shows this

Question 3

Describe how crude oil is separated into fractions by fractional distillation.

Accepted Answer

B1 the crude oil is heated in a furnace so that most of it vaporises, and the vapour is fed into the bottom of a column B1 the column has a temperature gradient, hottest at the bottom and coolest at the top B1 vapour rises and each hydrocarbon condenses on the tray where the temperature falls below its boiling point B1 fractions with similar boiling points are drawn off at different heights; short chains near the top, long chains near the bottom, and the residue leaves at the base

Question 4

Compare thermal cracking with catalytic cracking.

Accepted Answer

B1 thermal cracking uses 400 to 900 °C, whereas catalytic cracking runs at a lower temperature, about 450 °C; accept any single value in each range B1 thermal cracking uses a high pressure and no catalyst; catalytic cracking uses a slight pressure and a zeolite catalyst B1 thermal cracking goes through radicals, catalytic cracking through carbocations B1 thermal cracking gives a high proportion of alkenes, catalytic cracking mainly branched alkanes, cycloalkanes and aromatic hydrocarbons for petrol

Question 5

Give an equation for the reaction of carbon monoxide with nitrogen monoxide in a catalytic converter, and name a metal used as its catalyst. Give an equation for the removal of sulfur dioxide from flue gases by calcium carbonate, and name the useful product made from it.

Accepted Answer

B1 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g); ignore state symbols B1 platinum, palladium or rhodium; reject 'a transition metal' alone B1 CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g) B1 the calcium sulfite is oxidised to calcium sulfate, sold as gypsum for plasterboard

Question 6

Evaluate the claim that a car fitted with a catalytic converter produces a harmless exhaust.

Accepted Answer

B1 the converter removes most CO, NOₓ and unburnt hydrocarbons, converting them to CO₂, N₂ and H₂O B1 it adds carbon dioxide, a greenhouse gas, and does not remove any B1 it does not work until it is hot, so pollutants pass through on short or cold journeys; it does not remove soot particles from a diesel engine B1 the claim is therefore not justified: the exhaust is much less toxic, but not harmless

Question 7

Explain why 2,2-dimethylpropane boils at 10 °C while its isomer pentane boils at 36 °C.

Accepted Answer

B1 both have the formula C₅H₁₂ and so the same number of electrons, but 2,2-dimethylpropane is branched and more compact, closer to spherical B1 its molecules cannot pack as closely and have a smaller surface area in contact with each other B1 so the van der Waals forces between its molecules are weaker and less energy is needed to separate them; reject any answer in which covalent bonds break

Question 8

Calculate the mass of carbon dioxide formed when 500 g of hexane, C₆H₁₄, burns completely. Ar: H 1.0, C 12.0, O 16.0.

Accepted Answer

M1 Mr(C₆H₁₄) = 86.0, so the amount of hexane is 500 ÷ 86.0 = 5.814 mol M1 each mole of hexane gives 6 mol of CO₂, so 6 × 5.814 = 34.88 mol of CO₂ A1 mass = 34.88 × 44.0 = 1535 g = 1.53 × 10³ g, or 1.53 kg, to three significant figures

Question 9

Calculate the volume of oxygen needed for the complete combustion of 2.40 dm³ of propane, and the volume of carbon dioxide formed, both at room temperature and pressure.

Accepted Answer

M1 C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l) M1 equal volumes of gas hold equal amounts, so the volumes are in the mole ratio 1 : 5 : 3 A1 12.0 dm³ of oxygen and 7.20 dm³ of carbon dioxide, to three significant figures

Question 10

Explain how nitrogen monoxide is formed in a petrol engine, and give one environmental problem it causes.

Accepted Answer

B1 nitrogen and oxygen from the air drawn into the engine react; the nitrogen does not come from the fuel B1 the reaction needs the high temperature of the spark and the burning fuel: N₂(g) + O₂(g) → 2NO(g) B1 NO is oxidised in air to NO₂, which causes acid rain or contributes to photochemical smog

Question 11

Suggest why the catalyst in a catalytic converter is spread as a thin layer over a ceramic honeycomb, and why a converter removes little pollution during the first few minutes of a journey.

Accepted Answer

B1 the honeycomb gives a very large surface area, so more gas is adsorbed onto the catalyst at once B1 a thin layer means only a small mass of an expensive metal is needed B1 the converter is cold at first and the reactions on the surface are too slow until it heats up

Question 12

Suggest why calcium oxide removes sulfur dioxide from the flue gases of a coal-fired power station, give an equation for the reaction, and suggest why the sulfur is removed after burning rather than from the coal.

Accepted Answer

B1 sulfur dioxide is an acidic oxide and calcium oxide is a base, so they react together B1 CaO(s) + SO₂(g) → CaSO₃(s) B1 sulfur bound into a solid fuel cannot easily be removed before it burns, whereas petrol and diesel are liquids that are desulfurised at the refinery

Question 13

Explain how carbon dioxide in the atmosphere causes the greenhouse effect.

Accepted Answer

B1 the Earth's surface, warmed by the Sun, emits infrared radiation B1 carbon dioxide molecules absorb some of this infrared, because it makes their bonds vibrate B1 they re-emit it in all directions, some back towards the surface, so the lower atmosphere is warmer than it would otherwise be

Question 14

Explain why refineries reform straight-chain alkanes such as hexane into branched and cyclic hydrocarbons for use in petrol.

Accepted Answer

B1 straight-chain alkanes can ignite from compression alone, before the spark, causing knocking in a petrol engine B1 branched alkanes, cycloalkanes and arenes resist this early ignition and burn more smoothly B1 so the engine can run at a higher compression and the fuel is used more efficiently

Question 15

Explain, with equations, how nitrogen dioxide in the air acts as a catalyst in the formation of acid rain from sulfur dioxide.

Accepted Answer

B1 SO₂(g) + NO₂(g) → SO₃(g) + NO(g) B1 NO(g) + ½O₂(g) → NO₂(g), so the nitrogen dioxide is regenerated and is not used up B1 the sulfur trioxide dissolves in rain to give sulfuric acid: SO₃(g) + H₂O(l) → H₂SO₄(aq)

Question 16

Show that the complete combustion of 1.00 kg of octane, C₈H₁₈, releases about 48 MJ, given that the enthalpy of combustion of octane is −5470 kJ mol⁻¹. Ar: H 1.0, C 12.0.

Accepted Answer

M1 Mr of C₈H₁₈ = 8 × 12.0 + 18 × 1.0 = 114.0, so the amount of octane is 1000 ÷ 114.0 = 8.77 mol M1 energy released = 8.77 mol × 5470 kJ mol⁻¹, the negative sign of ΔH showing that the energy leaves the fuel A1 energy = 4.80 × 10⁴ kJ = 48.0 MJ, to three significant figures, which is about 48 MJ; error carried forward from the amount of octane

Question 17

Explain why the residue from the fractionating column is distilled again at reduced pressure.

Accepted Answer

B1 reducing the pressure lowers the boiling points of the long-chain hydrocarbons in the residue B1 so they can be vaporised and separated at a temperature below that at which they would decompose

Question 18

Complete the equation for the cracking of tetradecane into octane and propene only: C₁₄H₃₀(g) → C₈H₁₈(g) + ...

Accepted Answer

B1 the other product is propene, C₃H₆(g) B1 two molecules of it are needed: C₁₄H₃₀(g) → C₈H₁₈(g) + 2C₃H₆(g), with 14 carbon and 30 hydrogen atoms on each side

Question 19

Suggest an equation for the reforming of heptane, C₇H₁₆, into methylbenzene, C₇H₈, and suggest a use for the other product.

Accepted Answer

B1 C₇H₁₆(g) → C₇H₈(g) + 4H₂(g), with 7 carbon and 16 hydrogen atoms on each side B1 the hydrogen can be used at the refinery, for example to remove sulfur from fuels

Question 20

Suggest why nitrogen and oxygen in the air do not react at room temperature, although they do inside a car engine.

Accepted Answer

B1 the N≡N triple bond is very strong, 945 kJ mol⁻¹, and non-polar, so the activation energy is very high B1 only at the high temperature in the cylinder do enough collisions have energy equal to or above the activation energy

Alkane	Formula	Boiling point / °C
Propane	C₃H₈	−42
Pentane	C₅H₁₂	36
Octane	C₈H₁₈	126
Decane	C₁₀H₂₂	174
2-methylheptane	C₈H₁₈	118
2,2,4-trimethylpentane	C₈H₁₈	99