A-LevelChemistryHydrocarbons9701/22 · May/June 2019 · Q414 Marks

There are many different types of aliphatic and aromatic hydrocarbons.

(a)Name a naturally occurring source of aliphatic and aromatic hydrocarbons and outline how different hydrocarbons are separated from this source. name of source .......................... outline of separation of hydrocarbons ..........................[2]
(b)When alkanes are heated to high temperatures, in the absence of air, the molecules can break into smaller molecules.
(b)(i)Identify the type of reaction occurring.[1]
(b)(ii)Write an equation which describes the reaction occurring when heptane, C₇H₁₆, is heated in the absence of air, to form hexane, butane and ethene only.[1]
(c)The equation for the complete combustion of ethene is shown. C₂H₄ + 3O₂ → 2CO₂ + 2H₂O Calculate the volume, in dm³, of carbon dioxide formed in the complete combustion of 1.00 g of ethene at room temperature and pressure. volume of CO₂ = .......................... dm³[3]
(d)The table compares the reactivity of alkanes and alkenes with chlorine. alkanes | alkenes ---|--- name of the type of reaction with chlorine | substitution | addition and substitution name of the type of reacting species | free radical | electrophile and free radical
(d)(i)During the first stage in the substitution reaction chlorine forms chlorine free radicals. Explain what is meant by the term free radical.[1]
(d)(ii)Name and explain the type of bond breaking which occurs to form chlorine free radicals.[2]
(d)(iii)Name the stage of the reaction mechanism which occurs when a methane molecule reacts with a chlorine free radical.[1]
(d)(iv)Complete the equation for the reaction which occurs when a methane molecule reacts with a chlorine free radical. H H–C–H + •Cl → .......................... + .......................... H[1]
(d)(v)Carbon atoms can form σ and π bonds within hydrocarbon molecules. Explain the following statement with reference to σ and π bonds. Alkenes react with electrophiles but alkanes do not.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] name of source crude oil / petroleum
[1] outline of separation of hydrocarbons (separation of molecules according to) different boiling points
[1] cracking
[1] 2C₇H₁₆ → C₄H₁₀ + C₆H₁₄ + 2C₂H₄
[1] M1 1/28 (= 0.035714) OR 1:88 / 28 (= 3.14286)
[1] M2 2 × M1 (= 1/14 = 0.07143) OR M1 / 44 (= 0.071429)
[1] M3 M2 × 24 = 1.7 dm³
[1] unpaired electron(s)
[1] homolytic (fission)
[1] one electron goes to each chlorine / atom OR pair of electrons is shared out (equally between the two atoms)
[1] propagation
[1] CH₄ + (•)Cl → (•)CH₃ + HCl OR CH₄ + (•)Cl → HCl + CH₃(•)
[1] M1 identify bond types in alkanes and alkenes bonds in alkanes are (all) sigma / σ AND alkenes have (C = C made of) sigma AND pi / σ AND π
[1] M2 electrons in π (of the C = C) are responsible for the reaction electrophiles are attracted to / attack to electrons in pi / π OR electrophiles react with pi / π