O-LevelChemistryOrganic Chemistry5070/21 · May/June 2020 · Q1010 Marks

Fractional distillation and cracking are important processes in the conversion of petroleum (crude oil) into useful substances.

(a)Complete the sentence about petroleum (crude oil). Choose from the list. an alloy a compound an element a mixture a polymer a salt Petroleum (crude oil) is .................................................... of hydrocarbons.[1]
(b)Fractional distillation separates petroleum (crude oil) into fractions such as paraffin (kerosene) and naphtha. Give one use for the paraffin (kerosene) fraction.[1]
(c)(i)The naphtha fraction is used as a chemical feedstock. One of the hydrocarbons in naphtha has the molecular formula C10H22- The flow chart shows some compounds that can be made from C10H22. [Flowchart: C10H22 is cracked into C3H8 + C3H6 + C4H8. C3H8 reacts with Cl2 and uv light to form C3H7Cl. C3H6 reacts with Br2 to form C3H6Br2. C4H8 undergoes polymerisation to form poly(butene).] C3H8 is an alkane and C3H6 is an alkene. Explain why, in terms of their general formulae, C3H8 is an alkane and C3H6 is an alkene.[2]
(c)(ii)In the presence of uv light chlorine reacts with C3H8. Two of the products formed are HCl and C3H7Cl. What type of reaction takes place when C3H8 reacts with chlorine? Give the formula of one other product of this reaction.[2]
(c)(iii)Describe the colour change when C3H6 reacts with bromine.[1]
(d)(i)Suggest a possible structure for C4H8.[1]
(d)(ii)Draw the partial structure of poly(butene) that shows at least two repeat units.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] a mixture
[1] aviation fuel / heating (in the home) / cooking (stoves)
[2] alkane is CnH2n+2 which fits C3H8 if n = 3 (1) alkene is CnH2n which fits C3H6 if n = 3 (1)
[2] substitution (1) C3H6Cl2 / C3H5Cl3, etc. (1)
[1] orange to colourless / decolourises
[1] Any one from: H3C C=CH2 (with H3C on second carbon of double bond) H3C C=C H (with H on second carbon of double bond and H on third carbon of double bond) H3C-C(H)=C(H)-CH3 H2C=CH-CH2-CH3 (1)
[2] Any one from: partial structure of poly(butene) that shows at least two repeat units with correct backbone and side chains, e.g., showing extension bonds at end and no double bonds between carbon atoms (1); rest of structure (1)

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