Alkenes Exam Questions

(a) CnH2n [1]

(b) An unsaturated compound contains at least one carbon-carbon double bond (C=C) [1]

(c) Ethene, C2H4 [1]

Examiner note: Methene (CH2) does not exist because a carbon atom needs at least two bonds to other atoms. The simplest alkene is therefore ethene (2 carbons). Do not confuse the general formula CnH2n (alkenes) with CnH2n+2 (alkanes) — alkenes have two fewer hydrogen atoms because of the double bond.

• Add bromine water (orange/brown solution) to each compound and shake [1]
• Ethene: the bromine water is decolourised / turns from orange to colourless [1]
• Ethane: the bromine water remains orange/brown / no colour change [1]

Examiner note: “Turns clear” is acceptable for decolourised, but “turns white” is wrong. The bromine adds across the C=C double bond in ethene (addition reaction), removing the colour. Ethane has no double bond, so no reaction occurs with bromine water at room temperature.

(a) CH2=CH2 + Br2 → CH2BrCHBr2 / C2H4Br2 [1] Product: 1,2-dibromoethane [1]

(b) CH2=CH2 + H2O → CH3CH2OH / C2H5OH [1] Product: ethanol. Conditions: high temperature (300 degrees C), high pressure (60-70 atm), phosphoric acid catalyst [1]

Examiner note: In addition reactions, atoms add across the double bond and no other product is formed. The C=C breaks to become a C-C single bond. The hydration of ethene is the industrial method for manufacturing ethanol. Compare this with fermentation, which uses glucose and yeast.

(a) CH2=CH2 + H2 → CH3CH3 / C2H4 + H2 → C2H6 [1]

(b) Ethane [1]

(c) Addition reaction / hydrogenation [1]

Examiner note: Hydrogenation converts an unsaturated compound to a saturated one. This is the same process used to harden vegetable oils into margarine. The nickel catalyst and heat are essential conditions. After the reaction, the product no longer decolourises bromine water because the double bond has been removed.

(a) Addition polymerisation [1]

(b) Poly(ethene) / polyethylene [1]

(c) Poly(propene) / polypropylene [1]

(d) The repeating unit shows -CH2-CH2- with brackets and the letter n, derived from breaking the C=C double bond in ethene [1]

Examiner note: In addition polymerisation, many small alkene molecules (monomers) join together to form a long-chain polymer. The C=C double bond breaks open to form single bonds linking the monomers. No other product is formed (unlike condensation polymerisation). Draw the repeating unit with bonds extending beyond the brackets on each side.

• By cracking long-chain alkanes [1]
• The long-chain alkane is heated to a high temperature (over 500 degrees C) and passed over a catalyst (zeolite / aluminium oxide) [1]
• The large molecule breaks down into a shorter-chain alkane and an alkene [1]

Examiner note: Thermal cracking uses high temperatures and high pressure without a catalyst. Catalytic cracking uses a lower temperature with a catalyst. Both produce a mixture of shorter-chain alkanes and alkenes. An example: C10H22 → C8H18 + C2H4.

(a) CH2=CH2 + HBr → CH3CH2Br / C2H5Br [1]

(b) Bromoethane [1]. Structural formula: CH3-CH2-Br showing the bromine attached to the end carbon [1]

(c) Alkanes have no C=C double bond / are saturated, so there is no site for addition to occur [1]

Examiner note: The addition of HBr follows the same pattern as addition of Br2 or H2O — the double bond breaks and the two parts of HBr add across it. H adds to one carbon, Br to the other. Alkenes are more reactive than alkanes because the C=C double bond is a region of high electron density that attracts electrophiles.

(a) C2H4 + 3O2 → 2CO2 + 2H2O [2] (1 mark for correct products, 1 mark for balancing)

(b) Similarity: both produce carbon dioxide and water during complete combustion / both are exothermic [1] Difference: ethene burns with a smokier/more luminous/yellow flame than ethane because it has a higher carbon-to-hydrogen ratio / ethene is more likely to undergo incomplete combustion [1]

Examiner note: Alkenes tend to burn with smokier flames than alkanes of similar size because they have a higher proportion of carbon. This is why the flame test is not reliable for distinguishing them — always use the bromine water test instead.