OCR A Jan 2011 Paper 2 Q5 (with explained solutions) |

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OCR A Jan 2011 Paper 2 Q5

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This question is about halogenated hydrocarbons. 15 (a) Halogenoalkanes undergo nucleophilic substitution reactions with ammonia to form amines. Amines contain the NH2 functional group. For example, 1-bromopropane reacts with ammonia to form propylamine, CH3CH2CH2NH2. CH3CH2CH2Br + 2NH3 CH3CH2CH2NH2 + NH4Br (i) Iodoethane is reacted with ammonia. Write an equation for this reaction.[2] (ii) The first step in the mechanism of the reaction between CH3CH2CH2Br and NH3 is shown below. It is incomplete. CH3CH2 Br CH3CH2 + NH3Complete the mechanism. Include relevant dipoles, lone pairs, curly arrows and the missing product. [3]OCR 2011 Turn over<br />
(b) A student investigates the rate of hydrolysis of six halogenoalkanes. 16 The student mixes 5 cm3 of ethanol with five drops of halogenoalkane. This mixture is warmed to 50 C in a water bath. The student adds 5 cm3 of aqueous silver nitrate, also heated to 50 C, to the halogenoalkane. The time taken for a precipitate to form is recorded in a results table. The student repeats the whole experiment at 60 C instead of 50 C. halogenoalkane CH3CH2CH2CH2Cl CH3CH2CH2CH2Br CH3CH2CH2CH2I CH3CH2CHBrCH3 (CH3)2CHCH2Br (CH3)3CBr time taken for a precipitate to form / s at 50 C at 60 C 243 121 40 89 110 44 121 63 19 42 55 21 Describe and explain the factors that affect the rate of hydrolysis of halogenoalkanes. Include ideas aboutthe halogen in the halogenoalkanes the groups attached to the carbon of the carbonhalogen bond (the type of halogenoalkane) the temperature of the hydrolysis. In your answer you should link the evidence with your explanation.OCR 2011<br />
17[7] (c) Poly(tetrafluoroethene), PTFE, and poly(chloroethene), PVC, are halogenated plastics. (i) Write an equation, using displayed formulae, for the reaction to form PTFE from its monomer. [3] (ii) The combustion of waste polymers can be used for energy production. What problem is caused by disposing of PTFE and PVC in this way?[1] [Total: 16]OCR 2011 Turn over<br />

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Mark Scheme January 2011 Mark Guidance ALLOW CH3CH2I + NH3 (cid:198) CH3CH2NH2 + HI ALLOW CH3CH2I + NH3 (cid:198) CH3CH2NH3I Curly arrow must start from the lone pair on nitrogen and go to the carbon atom DO NOT ALLOW NH3OR NH3 ALLOWon the N atom of NH3 Curly arrow must start from the bond and go to the Br F322 Question Answer 5 (a) (i) CH3CH2I + 2NH3 (cid:198) CH3CH2NH2 + NH4I correct reactants (cid:57) correct products and balanced (cid:57) (ii) CH3CH2 (cid:71)(cid:14) C (cid:71)(cid:16) Br CH3CH2 NH3 Br NH3 Correct curly arrow from the lone pair of ammonia to the carbon atom of CBr (cid:57) Correct dipole on C+Br bond and curly arrow showing the heterolytic fission of the CBr bond (cid:57) Correct missing product: Br (cid:57) 15<br />
F322 Question Answer 5 (b) Mark Scheme January 2011 Mark Guidance 16 Effect of halogen in RX (3 marks) Any correct comparison of rate OR reaction time between at least TWO of chloroalkane, bromoalkane and iodoalkane (cid:57) Bond strength OR bond enthalpy/bond energy mentioned anywhere as a factor (even if reasoning is incorrect) (cid:57) Any correct comparison of bond strength OR bond enthalpy/energy OR bond length OR ease of breaking of at least TWO of CCl, CBr and CI (cid:57) ANNOTATE WITH TICKS AND CROSSES Examples chloroalkane reacts the slowest iodo compound reacts the fastest CI bond is hydrolysed faster than CBr CBr has shorter reaction time than CCl DO NOT ALLOW references to halogens as elements: ie chlorine is less reactive than bromine than iodine DO NOT ALLOW chloride, bromide and iodide ALLOW this mark if mentioned within effect of halogen, branching OR temperature Examples CI bond is weaker than CBr bond CI bond is the weakest CCl bond is shorter than CI bond CCl is strongest bond CBr is broken more easily than CCl<br />
F322 Question Answer 5 (b) Mark Scheme January 2011 Mark Guidance Effect of branching (2 marks) Any correct comparison of rate or reaction time between at least TWO of the bromoalkanes (cid:57) A sensible comparison of bond strength OR bond enthalpy/energy OR bond length OR ease of breaking of the CBr bond in at least TWO of the bromoalkanes (cid:57) Effect of temperature (2 marks) QWCUse of 50 C and 60 C using information in the table to show that rate increases with temperature (cid:57) At higher temperature, particles have more energy OR At higher temperature, particles move faster (cid:57) Tertiary hydrolyses faster than secondary OR reaction time is less with tertiary than primary OR secondary hydrolyses faster than primary OR branched hydrolyses faster than straight chains OR primary hydrolyses the slowest OR tertiary hydrolyses the fastest OR when halogen on carbon 1 is hydrolysed slower than when halogen is on carbon 2 (cid:57) DO NOT ALLOW short chains hydrolyse faster than long chains Examples CHal is weaker in tertiary halogenoalkane OR CBr bond is stronger when it is bonded to carbon 1 rather than carbon 2 ALLOW an explanation based on relative stabilities of tertiary, secondary and/or primary carbocations Answer must quote evidence from the table to get this mark Rate increases with temperature is NOT sufficient ALLOW more energy available to break the CHal bond OR more energy vibrates the CHal more so bond can break more easily ALLOW more successful collisions at higher temperature ALLOW more molecules exceed activation energy ALLOW ORA 17<br />
F322 Question Answer 5 (c) (i) Mark Scheme January 2011 Mark Guidance Polymer must have side links (do not have to cut through bracket) ALLOW a correct section of the polymer with side links ALLOW ECF from wrong monomer, including use of Fl for F n on LHS can be at any height to the left of formula AND n on the RHS must be a subscript (essentially below the side link) On the LHS, DO NOT ALLOW (C2F4)n (the n must be in front of the monomer) nC2F4 (cid:198) (C2F4)n scores 1 mark for the correct use of n ALLOW produces poisonous gases OR produces gases that can kill IGNORE HF, Cl2 and F2 Makes a dangerous or harmful gas is NOT sufficient IGNORE CO and CO2 are greenhouse gases IGNORE chlorine radicals and ozone depletion IGNORE causes pollution Correct monomer (cid:57) Correct polymer (cid:57) Balanced equationcorrect use of n in the equation (cid:57) (ii) (PVC) produces hydrogen chloride OR produces acidic gases OR (PVC) produces phosgene OR produces toxic gases OR (PVC) produces dioxins (cid:57) Total 16 18<br />

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