GCE. Chemistry A. Mark Scheme for January Advanced Subsidiary GCE Unit F322: Chains, Energy and Resources PMT

Transcription

1 GE hemistry A Advanced Subsidiary GE Unit F322: hains, Energy and Resources Mark Scheme for January 2012 xford ambridge and RSA Examinations

2 R (xford ambridge and RSA) is a leading UK awarding body, providing a wide range of qualifications to meet the needs of candidates of all ages and abilities. R qualifications include AS/A Levels, Diplomas, GSEs, R Nationals, Functional Skills, Key Skills, Entry Level qualifications, NVQs and vocational qualifications in areas such as IT, business, languages, teaching/training, administration and secretarial skills. It is also responsible for developing new specifications to meet national requirements and the needs of students and teachers. R is a not-for-profit organisation; any surplus made is invested back into the establishment to help towards the development of qualifications and support, which keep pace with the changing needs of today s society. This mark scheme is published as an aid to teachers and students, to indicate the requirements of the examination. It shows the basis on which marks were awarded by examiners. It does not indicate the details of the discussions which took place at an examiners meeting before marking commenced. All examiners are instructed that alternative correct answers and unexpected approaches in candidates scripts must be given marks that fairly reflect the relevant knowledge and skills demonstrated. Mark schemes should be read in conjunction with the published question papers and the report on the examination. R will not enter into any discussion or correspondence in connection with this mark scheme. R 2012 Any enquiries about publications should be addressed to: R Publications P Box 5050 Annesley NTTINGAM NG15 0DL Telephone: Facsimile: publications@ocr.org.uk

3 Annotations Annotation Meaning Benefit of doubt given ontradiction Incorrect response Error carried forward Ignore Not answered question Benefit of doubt not given Power of 10 error mission mark Rounding error Error in number of significant figures orrect response 1

4 Abbreviations, annotations and conventions used in the detailed Mark Scheme (to include abbreviations and subject-specific conventions). Annotation Meaning D NT ALLW Answers which are not worthy of credit IGNRE Statements which are irrelevant ALLW Answers that can be accepted ( ) Words which are not essential to gain credit Underlined words must be present in answer to score a mark EF Error carried forward AW Alternative wording RA r reverse argument Subject-specific Marking Instructions The following questions should be annotated with ticks to show where marks have been awarded in the body of the text: Q2 Q4 Q8 Q8 (b)(i) (c) (d) (e) All questions where an EF has been applied. hecking additional pages All the Additional Pages in the examination script must be checked to see if any candidates include any answers. When you open question 1(a) you will see a view of page 23 one of the Additional Pages. If the page is blank then, using the marking mode, annotate the page with an omission mark, ^. Scroll down to page 24 and annotate with a ^ if the page is blank. If pages 23 or 24 are not blank then use the paper clip icon to link the pages to the correct questions. You may need to contact your Team Leader if you do not know how to do this. 2

5 Generic comments RGANI STRUTURES For a structure or structural formula, ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) For an alkyl group shown within a structure, ALLW bond drawn to or, e.g.allw 3, 2, 3 7, etc ALLW vertical bond to any part of an alkyl group For an group shown within a structure, D NT ALLW formula with horizontal R ALLW vertical bond to any part of the group For a group shown within a structure, D NT ALLW For a 3D structure, For bond in the plane of paper, a solid line is expected: For bond out of plane of paper, a solid wedge is expected: For bond into plane of paper, ALLW: ALLW a hollow wedge for in bond R an out bond, provided it is different from the other in or out wedge e.g.:, 3

6 NAMES Names including alkyl groups: ALLW alkanyl, e.g. ethanyl (i.e. IGNRE an ) D NT ALLW alkol, e.g. ethol (ie an is essential) Names of esters: Two words are expected, e.g. ethyl ethanoate ALLW one word, e.g. ethylethanoate Names with multiple numbers and hyphens: Use of e ALLW superfluous e, e.g. propane-1-ol ( e is kept if followed by consonant) ALLW absence of e, e.g. propan-1,2-diol ( e is omitted if followed by vowel) yphens separate name from numbers: ALLW absence of hyphens, e.g. propane 1,2 diol Multiple locant numbers must be clearly separated: ALLW full stops: e.g. 1.2 R spaces: 1 2 D NT ALLW e.g. 12 Locant numbers in formula must be correct D NT ALLW propan-3-ol rder of substituents should be alphabetical: ALLW any order (as long as unambiguous), e.g. 2-chloro-3-bromobutane ABBREVIATINS van der Waal s forces ALLW vdw forces R VDW forces (and any combination of upper and lower cases) 4

7 1 (a) (a compound) with no double bond (or triple bond) 2 ALLW contains single bonds only containing hydrogen and carbon only ALLW it contains just carbon and hydrogen D NT ALLW a mixture of carbon and hydrogen R only carbon and hydrogen molecules (b) 2 1 ALLW 2 (c) D and I R F and G R F and 1 D NT ALLW G and (d) (i) yclic hydrocarbons have more efficient combustion 1 The answer must relate to combustion or burning Assume they refers to the cyclic hydrocarbons ALLW cyclic hydrocarbons allow smoother burning R cyclic hydrocarbons increase octane number R cyclic hydrocarbons reduce knocking R cyclic hydrocarbons are less likely to produce pre-ignition R cyclic hydrocarbons are more efficient fuels R cyclic hydrocarbons burn better R easier to burn R cyclic hydrocarbon combust more easily R improves combustion D NT ALLW cyclic hydrocarbons ignite more easily ALLW ora for straight chain hydrocarbons IGNRE cyclic hydrocarbons increase volatility of fuel IGNRE cyclic hydrocarbons have a lower boiling point cyclic hydrocarbons are a better fuel on their own is NT sufficient cyclic hydrocarbons burn more cleanly on their own is NT sufficient 5

8 1 (d) (ii) ALLW molecular formulae R correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) D NT ALLW cycloheptane structure in equation (e) 2 Both answers need to be comparisons Assume it refers to D D has more surface (area of) contact R D is a bigger molecule D has more van der Waals forces R have fewer van der Waals forces ALLW has more electrons R longer (carbon) chain R higher molecular mass IGNRE surface area ALLW RA ALLW D has stronger van der Waals forces / larger VDW / greater VDW R has weaker van der Waals forces R has smaller VDW ALLW more VDW forces More intermolecular forces is not sufficient D NT ALLW reference to bonds breaking or more bonds present unless it is clear that that the bonds are VDW (f) Same structural formula 2 ALLW have the same structure / displayed formula / skeletal formula Stereoisomers have the same formula or molecular formula is not sufficient Different arrangement of groups around a double bond R different arrangement (of atoms) in space ALLW different three dimensional arrangement 6

9 1 (g) orrect reactants and products Balancing 2 ALLW molecular formulae R correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) ALLW any correct multiple IGNRE state symbols Balancing is dependent on correct formulae (h) ALLW molecular formulae R correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) ALLW any correct multiple ALLW structural R displayed R skeletal formulae in equation ALLW but-1-ene IGNRE state symbols (i) (i) Group of atoms (in a molecule or compound) that is responsible for the reactions 1 ALLW the 'part' (of the molecule or compound) that reacts ALLW the group of atoms that gives the chemical properties ALLW group of atoms which indicates the homologous series (ii) 8 1 (iii) has an unpaired electron 1 ALLW plural i.e. unpaired electrons has a lone R single R free electron is not sufficient Total 16 7

10 2 (a) Any three from: 3 Assume it refers to Process 1 Process 1 has a high atom economy R has 100% atom economy R a greater atom economy R makes only the desired product ALLW process 1 has no waste R process 1 has no coproducts R process 1 needs less separation R process 1 has fewer other products R gives only one product ALLW RA if process 2 is specified Process 1 has a very efficient conversion of reactants to products R not much waste of starting material ALLW RA if process 2 is specified high percentage yield is not sufficient D NT ALLW if percentage yield is explicitly linked to more waste (products) e.g. process 1 has a high percentage yield so makes little waste (product) scores 0 marks but process 1 makes no waste (product) and it has a high percentage yield scores 1 mark Process 1 uses a lower pressure ALLW RA if process 2 is specified Process 1 uses up toxic carbon monoxide Process 1 uses methanol which can be produced from biomass IGNRE process 2 comes from crude oil a non-renewable source ALLW process 1 starts from a renewable source if the source is specified e.g. wood, municipal waste or sewage IGNRE reference to catalyst and rate of reaction 8

11 2 (b) (i) 5 ANNTATE ANSWER WIT TIKS AND RSSES ET ontains = bond because of absorption between 1700 and 1740 cm 1 (from the spectrum) ALLW contains a carbonyl group because of absorption within range cm 1 R contains an aldehyde, ketone or carboxylic acid because of absorption within range cm 1 Mention of only an aldehyde or a ketone is not sufficient it needs reference to the wavenumber LK FR TIS MARK N TE SPETRUM does not contain an bond ALLW not a carboxylic acid ALLW does not have any other characteristic absorbance due to other functional groups (So was a) ketone R aldehyde ALLW (so was a) carbonyl compound ALLW this mark if a structure of an aldehyde or a ketone is given even if the structure has an incorrect number of carbon atoms M r = 86 orrect structure ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) LK FR AN ALDEYDE or KETNE with FIVE carbon atoms R a DIALDEYDE, DINE R an XALDEYDE with FUR carbon atoms a comprehensive list of correct structures is shown on page 34 IGNRE incorrect name D NT ALLW for an aldehyde 9

12 ALLW as a slip one stick with no on in a displayed formula R pentanal 3 2-methylbutanal R pentan-3-one 10

13 2 (b) (ii) orrect structure 2 ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) All bonds and all hydrogen atoms must be shown in a displayed formula within this question Name of the structure drawn Name must correspond to the correct structure for two marks ALLW butanoic acid or 2-methylpropanoic acid if the structure drawn is incorrect There is no EF in this question ALLW R butanoic acid 3 ALLW ( 3 ) 2 2-methylpropanoic acid ALLW methylpropanoic acid 11

14 2 (c) Use of propan-1-ol 4 ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) ALLW from the equation propanol R 3 7 is not sufficient orrect formulae for the ester orrectly balanced equation ALLW molecular formula R correct structural R displayed R skeletal formula R mixture of the above ALLW propan-2-ol in the equation Add 2 S 4 R acid catalyst R + ALLW conditions mark over the arrow in the equation Total 14 12

15 3 (a) (i) FIRST, EK TE ANSWER N ANSWER LINE IF answer = 2.68 (kj) award 2 marks 2 IF there is an alternative answer, check to see if there is any EF credit possible using working below IF EF, ANNTATE WIT TIKS AND RSSES, etc ALLW ecf only from using mass of q = mc T R = ALLW J IGNRE sign If mass used is answer is = 2.68 (kj) ALLW 2.7 R R D NT ALLW 3 IGNRE sign If mass used is answer is 2.7, 2.70, up to calculated value of correctly rounded ALLW one mark for using 4.2 and correctly calculating q in kj to at least 2 sig figs (ii) amount = 0.02(00) (mol) 1 ALLW 1 50 IGNRE trailing zeroes 13

16 3 (a) (iii) FIRST, EK TE ANSWER N ANSWER LINE IF answer = -134 (kj) award 3 marks IF answer = +134 (kj) award 2 marks 3 IF there is an alternative answer, check to see if there is any EF credit possible using working below IF EF, ANNTATE WIT TIKS AND RSSES, etc ALLW EF i.e. ( i) ( ii) orrectly calculates the value to 3 sig figs This is dependant on the previous mark ALLW EF If 2.68, or and moles of 0.02answer is (-)134 If mass of magnesium included answer is (-)135 If 2.7 kj and moles of 0.02 used answer is (-)135 ALLW only answers to three significant figures sign (b) (i) (Enthalpy change) when one mole of a compound is formed from its elements at 25 o /298 K AND 1 atmosphere/101 kpa sign is independent of answer 3 ALLW energy required R energy released ALLW (energy change) when one mole of a substance/molecule/product D NT ALLW enthalpy change for one mole of products D NT ALLW one mole of reactants ALLW any stated temperature and 1 bar/1000/mb/100kpa/100000pa/101000pa/101000nm -2 etc IGNRE reference to concentration 14

17 3 (b) (ii) orrect labelling of enthalpy cycle 3 IF there is an alternative answer, check to see if there is any EF credit possible using working below IF EF, ANNTATE WIT TIKS AND RSSES, etc Two or three boxes correct BUT all four boxes correct f 792 (kj mol 1 ) ALLW EF from wrong enthalpy changes in the boxes Total 12 15

18 4 (a) FIRST, EK TE ANSWER N ANSWER LINE IF answer = (kj mol -1 ) award 2 marks 2 IF there is an alternative answer, check to see if there is any EF credit possible using working below IF EF, ANNTATE WIT TIKS AND RSSES, etc Energy required to break bonds = (+)679 kj so bond enthalpy = (+)431.5 ALLW (+)432 ALLW one mark in this question for R (+)863 ALLW ecf for bond enthalpy = 0.5 x ( energy required to break bonds) (b) 2 Must state somewhere in the answer that the rate is faster for full marks ALLW RA if lower pressure is specified more concentrated (particles) R more particles per (unit) volume ALLW particles are closer together R more crowded particles R more particles in the same space R same number of particles in a smaller volume ALLW molecules for particles but D NT ALLW atoms D NT ALLW area instead of volume more collisions per second R more frequent collisions ALLW collisions more often R increased rate of collision R collisions are more likely R there is a greater chance of collisions More collisions is not sufficient IGNRE successful 16

19 4 (c) 5 ANNTATE ANSWER WIT TIKS AND RSSES ET Assume answers refer to lower temperature and rate decreases unless specified otherwise ALLW RA i.e. correct explanation for why higher temperatures increase rate if clearly specified ALLW particles instead of molecules throughout question D NT ALLW atoms the first time it appears in the answer y-axis label is '(number of) molecules' AND x-axis label is 'energy' AND one correct curve orrect curve for lower temperature (labelled) Boltzmann distribution - must start at origin and must not end up at 0 on y-axis i.e. must not touch x-axis Maximum of curve to left AND higher than maximum of higher temperature curve AND below higher temp line at higher energy as shown in diagram below IGNRE minor point of inflexion of both curves Activation energy does not change R clearly labelled on diagram, e.g. E a R E Fewer molecules have energy above activation energy R fewer molecules have enough energy to react So fewer successful collisions ALLW RA for higher temperature if specified Fewer molecules have enough energy to collide successfully is worth one mark Fewer collisions per second is not sufficient 17

20 lower temperature (number of) molecules E a energy fewer molecules with energy above E a (d) (i) l 2 2l 1 No need to show radicals with a dot (ii) (iii) l + l + l Any two from: + 2 l + l l 2 + l l Total 14 2 No need to show radicals with a dot 2 No need to show radicals with a dot 18

21 5 (a) nly one (desired) product formed 1 ALLW no waste products R no co-product R all atoms on left hand side are in the desired product R sulfuric acid is the only product IGNRE it is an addition reaction (b) FIRST, EK TE ANSWER N ANSWER LINE IF answer = 94% award 3 marks Moles of sulfur reacted or theoretical moles of 2 S 4 = Actual moles of 2 S 4 = % yield = 94 (c) (i) Position of equilibrium unchanged 3 IF there is an alternative answer, check to see if there is any EF credit possible using working below 2 ALLW to the calculator value correctly rounded ALLW 1.60 up to calculator value correctly rounded ALLW to the calculator value correctly rounded ALLW 1.5 up to calculator value correctly rounded ALLW theoretical mass of 2 S 4 = 157 (tonnes) up to the calculator value of correctly rounded for two marks ALLW EF for a percentage yield from wrong moles above but answer must have two significant figures Rate of backward reaction decreases 19

22 5 (c) (ii) 1 Both position of equilibrium AND explanation needed for one mark (equilibrium position shifts) to the left because (forward) reaction is exothermic R equilibrium position shifts) to the left because reverse reaction is endothermic Note: ALLW suitable alternatives for to left, e.g. towards S 2 or 2 / towards reactants R in backward direction R in reverse direction R decreases yield of S 3 /products ALLW favours the left, as alternative for shifts equilibrium to left ALLW reaction gives out heat for exothermic ALLW reaction takes in heat for endothermic ALLW moves to the left in the endothermic direction ALLW RA if specified IGNRE responses in terms of rate (iii) 1 Both position of equilibrium AND explanation needed for one mark (equilibrium position shifts) to the left because there are more moles (of gas) on the reactant side R (equilibrium position shifts) to the left because there are fewer moles (of gas) on product side Note: ALLW suitable alternatives for to left, e.g.: towards S 2 or 2 / towards reactants R in backward direction R in reverse direction R decreases yield of S 3 /products ALLW favours the left, as alternative for shifts equilibrium to left ALLW correct reference to volume of gases e.g. shifts to the left because there is a smaller volume of gas on the product side ALLW RA if specified IGNRE responses in terms of rate 20

23 5 (d) (i) orrect structure 1 ALLW correct structural R displayed R skeletal formula 3 R mixture of the above (as long as unambiguous) ALLW bonds going to any part of the 3, 2 and bonds R 3 ALLW vertical bond to any part of the group D NT ALLW horizontal in the formula R 3 ALLW as a slip one stick with no on in a displayed formula IGNRE name 3 21

24 5 (d) (ii) orrect structure for L 3 ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) for L, M 2 3 and N e.g. 3 orrect structure for M orrect structure for N L or M N L or M Answers to L and M are interchangeable IGNRE cis/trans R E/Z labels ALLW as a slip one stick with no on in a displayed formula ALLW 2 marks if three correct structures are drawn but some are in the wrong boxes ALLW 1 mark if two correct structures are drawn but in the wrong boxes 22

25 5 (d) (iii) 1 ALLW correct structural R displayed R skeletal formula 3 R mixture of the above (as long as unambiguous) ALLW vertical bond to any part of the group D NT ALLW horizontal in the formula ALLW as a slip one stick with no on in a displayed formula Total 13 23

26 6 (a) (i) (m/z =) 46 1 (ii) 3 + R MUST show '+' (iii) ALLW 2 2 (b) R R A r = 63.6 ALLW two marks for 63.6 with no working out opper / u Total 6 24

27 7 (a) Shape tetrahedral 2 Bond angle o (b) (i) Volatile R non-toxic R non-flammable R easily vaporised ALLW o 1 ALLW not carcinogenic / not an irritant / not harmful / not hazardous IGNRE cheap / not dangerous / gas / low boiling point D NT ALLW inflammable (ii) ( F or l ) bonds need a large amount of energy to break 1 ALLW (the F or l) bonds are strong / bonds have a large bond enthalpy ALLW the molecule is not polar enough / non-polar molecule is not sufficient ALLW the activation energy is too high D NT ALLW dissolves IGNRE references to hydrogen bonding (c) F 2 l 2 F 2 l + l 3 ALLW F 2 l 2 (breaks down to) produces chlorine atoms/radicals ALLW equation with any F AND ANY TW FRM l catalyses the decomposition of ozone l + 3 l + 2 l + l + 2 ALLW l + 3 l ALLW R for one mark if the two equations for the steps have not been given IGNRE other propagation equations 25

28 7 (d) Because (more) UV will reach the Earth s surface and risk of (skin) cancer increased/risk of cataracts/crop mutation increased 1 D NT ALLW global warming ALLW protects from UV which causes skin cancer etc (e) Ideas related to uses Fs are still entering the atmosphere (from disused items) R Fs are still used (for some purposes and by some countries) 2 ALLW 'stratosphere' for 'upper atmosphere' ALLW Fs are still entering the ozone layer Ideas relating to lifetime within the atmosphere Fs have a long lifetime in the atmosphere R it takes a long time for Fs to reach upper atmosphere R Fs are inert Total 10 26

29 8 (a) 3 ALLW structures with missing hydrogen atoms on the carbon atoms that do not take part in the reaction. i.e. all hydrogen atoms must be shown in Q but not in P and R For example for the structures of P and R compound P Br Br compound Q Br Br compound R 27

30 8 (b) range R brown to colourless 1 ALLW shades of orange R yellow R brown D NT ALLW red alone D NT ALLW any response that includes precipitate R solid, irrespective of colour (c) Two or more repeat units ALLW correct structural R displayed R skeletal formula R mixture of the above (as long as unambiguous) Must have at least two repeat units and the free bonds at the end ALLW free bonds with dotted lines All carbon carbon bonds in the polymer chain must be shown IGNRE any brackets drawn IGNRE any missing hydrogen atoms on the 2 groups ALLW skeletal formula 28

31 8 (d) urly arrow from double bond to attack hydrogen of l and breaking of l bond 5 ANNTATE ANSWER WIT TIKS AND RSSES ET urly arrow must start from the double bond and not a carbon atom; other curly arrow must start from l bond D NT ALLW dipoles on double bond orrect dipole shown on l orrect carbonium ion drawn urly arrow from l to the carbonium ion Dipole must be partial charge and not full charge arbocation needs a full charge and not a partial charge (charges do not need to be in a circle) l curly arrow must come from one lone pair on l ion R from minus sign on l ion Lone pair does not need to be shown on l ion ALLW structures with missing hydrogen atoms on the 2 groups l l intermediate orrect product product l 29

32 8 (e) 5 ANNTATE ANSWER WIT TIKS AND RSSES ET Nucleophilic substitution eterolytic (fission) spelt correctly dipole shown on l bond, δ+ and l δ curly arrow from to carbon atom of l bond curly arrow from l bond to the chlorine atom and formation of l - Dipole must be partial charge and not full charge curly arrow must come from one lone pair on of ion R from minus sign on ion curly arrow must start from l bond and not from atom l l - ALLW structures with missing hydrogen atoms on the 2 groups - ALLW S N 1 mechanism dipole shown on l bond, δ+ and l δ curly arrow from l bond to the l atom and l - shown curly arrow from to correct carbonium ion Total 15 30

33 APPENDIX

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