Mark Scheme (Results) January 2007

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1 Mark Scheme (Results) January 2007 GCE GCE Chemistry (6244/01) Edexcel Limited. Registered in England and Wales No Registered Office: One90 igh olborn, London WC1V 7B

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3 General Guidance on Marking Examiners should look for qualities to reward rather than faults to penalise. This does NOT mean giving credit for incorrect or inadequate answers, but it does mean allowing candidates to be rewarded for answers showing correct application of principles and knowledge, and for critical and imaginative thinking. Examiners should therefore read carefully and consider every response: even if it is not what is expected it may be worthy of credit. Using the mark scheme The mark scheme gives you: an idea of the types of response expected how individual marks are to be awarded the total mark for each question examples of responses that should NOT receive credit. Candidates must make their meaning clear to the examiner to gain the mark. Make sure that the answer makes sense. Do not give credit for correct words/phrases which are put together in a meaningless manner. Answers must be in the correct context. / means that the responses are alternatives and either answer should receive full credit. ( ) means that a phrase/word is not essential for the award of the mark, but helps the examiner to get the sense of the expected answer. [ ] words inside square brackets are instructions or guidance for examiners. Phrases/words in bold indicate that the meaning of the phrase or the actual word is essential to the answer. CQ (consequential) means that a wrong answer given in an earlier part of a question is used correctly in answer to a later part of the same question. There is space at the bottom of each page of this mark scheme for examiners to write their notes. Note: If a candidate has crossed out an answer and written new text, the crossed out work should be ignored. If the candidate has crossed out work, but written no new text, the crossed out work for that question or part question should be marked, as far as it is possible to do so. 1

4 1. IGNE s.f. throughout this question (a) Acid Proton or + donor Or forms + or 3 O + Weak dissociates to a small extent Or ionises to a small extent few molecules dissociate Or incomplete dissociation Or partial dissociation not fully dissociated Or not dissociated fully (b) 2COO(aq) + Na 2 CO 3 (aq) 2COONa(aq) + CO 2 (g) + 2 O(l) Or COO(aq) + Na 2 CO 3 (aq) COONa(aq) + NaCO 3 (aq) Species + balancing State symbols Consequential on correct species 2COONa(aq) + 2 CO 3 (aq) CO 2 for the acid CO 2 Na or COO - Na + for salt 1

5 (c) (i) one acid: COO Conjugate base: COO 1 mark for both Correct acids and conjugate bases in either order ACCEPT CO 2 and CO 2 O C O O C O - other acid: 3 O + Conjugate base: 2 O 1 mark for both + for 3 O + (ii) (K a ) = [COO ][ 3 O + ] [COO] Must use square brackets + [ ] instead of O ] [ 3 [CO 2 ] and [CO 2 ] + (1 mark) 2

6 (iii) [ + ] 2 = K a [COO] K a = [ + ] 2 [COO] [ + ] 2 = [ + ] = = (mol dm ) IGNE sig figs Max 1 if [ + ] 2 expression incorrect Any correct expression with [ + ] 2 or correct numbers If [ + ] = (K a c) quoted Scores first two marks p = 4.8 scores (2) as square root has not been taken + p = log 10 [ ] p = Alternative method pka = p = pka log[ acid] 2 2 p = 1.90 ( 0.50) any p value consequential + on [ ], provided p < 7 p = 2.39 (is a rounding error) so no third mark p = p = 2.39 (is a rounding error) so no third mark (3 marks) 3

7 (d) (i) + [ acid] [ ] = Ka [ salt] [ ] = = (mol dm ) p = 4.40 IGNE sig figs [ COO ] p = pka log 10 [ COO ] p = log10 ( ) log (rounding error) so no third mark p = 3.80 ( 0.60) p = 4.40 IGNE sig figs 4.39 (rounding error) so no third mark (3 marks) 4

8 (ii) Addition of + ions: + COO + COO Addition of O - ions: COO + O COO + 2O If the ionisation of sodium methanoate shown with then max out of 2 for above equations If described in terms of A + + A shifting to left Addition of O - ions: + + O 2O must be followed by more dissociation + of COO (to restore [ ] ) molecular equations or equations described in words or notation involving A, + and A. (buffer solution has) high concentrations Or a large reservoir of both COO and + relative to added / O (hence virtually no change in [ + ] ) COO Just large reservoir of both COO and COO (3 marks) Total 16 Marks 5

9 2 (a) IGNE s.f. throughout this question (i) moles SO 2 ( ) = 1.00 (mol) moles O 2 ( ) = (mol) moles SO (mol) all 3 correct (2) 2 correct (ii) All three total number of moles i.e X SO = ( = ) or 2 / X O = ( = ) or 1 / X SO = ( = 0.857) or 18 / 3 21 or 6 / Mark consequential on (a)(i) Multiples of the stated moles Rounding to 1 sig fig (1 mark) 6

10 (iii) All three total pressure i.e pso = or 4 / = 0.190( atm) (iv) po 2 = 2.00 or 2 / = ( atm) 9.00 pso 3 = 2.00 or 36 / 21 or 12 / = 1.71( atm) Mark consequential on (a)(ii) 2 (1.71) K p = 2 (0.190) (0.0952) (1 mark) K = 851 p 1 atm Mark consequential on (a)(iii) and (a)(iv) Answer with units and no working (2) Correct answers between 845 and 855 as this covers rounding up etc Wrong units e.g. mol -1 dm 3 7

11 (b) (i) (K p ) decreases (1 mark) (ii) (K p decreases so) p SO3 fraction / quotient 2 p SO2 po2 has to decrease (to equal new k p ) 2 so shifts to left hand side this mark only available if (b)(i) answer was k p decreases. Any Le Chatelier argument (this prevents access to 1 st mark) Shifts to right, even if answer to (b)(i) was k p increases (as p SO decreases 3 whereas p SO and p 2 O2 increase) NOTES: 8

12 (c) (i) No effect/none/zero (effect) (1 mark) (ii) Increases more SO 3 /more sulphur trioxide increases amount of SO 3 /sulphur trioxide (1 mark) (d) (i) No effect/none/zero (effect) (1 mark) (ii) No effect/none/zero (effect) (1mark) Total 13 marks 9

13 3 (a) Compound A C C C C O a branched chain isomer C O C C C -C 3 as side chain Compound B O C C C C Penalise compressed formula once only e.g. C 3 C 2 C 2 CO C 3 COC 2 C 3 -CO for aldehyde 10

14 (b) O 2 N C 3 C 2 C 2 C N N NO 2 C 3 7 C 2 5C 2 for C 3C 2C 2 C N linkage C=N Remainder of the molecule NO 2 groups in wrong position for remainder of molecule mark Lack of circle in benzene ring for second mark Mark consequential on structure given for Compound A in (a). (c) (i) triiodomethane Iodoform Or triodomethane CI 3 or I C I C 3 I I 11

15 (ii) butan(-)2(-)ol IGNE punctuation C ( C or 3 C O ) 2 5 O 2(-)butanol Or iso-butanol Or butane-2-ol But-2-ol 2-hydroxybutane C C C C Only penalise if bond is clearly shown pointing to ie O is OK O is wrong (d) (i) (It is not) superimposable on its mirror image OWTTE Or Does not have a plane of symmetry Or does not have a centre of symmetry Just four different groups on the same molecule Just (has an) asymmetric C atom (1 mark) 12

16 (ii) Rotate (the) plane (of plane ) polarised (monochromatic) light (equally) in opposite directions pass polarised light through sample use a polarimeter rotates the plane (equally) in opposite directions (1 mark) (e) 3 C C 2 O 90 formula C C e.g. and C C C 2 O C C C 3 1 mark for each 2 nd isomer cq if first isomer is carboxylic acid C C cis and trans acid scores consequentially Total 12 marks 13

17 4 (a) (i) ½Br 2 Br state symbols ½ Br 2 (g) Br(g) scores only one Wrong halogen or use of X (0) e.g. ½Br 2 (l) Br(g) (2) Br 2 (l) 2Br(g) ie for state symbols Br 2 Br (0) (ii) Energy change when 1 mol eat or enthalpy for energy; energy released instead of energy change energy required of a solid/crystal/lattice is formed from its (isolated) gaseous ions IGNE standard states Just balanced equation e.g. Na + (g) + Cl (g) NaCl(s) can score only last two marks (3 marks) 14

18 (b) (i) f = a [Mg] + IE 1 [Mg] + IE 2 [Mg] + 2 a [chlorine]+ 2EA[Cl] + LE[MgCl 2 ] Or this in words Or 2EA = (2 +122) (+1450) (+736) (+150) + ( 642) ( 2526) 1 = 696(kJ mol ) cq on first mark 696 EA = 2 1 = 348(kJ mol ) must 2 [some likely outcomes but working must be shown] -348 scores (3) -696 or -287 or (+) 348 scores (2) -574 or (+) 696 or (+) 287 scores (+) 574 scores (0) (3 marks) 15

19 (ii) MgCl 2 has (a degree of ) covalent character Mention of atoms or molecules scores (0) for all of (b)(ii) (c) due to polarisation of the anion 2+ (by Mg cation) As group descended, radius of M cation increases Charge on ions remains the same/2+ 2+ (ion) increases (down group) weaker forces of attraction between ions Reverse arguments size instead of radius Correct formulae of cations for charge mark charge density decreases scores one of the first two marks Just Mg 2+ (strongly) polarising Mention specifically of atoms (e.g. Mg atoms) or molecules (MgCl 2 molecules) scores (0) for all of part (c) weaker bonds weaker bonding (3 marks) Total 13 marks 16

20 5 (a) Na Mg Al Si P NaCl MgCl 2 AlCl 3 Al 2 Cl 6 PCl 3 PCl 5 All 4 (2) 3 None, one or two correct (0) PCl 4 +, PCl 6 (b) (i) NaCl(s) + aq Na + (aq) + Cl (aq) Or NaCl(s) + 2 O(l) Na + (aq) + Cl (aq) NaCl(s) + 2 O(l) NaO(aq) + Cl (aq) (ii) PCl + O PO 3Cl PCl5 + 2O POCl3 + 2Cl PCl O 3PO4 + 5Cl NaCl: Ionic (so) dissolves (in water) both needed PCl x : Covalent (so) reacts (in water) hydrolyses (in water) both needed 17

21 (c) SiCl 4 reacts/hydrolyses, CCl 4 does not [This must be clearly stated and not just implied] (lone) pair of electrons (from the oxygen atom) in a water molecule lone pair for (lone) pair of electrons cannot form a bond with/be donated to the C atom Or cannot be accepted by C atom because C has no available orbital no 2d orbitals in C C is a small atom surrounded by Cl atoms Cl atoms are large and surround C atom (so attack is sterically hindered) reverse argument for Si atom CCl 4 has no d orbitals [see below] Or CCl 4 has no 2d orbital(s) Just C has no d-orbital(s) Si has (available) 3d orbital(s) (d) (i) PbO 2 + 4Cl PbCl O + Cl2 Multiples SiCl 4 has available 3d orbitals (but penalise this only once) (5 marks) Species and balancing (1 mark) (ii) +2 (oxidation state) becomes more stable down the group relative to +4 Relative stabilities of Pb and Si oxidation states MUST have comparison of +2 and +4 oxidation states (1 mark) Total 13 marks 18

22 6 (a) (i) Ethanenitrile Methyl cyanide ethanitrile ethanonitrile IGNE any formula phonetic spelling e.g. ethanenitrille Ethenenitrile (1 mark) (ii) (Acid) hydrolysis IGNE word acid before hydrolysis phonetic spelling e.g. hydrolisis (1 mark) (iii) Step 1: any named mineral acid (eg. hydrochloric acid) or formula Using a named alkali or formula, then acidify Just Cl or 2 SO 4 Conc 2 SO 4 Step 2: PCl 5 / SOCl2 PCl 3 Cl 2 (iv) C COCl C N C CONC + Cl C 3 COCl + 2C 3 N 2 C 3 CONC 3 + C 3 N 3 Cl (1 mark) 19

23 (b) L: C C C M: C C C C C O- Only penalise if bond is clearly shown pointing to atom Allow O N: O C C C C -O Penalise omissions of atoms once only If C 3, C 2 are used max (2) Allow O COO Or CO 2 (3 marks) Total 8 marks 20

Mark Scheme (Results) January 2008

Mark Scheme (Results) January 008 GE GE hemistry (644) Paper Edexcel Limited. Registered in England and Wales No. 4496750 Registered ffice: ne90 igh olborn, London WV 7B General Marking Guidance All candidates