Mark Scheme (Results) January 2008

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

2 General Marking Guidance All candidates must receive the same treatment. Examiners must mark the first candidate in exactly the same way as they mark the last. Mark schemes should be applied positively. andidates must be rewarded for what they have shown they can do rather than penalised for omissions. Examiners should mark according to the mark scheme not according to their perception of where the grade boundaries may lie. There is no ceiling on achievement. All marks on the mark scheme should be used appropriately. All the marks on the mark scheme are designed to be awarded. Examiners should always award full marks if deserved, i.e. if the answer matches the mark scheme. Examiners should also be prepared to award zero marks if the candidate s response is not worthy of credit according to the mark scheme. Where some judgement is required, mark schemes will provide the principles by which marks will be awarded and exemplification may be limited. When examiners are in doubt regarding the application of the mark scheme to a candidate s response, the team leader must be consulted. rossed out work should be marked UNLESS the candidate has replaced it with an alternative response. Using the mark scheme / 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. 3 [ ] words inside square brackets are instructions or guidance for examiners. 4 Phrases/words in bold indicate that the meaning of the phrase or the actual word is essential to the answer. 5 ecf/te/cq (error carried forward) 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.

3 .(a)(i) Ionic Giant ionic or electrovalent.(a)(ii) ovalent Giant covalent onvalent.(b)(i) Basic Base or alkali or alkaline.(b)(ii) Acidic Acid Weakly acidic Weak acid.(c)(i) 3Na + 3 P 4 Na 3 P Na + 3 P 4 Na P 4 + Na + 3 P 4 Na P 4 + () Ignore state symbols.(c)(ii) Si + Na Na Si 3 + () Ignore state symbols Si + Si3 +

4 .(d) First mark: Al 3(S) (aq) Al 3+ (aq) + 3 (l) () This mark is for correct species and balancing Second mark: Two correct molecular equations with correct state symbols scores () 3 Al 3(S) + - (aq) + 3 (l) Al() - 4 (aq) Al 3(S) (aq) + 3 (l) Al() 3-6 (aq) Al 3(s) + - (aq) Al - (aq) + (l) () This mark is for correct species and balancing Third mark is for the state symbols () orrect state symbols in either equation, but all species must be correct. This mark may be awarded from an unbalanced equation..(e) Pb + 4l Pbl + l + Ignore state symbols Pb + 6l Pbl 4 l +.(f) First mark: Tin more stable in the +4 oxidation state (than the + oxidation state) whereas lead more stable in the + oxidation state (than in the +4 oxidation state) + oxidation state becomes more stable relative to +4 oxidation state as group descended. () Sn + ions less stable than Pb + Pb(II) is more stable than Sn(II) Second Mark: (So) I reduced to I (by Sn + ) Sn + + I Sn 4+ + I - Therefore tin(ii) is a strong(er) reducing agent (than lead(ii)) () redox reaction between Sn + and I Sn + oxidised (to Sn 4+ ) Sn(II) acts as (a strong) reducing agent

5 .(a) IGNE alkane in any answer 3 X : ester () Y : both alkene and alcohol or hydroxyl () Z : both alcohol or hydroxyl and aldehyde () carbon-carbon double bond hydroxy hydroxy carbonyl or hydroxide or hydroxide or carbonyl Just the formula.(b) X : no reaction () 3 Y : no reaction () Z : () do not award if the bond from the carbon atom is clearly to the of the group - - Na + or -Na Any formula with the alcohol group oxidised

6 .(c)(i) 3 Na/ 3 Na + / arboxylic acid Allow 3 7 / 5 Na () 3 - Na + / 3 / 3 Na / 3 Na + r Na + 3 /.(c)(ii) I 3 / () I I I () Na / Na / Na / / Na + / Na + / =Na / = Na / = Na + / = Na + () Allow carboxylic acid as product e.g..(c)(iii) N / () (N) / () ()N ()

7 3.(a)(i) To slow down the reaction/to stop the reaction to quench the reaction to freeze the (position of) equilibrium WTTE () To stop equilibrium shifting to the left so that the (equilibrium) concentrations/amounts do not change () 3.(a)(ii) First mark: I ( g ) = ( g ) If [I] not squared, first mark only. If first mark not awarded, total (0). 3 Use of ( ) () Second mark: ( 4 I ( g ) = ) = ( ) [I (g) ] [I(g)] = (5.0 x 0-4 ) () 0.09 Third mark: I ( g ) = 3.6 x 0-3 (mol dm -3 ) () orrect answer scores 3 marks. Ignore state symbols. Ignore units unless wrong. Ignore s.f.

8 3.(b)(i) K p = p pi p I [ ] scores (0) Ignore position of any ( ) 3.(b)(ii) Each step of this calculation must be looked at. st mark is for calculating equilibrium moles Η = 0. Ι = 0. ΗΙ =.6 () Mark consequentially 4 nd mark is for dividing these by (to get mole fractions) 0. x = = x I = = x I = = 0.8 ().0 3 rd mark is for multiplying by. (to get partial pressures) 0. Ρ =..0 = 0. (atm) 0. Ρ I =..0 = 0. (atm).6 Ρ I =..0 = 0.88 (atm) () 4 th mark is for substituting into their expression and calculating K p ( 0.88) K P = ( 0. ) (0.) = 64 () Ignore s.f. orrect answer with no working scores () Mark consequentially Mark consequentially If moles I given as 0.8, K p = 6 max (3)

9 3.(b)(iii) 4.(a)(i) Same number of moles on each side (Total) pressure cancels (Pressure) units cancel (May be shown by crossing out etc. in b(ii)) Powers cancel They cancel Same number of molecules on each side Partial pressures cancel mol dm -3 cancel 6 4.(a)(ii) 4.(b) Either f = f = (+78)+(735)+ (+8)+ (-73) + (-389) = 86 (kj mol - ) () orrect answer with no working () Ignore kj [First mark only if doubles both at and electron affinity for hydrogen] [nd mark is only consequential on failure to multiply either at or electron affinity or both giving: 404 / 3 / 33 (kj mol-)] +86 scores (0) +404 / +3 /+33 scores (0)

10 4.(c) EITER First mark: Magnesium/Mg ion smaller (radius) than calcium/a ion r the sum of the ionic radii in Mg smaller (than in a ) () Magnesium ion has greater charge density than calcium ion for first mark. Reference to atoms or molecules or scores zero overall. 3 Second mark: but charges the same () Third mark: (so) stronger (forces of) attraction between ions (in Mg ) () [orrect reverse arguments can score both marks] First and second mark combined: + Mg ( ion ) or + Mg ( cation) smaller (radius) than a + () Third mark: (so) stronger (forces of) attraction between ions (in Mg ) () [orrect reverse arguments can score both marks] Ignore references to polarisation of the hydride ion or covalent character in the hydrides. Ignore references to energy required to separate ions/break bonds stronger ionic bonding for 3 rd mark in either case. If + ions or hydrogen ions referred to, 3 rd mark cannot be awarded in either case If just stronger bonding in Mg, 3 rd mark cannot be awarded in either case

11 4.(d)(i) Enthalpy/energy/heat change when eat released.. Any implication of mol of gaseous ions () X + (g) + aq X + (aq) and endothermic, do not statement of energy award st mark change per mole for first mark. Is dissolved in (a large) excess of water r Is dissolved until further dilution causes no further heat change () Ignore any reference to standard conditions Mark independently Added to water or reacts with water instead of dissolved Infinitely dilute solution Is completely surrounded by water molecules Dissolves completely 4.(d)(ii) δ (in water) attracted to positive ions/cations () δ + (in water) attracted to negative ions/anions () forms (dative) bonds instead of attracted Just attraction between water (molecules) and ions ( max) Reference to full charges on water molecules scores zero overall energy required or implication of an endothermic process scores (0) overall. Dipole-dipole attractions and/or polarisation scores zero overall

12 5.(a)(i) ne acid: 3 (aq) onjugate base: 3 (aq) () ther acid: 3 + (aq) onjugate base: (l) () Accept correct acids with conjugate bases in either order Ignore state symbols 5.(a)(ii) WEAK: dissociates/ionises to a small extent () WTTE Few molecules dissociate Incomplete or partial dissociation Does not fully dissociate ions partially dissociate AID: proton donor () Produces 3 + / hydrogen / + ions Just contains (b)(i) + [ 3 ][ 3 ] Ka = [ + ] instead of [ 3 + ] [ ] 3 Any expression containing [ ] 5.(b)(ii) ([ + ] =) 3.63 x 0-4 (mol dm -3 ) () r If K a expression incorrect in (b)(i) or 5 [ + ] not squared, only [ 3 ] = [ + ] st mark available.30 x 0-5 r 4 ( ) [ 3 ] = () = 0.00 () (mol dm -3 ) () ASSUMPTINS: First assumption mark: negligible [ + ] from ionisation of water r [ 3 ] = [ + ] () Second assumption mark: Ionisation of the (weak) acid is negligible r x [ + ] x where x is initial concentration of 3 r [+]<<[A] () No other source of + ions Very slight ionisation the initial [A] = equilibrium [A] Just 3 = + (ie no square brackets) Any mention of nonstandard conditions or temperature not at 98 K

13 5.(c) 3 + / 3 + r 3 Na + / 3 + Na () and causes the following eqm to shift to the right + + ions produced cause the solution to be alkaline () Mark independently ausing an excess of ions () ions from water 5.(d)(i) Ignore A solution of known p which. maintains nearly constant p resists change in p () WTTE on adding small amounts of acid or alkali () Mark independently

14 5.(d)(ii) Working MUST be checked First mark: 3 [ + ] = K a [acid] () [salt] K a = [ + ]x [salt] [acid] Second mark: orrect [acid] = and [salt] = () Third mark: alculation of p correct consequential on [acid] and [salt] used [ + ] = = (mol dm -3 ) p = 5.06 () Ignore sig fig First mark: [ acid] p = pka log 0 [ salt ] () Second mark: orrect [acid] = and [salt] = () If [salt] and [acid] inverted, p is 4.7 ( marks) Inverted with the original concentrations, p = 5.9 ( mark) In both cases, if [acid] = [0.000] and [salt] = [ ], p = 4.59 ( marks) Third mark: alculation of p correct consequential on [acid] and [salt] used. [0.005] p = 4.89 log 0 [ ] () = 4.89 ( 0.8) = 5.07 () 5.06 Ignore sig fig

15 6.(a)(i) Delocalised carboxylate group with a negative charge shown ompressed structural formula N Positive charge must be on the N atom The minus charge must be on the in the if no delocalisation shown 6.(a)(ii) 6.(a)(iii) ( + from) (group) protonates the N (group) Transfer of + from to N r self-protonation Just protonation Just acid-base reaction Read the whole answer! igh energy needed to overcome (strong) ionic attractions () between zwitterions () Ignore reference to molecules if clearly used in the context of attraction between ions ionic bonds or ionic lattice instead of ionic attractions between adjacent species Just intermolecular forces r bonding r van der Waals forces etc award zero overall

16 6.(b)(i) + N 3 / + 3 N / + 3 N written right to left N l + 3 Molecular formula r N 3 l N 3 6.(b)(ii) N / N / Na Molecular formula or Na + N 6.(b)(iii) 3 N / 3 N Molecular formula no reaction () N 6.(b)(iv) N 3 / N 3 N

17 6.(c)(i) (Glutamic acid molecule) has four different groups attached to a (atom) r (Glutamic acid molecule) has four different groups attached to a chiral centre ontains an asymmetric carbon (atom) r molecule has no plane of symmetry Just has a chiral centre r Just the molecule is asymmetrical has mirror images which are not superimposable 6.(c)(ii) (the isomers) rotate the plane (or polarisation) of (plane-) polarised light ().rotate plane polarised light in opposite directions () Just in different directions Ignore any reference to polarimeter 6.(d) N( ) 6 N () N ( ) 6 N l( ) 4 l / ( ) 4 / ( ) 4 / l( ) 4 l () [Monomers can be given in either order] ( ) 4 ( ) 4 r l( ) 4 l

Mark Scheme (Results) January 2007

Mark Scheme (Results) January 2007 GCE GCE Chemistry (6244/01) Edexcel Limited. Registered in England and Wales No. 4496750 Registered Office: One90 igh olborn, London WC1V 7B 1 General Guidance on Marking