Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level. Published

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1 Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level CHEMISTRY 970/ Paper AS Level Structured Questions March 07 MARK SCHEME Maximum Mark: 60 Published This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge will not enter into discussions about these mark schemes. Cambridge is publishing the mark schemes for the March 07 series for most Cambridge IGCSE, Cambridge International A and AS Level components and some Cambridge O Level components. IGCSE is a registered trademark. This document consists of 5 printed pages. UCLES 07 [Turn over

2 970/ Cambridge International AS/A Level Mark Scheme March 07 (a)(i) max O.N. + (+) (+)3 (+)5 (+)6 +7 (a)(ii) (from Na to Cl ) nuclear charge increases electrons are in the same shell / have same shielding greater / stronger attraction (of electrons to nucleus) (a)(iii) Mg + AND S ion of Mg / Mg + has one fewer shell (than ion of S / S ) (b)(i) P 4 + 5O P 4 O 0 / P O 5 (b)(ii) any from: yellow / green colour (of chlorine gas) disappears white flame white solid solid melts (b)(iii) phosphoric(v) acid (c)(i) diagram showing regular arrangement of (positive) ions surrounded by / sea of (delocalised) electrons (c)(ii) any from: high melting / boiling / sublimation point electrical / thermal insulator hard / rigid retains strength at high temperature / pressure UCLES 07 Page of 5

3 970/ Cambridge International AS/A Level Mark Scheme March 07 (c)(iii) M % abundance of fourth isotope = 00 ( ) =.4 M ( ) + ( ) + ( ) + (.4 RIM) 00 = 40.6 ( ) = =.4 RIM M3 RIM = = 4.95 (a)(i) (a)(ii) bond in which the centres of positive and negative charges do not coincide OR electron distribution is asymmetric / unequal OR two (bonded) atoms are partially charged HF has the strongest (permanent) dipole dipole / van der Waals (forces) / HF has hydrogen bonding requires more energy to overcome (than weaker (permanent) dipole dipole/ van der Waals forces between other hydrogen halides) (a)(iii) thermal stability of the hydrogen halides decreases down group (7) larger (halogen) atoms / atomic radius (down group) / increased shielding bond energies decrease / less energy required to break H X (b)(i) M base is Cl AND conjugate acid is HCl OR base is HSO 4 AND conjugate acid is H SO 4 M Cl / HSO 4 / base is a proton acceptor OR HCl / H SO 4 / (conjugate) acid has one more H + (b)(ii) H SO 4 is (too strong) an oxidising agent I would be formed instead UCLES 07 Page 3 of 5

4 970/ Cambridge International AS/A Level Mark Scheme March 07 (c)(i) r H = r H{products} r H{reactants} = ( 4) 4 ( 9) = 6 (sign AND answer) (c)(ii) heterogeneous (catalyst) provides an alternative reaction pathway of lower activation energy (c)(iii) reaction is exothermic (increased temperature) shifts equilibrium to the left AND decreases yield of products (Cl and / or H O) / less product formed (c)(iv) HCl O Cl H O 3 initial number of moles M eqm number of moles M mole fraction M3 partial pressure = ½ = ptot = (c)(v) ( 4 ) ( ) K p = ( ) = units = Pa (c)(vi) K p would not change 3(a)(i) 3(a)(ii) reaction = HCl (aq) reaction = (conc.) NaOH / KOH AND ethanol UCLES 07 Page 4 of 5

5 970/ Cambridge International AS/A Level Mark Scheme March 07 3(a)(iii) C C backbone with dangling bonds rest of structure 3(b) 3 lone pair on O AND curly arrow from O to C of C Br dipole on C Br AND curly arrow from C Br to Br product (butan--ol) 3(c)(i) (electrophilic) addition 3(c)(ii) S has CH 3 CHOH OR methyl / CH 3 group next to CHOH 3(c)(iii) 3(c)(iv) positive inductive effect of more alkyl groups / more alkyl groups donate electron density secondary carbocation / secondary intermediate is more stable (than primary) S = T = U = 3(c)(v) CH 3 CHOHCH CH 3 + [O] CH 3 COCH CH 3 + H O 3(d)(i) methyl pentanoate 3(d)(ii) (compound V is) spectrum X spectra X and Z show a C=O (stretch) at 730 (cm ) spectra Y and Z show O H (stretches) above 500 (cm ) V has a C=O (bond) and no O H (bond) UCLES 07 Page 5 of 5