Cambridge A Level Chemistry 9701 — 2010 May/June Paper 2 · Variant 1

This document consists of 11 printed pages and 1 blank page. DC (SHW 00422 3/09) 11740/2 © UCLES 2010 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Subsidiary Level and Advanced Level READ THESE INSTRUCTIONS FIRST Write your name, Centre number and candidate number on all the work you hand in. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs, or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. DO NOT WRITE ON ANY BARCODES. Answer all questions. You may lose marks if you do not show your working or if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. * 1 5 6 5 0 7 6 5 4 3 * CHEMISTRY 9701/21 Paper 2 Structured Questions AS Core May/June 2010 1 hour 15 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet For Examiner’s Use 1 2 3 4 5 Total

9701/21/M/J/10 © UCLES 2010 For Examiner’s Use Answer all the questions in the spaces provided. 1 Elements and compounds which have small molecules usually exist as gases or liquids. (a) Chlorine, Cl 2, is a gas at room temperature whereas bromine, Br 2, is a liquid under the same conditions. Explain these observations. … … … [2] (b) The gases nitrogen, N2, and carbon monoxide, CO, are isoelectronic, that is they have the same number of electrons in their molecules. Suggest why N2 has a lower boiling point than CO. … … … [2] (c) A ‘dot-and-cross’ diagram of a CO molecule is shown below. Only electrons from outer shells are represented. C O In the table below, there are three copies of this structure. On the structures, draw a circle round a pair of electrons that is associated with each of the following. (i) a co-ordinate bond (ii) a covalent bond (iii) a lone pair C O C O C O [3] 2

3 9701/21/M/J/10 © UCLES 2010 [Turn over For Examiner’s Use (d) Hydrogen cyanide, HCN, is a gas which is also isoelectronic with N2 and with CO. Each molecule contains a strong triple bond with the following bond energies. bond bond energy / kJ mol–1 –CN in HCN 890 NN 994 CO 1078 Although each compound contains the same number of electrons and a strong triple bond in its molecule, CO and HCN are both very reactive whereas N2 is not. Suggest a reason for this. … … [1] (e) HCN reacts with ethanal, CH3CHO. (i) Give the displayed formula of the organic product formed. (ii) What type of reaction is this? … (iii) Draw the mechanism of this reaction. You should show all full and partial charges and represent the movement of electron pairs by curly arrows. [5] [Total: 13]

4 9701/21/M/J/10 © UCLES 2010 For Examiner’s Use 2 The diagram below shows, for a given temperature T, a Boltzmann distribution of the kinetic energy of the molecules of a mixture of two gases that will react together, such as nitrogen and hydrogen. The activation energy for the reaction, Ea, is marked. Ea energy number of molecules (a) On the graph above, (i) draw a new distribution curve, clearly labelled T, for the same mixture of gases at a higher temperature, T; (ii) mark clearly, as H, the position of the activation energy of the reaction at the higher temperature, T. [3] (b) Explain the meaning of the term activation energy. … … … … [2]

5 9701/21/M/J/10 © UCLES 2010 [Turn over For Examiner’s Use The reaction between nitrogen and hydrogen to produce ammonia in the Haber process is an example of a large-scale gaseous reaction that is catalysed. (c) (i) State the catalyst used and give the operating temperature and pressure of the Haber process. catalyst … temperature … pressure … (ii) On the energy axis of the graph opposite, mark the position, clearly labelled C, of the activation energy of the reaction when a catalyst is used. (iii) Use your answer to (ii) to explain how the use of a catalyst results in reactions occurring at a faster rate. … … … [3] (d) Two reactions involving aqueous NaOH are given below. CH3CHBrCH3 + NaOH CH3CH(OH)CH3 + NaBr reaction 1 HCl + NaOH NaCl + H2O reaction 2 In order for reaction 1 to occur, the reagents must be heated together for some time. On the other hand, reaction 2 is almost instantaneous at room temperature. Suggest brief explanations why the rates of these two reactions are very different. reaction 1 … … … reaction 2 … … … [4] [Total: 12]

6 9701/21/M/J/10 © UCLES 2010 For Examiner’s Use 3 This question refers to the elements shown in the portion of the Periodic Table given below. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr (a) From this table, identify in each case one element that has the property described. Give the symbol of the element in each case. (i) The element that has a molecule which contains exactly eight atoms. … (ii) The element that forms the largest cation. … (iii) An element that floats on water and reacts with it. … (iv) An element that reacts with water to give a solution that can behave as an oxidising agent. … (v) An element whose nitrate gives a brown gas on thermal decomposition. … [5]

7 9701/21/M/J/10 © UCLES 2010 [Turn over For Examiner’s Use (b) (i) Give the formula of the oxide of the most electronegative element. … (ii) Several of these elements form more than one acidic oxide. Give the formulae of two such oxides formed by the same element. … and … [3] The formulae and melting points of the fluorides of the elements in Period 3, Na to Cl, are given in the table. formula of fluoride NaF MgF2 Al F3 SiF4 PF5 SF6 Cl F5 m.p. / K 1268 990 1017 183 189 223 170 (c) (i) Suggest the formulae of two fluorides that could possibly be ionic. … (ii) What is the shape of the SF6 molecule? … (iii) In the sequence of fluorides above, the oxidation number of the elements increases from NaF to SF6 and then falls at Cl F5. Attempts to make Cl F7 have failed but IF7 has been prepared. Suggest an explanation for the existence of IF7 and for the non-existence of Cl F7. … … … [4] [Total: 12]

8 9701/21/M/J/10 © UCLES 2010 For Examiner’s Use 4 (a) Complete the following reaction scheme which starts with propene. In each empty box, write the structural formula of the organic compound that would be formed. A CH3CH=CH2 KMnO4 /H+ Br2 cold, dilute NH3 in an excess KCN in aqueous ethanol B D C F E G H2SO4(aq) heat under reflux NaOH(in ethanol) heat under reflux HBr [7]

9 9701/21/M/J/10 © UCLES 2010 [Turn over For Examiner’s Use (b) Under suitable conditions, compound E will react with compound B. (i) What functional group is produced in this reaction? … (ii) How is this reaction carried out in a school or college laboratory? … … [3] [Total: 10]

10 9701/21/M/J/10 © UCLES 2010 For Examiner’s Use 5 Isomerism occurs in many organic compounds. The two main forms of isomerism are structural isomerism and stereoisomerism. Many organic compounds that occur naturally have molecules that can show stereoisomerism, that is cis-trans or optical isomerism. (a) (i) Explain what is meant by structural isomerism. … … (ii) State two different features of molecules that can give rise to stereoisomerism. … … [3] Unripe fruit often contains polycarboxylic acids, that is acids with more than one carboxylic acid group in their molecule. One of these acids is commonly known as tartaric acid, HO2CCH(OH)CH(OH)CO2H. (b) Give the structural formula of the organic compound produced when tartaric acid is reacted with an excess of NaHCO3. [1] Another acid present in unripe fruit is citric acid, HO2CCH2CCH2CO2H CO2H OH (c) Does citric acid show optical isomerism? Explain your answer. … … … [1]

11 9701/21/M/J/10 © UCLES 2010 For Examiner’s Use A third polycarboxylic acid present in unripe fruit is a colourless crystalline solid, W, which has the following composition by mass: C, 35.8%; H, 4.5%; O, 59.7%. (d) (i) Show by calculation that the empirical formula of W is C4H6O5. (ii) The Mr of W is 134. Use this value to determine the molecular formula of W. [3] A sample of W of mass 1.97 g was dissolved in water and the resulting solution titrated with 1.00 mol dm–3 NaOH. 29.4 cm3 were required for complete neutralisation. (e) (i) Use these data to deduce the number of carboxylic acid groups present in one molecule of W. (ii) Suggest the displayed formula of W. [5] [Total: 13]

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UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2010 question paper for the guidance of teachers 9701 CHEMISTRY 9701/21 Paper 2 (AS Structured Questions), maximum raw mark 60 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 must be read in conjunction with the question papers and the report on the examination. • CIE will not enter into discussions or correspondence in connection with these mark schemes. CIE is publishing the mark schemes for the May/June 2010 question papers for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Page 2 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 1 (a) fewer electrons in Cl 2 than in Br2 (1) smaller van der Waals’ forces in Cl 2 or stronger van der Waals’ forces in Br2 (1) [2] (b) CO has a permanent dipole or N2 does not (1) permanent dipole-permanent dipole interactions are stronger than those from induced dipoles (1) [2] (c) (i) a co-ordinate bond (1) (ii) a covalent bond (1) or (iii) a lone pair (1) or penalise any groups of 3 or 4 electrons that are circled [3] (d) CO and HCN both have a dipole or N2 does not have a dipole (1) [1]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 (e) (i) H H   HCCOH   H C≡N C≡N must be shown (1) (ii) nucleophilic addition (1) (iii) H H δ+ δ– |  CH3C=O CH3CO– HCN CH3COH + CN–  |  H CN– CN CN C=O dipole correctly shown or correct curly arrow on C=O (1) attack on Cδ+ by C of CN– (1) correct intermediate (1) CN– regenerated (1) [5 max] [Total: 13]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 2 (a) (i) new graph has lower maximum (1) maximum is to the right of previous maximum (1) (ii) H is at Ea (1) [3] (b) the minimum amount of energy molecules must have or energy required (1) in order for the reaction to take place (1) [2] (c) (i) iron or iron oxide (1) 100 to 500 atm and 400–550°C units necessary – allow other correct values and units (1) (ii) C is placed to the left of H (1) (iii) more molecules now have energy >Ea (1) [4] (d) reaction 1 has greater Ea (1) because energy is needed to break covalent bonds (1) reaction 2 has lower Ea or actual reaction is H+ + OH– → H2O or reaction involves ions (1) opposite charges attract (1) [4] [Total: max 12]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 3 (a) Accept only symbols. (i) S or S8 (1) (ii) K or K+ (1) (iii) Na – allow K or Li (1) (iv) Cl or Br or F (1) (v) Mg or Ca or Li allow Ni, Cu, or Zn (1) [5] (b) Accept only formulae. (i) F2O (1) (ii) SO2 and SO3 or P2O3/P4O6 and P2O5/P4O10 or any two from N2O3, NO2/N2O4, N2O5 or any two from Cl 2O, ClO2, ClO3, Cl 2O7 (1+1) [3] (c) (i) NaF, MgF2, AlF3 – any two (1) (ii) octahedral (1) (iii) I atom is larger than Cl atom (1) (iv) cannot pack 7 F atoms around Cl atom or can pack 7 F atoms around I atom (1) [4] [Total: 12]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 4 (a) Br2 KMnO4/H+ cold, dilute HBr KCN in NH3 aqueous in an ethanol excess H2SO4(aq) NaOH(in ethanol) heat under heat under reflux reflux give 1 for each correct structure (7 × 1) [7] (b) (i) ester (1) (ii) heat under reflux (1) trace of conc. H2SO4 or presence of HCl (g) (1) [3] [Total: 10] CH3CH=CH2 CH3CHBrCH2Br A CH3CH(OH)CH2OH B CH3CH2CH2CN or CH3CH(CN)CH3 D CH3CH2CH2Br or CH3CHBrCH3 C CH3CH2CH2CO2H or CH3CH(CO2H)CH3 E CH3CH2CH2NH2 or CH3CH(NH2)CH3 F CH3CH=CH2 G

Page 7 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 5 (a) (i) same molecular formula but different structural formula/structure (1) (ii) asymmetric C atom/chiral centre present (1) >C=C< bond present (1) [3] (b) NaO2CCH(OH)CH(OH)CO2Na (1) [1] (c) no because there is no chiral carbon atom present (1) [1] (d) (i) C : H : O = 12 35.8 : 1 4.5 : 16 59.7 this mark is for correct use of Ar values (1) C : H : O = 2.98 : 4.5 : 3.73 C : H : O = 1 : 1.5 : 1.25 this mark is for evidence of correct calculation (1) gives empirical formula of W is C4H6O5 (ii) C4H6O5 = 12 × 4 + 1 × 6 + 16 × 5 = 134 molecular formula of W is C4H6O5 (1) [3]

Page 8 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2010 9701 21 © UCLES 2010 (e) (i) n(OH–) = 1000 100 29.4 × = 0.0294 (1) n(W) = 134 1.97 = 0.0147 (1) no. of –CO2H groups present in one molecule of W = 0.0147 0.0294 = 2 (1) or n(OH–) = 1000 1.00 29.4 × = 0.0294 (1) 1.97 g W ≡ 0.0294 mol NaOH 134 g W ≡ 1.97 134 0.0294 × = 1.999 ≈ 2 mol NaOH (1) no. of –CO2H groups present in 1 molecule of W = 2 (1) [3] (ii) H H H—O   O—H C—C—C—C O   O H O—H or CH3 H—O  O—H C —C—C O  O O—H or OH  H—C—H H—O  O—H C—C—C O  O H one correct structure (1) correctly displayed (1) allow any correct ether [2] [Total: 13]