Cambridge A Level Chemistry 9701 — 2005 May/June Paper 6 · Variant 1

This document consists of 8 printed pages. SPA (MML 8078 3/04) S78179/3 © UCLES 2005 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Subsidiary Level and Advanced Level CHEMISTRY 9701/06 Paper 6 Options May/June 2005 1 hour Additional Materials: Answer paper Data Booklet Graph Paper (1 sheet) READ THESE INSTRUCTIONS FIRST Write your name, Centre number and candidate number on the front of any work handed 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. Write your answers on the separate answer paper provided. Answer all questions on two of the Options. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. You may use a calculator.

BIOCHEMISTRY Answer both questions on the paper provided. 1 The structures of four amino acids may be represented as shown below. (a) (i) Which functional groups are involved when a peptide chain is formed from amino acids? (ii) Draw a diagram to show how A and C form a dipeptide. [2] (b) (i) Draw the structure of B at pH 12. (ii) Draw the structure of D at pH 2. [2] (c) Which side-chains in the examples A to D, are most likely to be involved in the denaturing of proteins by (i) changes in pH, and (ii) addition of heavy metal ions? Explain your answers, giving equations or structures where possible. [6] H C CO2H A CH2 CH3 CH H2N H3C H C CO2H CO2H B CH2 H2N H C CO2H NH2 D (CH2)2 H2N H C CO2H C CH2OH H2N 2 9701/06/M/J/05 © UCLES 2005

2 (a) The diagram represents a section of the structure of a nucleic acid. (i) Explain why the nucleic acid is most likely to be DNA. (ii) Identify the blocks labelled X and Y. [4] (b) Analysis of a sample of DNA showed that 29% of the nitrogenous bases present were adenine, A. Calculate the percentages of each of the other bases in the DNA. [2] (c) Outline briefly how the code in m-RNA is translated to give the primary structure of the protein. [4] X X X X X A T X Y Y Y Y Y Y C G G C 3 [Turn over 9701/06/M/J/05 © UCLES 2005

ENVIRONMENTAL CHEMISTRY Answer both questions on the paper provided. 3 In hot summers there may be a significant increase in the concentration of tropospheric (low-level) ozone. Pollution from car exhausts is considered to be a significant contributor to the problem. (a) Give two examples of the adverse effects of ozone in the lower atmosphere. [2] (b) Nitrogen monoxide may be involved in the destruction of both tropospheric ozone and stratospheric ozone. Explain, using equations, how nitrogen monoxide is particularly effective in this process. [3] (c) The concentration of low-level ozone may be increased if hydrocarbons such as unburnt fuel or methane enter the atmosphere. This results from a complex series of gas phase free- radical reactions illustrated by the following equations which involve methane. OH• + CH4 →H2O + CH3• CH3• + O2 →CH3O2• CH3O2• + NO →CH3O• + NO2 CH3O• + O2 →HCHO + HO2• HO2• + NO →NO2 + OH• (i) Give an equation to show how OH• free radicals may be formed in the troposphere. (ii) Use the reaction sequence provided to explain carefully how methane can cause an increase in ozone in the troposphere. (iii) What other polluting gases could be present as a result of these reactions? [5] 4 (a) Gaseous oxygen maintains ions such as iron(III), sulphate and nitrate in their oxidised state within soils. Use the Data Booklet to give the half-equation for the reaction of oxygen which explains this. [1] (b) In normal soil conditions however the E of this half-equation is approximately +0.4 V lower than its value under standard conditions. State two reasons for this. [2] (c) If this lower value is decreased even further the iron(III) tends to become reduced. (i) Explain two circumstances which might cause such a decrease in the redox potential. (ii) With reference to the appropriate electrode potential, explain the reduction of the iron(III) ions. [4] (d) (i) Explain the effect of extreme reducing conditions on the sulphate ion and write a half-equation to illustrate your answer. (ii) What effect would such conditions have on plant growth? Explain your answer. [3] 4 9701/06/M/J/05 © UCLES 2005

PHASE EQUILIBRIA Answer both questions on the paper provided. 5 (a) The amounts of oxygen available to mammals and to fish in their respective environments are different. Oxygen (Mr = 32) obeys Henry’s Law, and may be considered as comprising 20% by volume of air. (i) State Henry’s Law. (ii) Calculate the mass of oxygen in 1 dm3 of air at 25 °C and 1 atm. (iii) The solubility of oxygen in water at 25 °C and a partial pressure of 0.2 atm is 0.031 cm3 per cm3 of water. Calculate the mass of oxygen dissolved in 1 dm3 of water. [3] (b) Henry’s Law does not apply to the distribution of oxygen and of carbon dioxide between the blood of a human and the gases in the atmosphere. Give reasons, with detailed explanations, why this is the case. [4] (c) Deep water divers rely on breathing pressurised air at about 5 atm. For this question, you may assume the volume of blood in a diver’s body to be 5 dm3. (i) What mass of oxygen from air at a pressure of 5 atm would dissolve in 5 dm3 of water at 25 °C? When a diver returns to the surface, there may be problems if bubbles of gas form in the blood vessels, as these restrict the flow of blood through valves and capillaries. (ii) Explain why these bubbles cannot be oxygen, and suggest which gas causes the problem. (iii) Explain briefly why carbon dioxide causes no real problem as the diver surfaces. [3] 6 Bismuth, m.p. 273 °C, and cadmium, m.p. 323 °C, form a eutectic at 140 °C of composition 41% by mass of cadmium. (a) Use graph paper to construct the bismuth/cadmium system, using a ruler to join the points. Label the four areas of the phase diagram. [3] (b) A solid sample of the mixture containing 20% by mass of cadmium is heated slowly. (i) At what temperature will liquid first appear? (ii) What will be the composition (% cadmium by mass) of this liquid? [2] (c) A 200 g of a sample of liquid containing 80% by mass of cadmium is cooled to 217 °C, and that temperature is maintained long enough for equilibrium to be established. What will be the compositions (% cadmium by mass) of the liquid and solid phases, and the masses of each phase present? [3] (d) Explain why bronze, an alloy of copper and tin, shows different properties to those of the two metals. [2] 5 [Turn over 9701/06/M/J/05 © UCLES 2005

SPECTROSCOPY Answer both questions on the paper provided. 7 (a) Describe the role of d orbitals in producing colour in transition metal complexes. You may use the example of water being progressively added to anhydrous copper sulphate in your explanation. [4] (b) The uv/visible spectrum shown below was obtained from a solution containing [M(H2O)6]3+, where M is a transition metal. Predict the colour of the solution, explaining your reasoning. [2] (c) Each of the following molecules absorbs energy in the uv/visible region of the spectrum as a result of electronic transitions. For each molecule, state the transition(s) responsible. (i) CH3CH2NH2 (ii) CH3CH=CH2 (iii) CH3CHO [4] 0 2 4 6 300 400 500 600 700 800 molar absorbtivity wavelength / nm 6 9701/06/M/J/05 © UCLES 2005

8 The spectra shown below were each obtained from compound Y. Use evidence from each of them to suggest a structure for the compound. Explain how absorptions shown in each of the spectra lead to your suggested structure. [10] 4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 absorbance wavenumber / cm–1 10 9 8 7 6 5 10 4 3 2 4 1 0 absorbance chemical shift
200 150 100 50 m/e m/e relative abundance relative abundance M M + 1 210 211 0.65 0.11 7 [Turn over 9701/06/M/J/05 © UCLES 2005

TRANSITION ELEMENTS Answer both questions on the paper provided. 9 (a) Briefly describe the biochemical importance of cobalt. [2] (b) (i) For the electrode reaction [Co(NH3)6]3+ + e– →[Co(NH3)6]2+, E = +0.10 V Use this information and the Data Booklet to explain why aqueous solutions of Co(II) salts are indefinitely stable to air, but when ammonia is added, oxidation readily occurs. (ii) Aqueous cobalt ions have very pale colours which can be ignored. Suggest the reaction that might occur, and hence the colour change that would be seen, when a solution containing Co3+(aq) is added to an acidified solution of Cr3+(aq). Write a balanced equation for the reaction. [5] (c) State a major use of chromium. [1] (d) When ammonium dichromate(VI) is heated, an exothermic reaction is initiated which results in the evolution of two neutral gases, and leaves as the only solid residue a grey-green powder, Cr2O3. Suggest the identity of the gases evolved, and construct a balanced equation for the decomposition. [2] 10 (a) Major impurities in copper ores are compounds of zinc. Describe in outline how zinc is removed during the electrolytic purification of copper. [4] (b) Explain the chemistry behind the use of alkaline Cu(II) complexes as a test for aldehydes. [3] (c) A 0.50 g sample of Palestinian brass dated 1200 BC was dissolved in nitric acid. After evaporating to remove the excess of nitric acid, the resulting nitrates were dissolved in water and an excess of KI(aq) added. After suitable treatment, the precipitated CuI(s) weighed 1.16 g. (i) Calculate the percentage of copper in the brass. (ii) What metal was the other major component in the brass? [3] 8 9701/06/M/J/05 © UCLES 2005 Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. University of Cambridge International Examinations is part of the University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Level MARK SCHEME for the June 2005 question paper 9701 CHEMISTRY 9701/06 Paper 6 (Options), maximum raw mark 40 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 Examiners were initially instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began. Any substantial changes to the mark scheme that arose from these discussions will be recorded in the published Report on the Examination. 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 must be read in conjunction with the question papers and the Report on the Examination. • CIE will not enter into discussion or correspondence in connection with these mark schemes. CIE is publishing the mark schemes for the June 2005 question papers for most IGCSE and GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Grade thresholds for Syllabus 9701 (Chemistry) in the June 2005 examination. minimum mark required for grade: maximum mark available A B E Component 6 40 23 20 11 The thresholds (minimum marks) for Grades C and D are normally set by dividing the mark range between the B and the E thresholds into three. For example, if the difference between the B and the E threshold is 24 marks, the C threshold is set 8 marks below the B threshold and the D threshold is set another 8 marks down. If dividing the interval by three results in a fraction of a mark, then the threshold is normally rounded down.

June 2005 GCE A LEVEL MARK SCHEME MAXIMUM MARK: 40 SYLLABUS/COMPONENT: 9701/06 CHEMISTRY Paper 6 (Options)

Page 1 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 Biochemistry 1 (a) (i) Carboxylic acid and amino/amine groups (formulae accepted) (1) (ii) (1) [2] (b) (i) (1) (ii) (1) [2] (c) (i) B will form -CO2 - at high pH (1) D will form -NH3 at low pH (1) (ii) B will form e.g. -CO2Ag (other heavy metals inc Hg, Cd, Pb) (1) C will form salts or ‘alcohoates’ e.g. -CH2O-Ag+ (1) D will form complex ions (1) -CH2NH2 → Cu2+ (or equiv) (1) [6] [Total: 10]

Page 2 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 2 (a) (i) T is present in DNA not RNA (or U present in RNA) (1) DNA is double helix/RNA usually single strand (1) (ii) X is deoxyribose (1) Y is phosphate/phosphorus (1) [4] (b) Since A is 29%, T must also be 29% (1) G = C = 2 ) 58 100 ( − = 21% (1) [2] (c) Sequence of 3 bases in m-RNA/triplet code/codon (1) Corresponds to a particular amino acid (1) m-RNA is complementary to section of 1 strand of DNA1 (1) Base sequence of m-RNA/DNA determines the primary structure (1) Other codons are for initiation or termination (1) [4 max] [Total: 10]

Page 3 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 Environmental Chemistry 3 (a) Formation of photochemical smog (1) Compounds irritate mucous membranes/respiratory system (1) Photosynthesis is adversely affected (1) Increases ‘greenhouse effect’ (1) [Any 2] (b) NO + O3 → NO2 + O2 O3 → O• + O2 3 eqns => 2 marks 2 eqns => 1 mark (2) NO2 + O• → NO + O2 NO is regenerated in the third reaction so reaction continues (1) [3] (c) (i) O3 + H2O → O2 + 2OH• (or other sensible eqns) (1) (ii) NO is used up thus preventing the continued destruction of ozone (1) OH• is regenerated so the reaction continues (1) Some comment about hydrocarbons providing an alternative oxidation pathway without using ozone (1) (iii) HCHO or NO2 (1) [5] [Total: 10] 4 (a) O2 + 4H+ + 4e- ⇋ 2H2O Eo = 1.23 V (1) [1] (b) The oxygen concentration is lower (1) The pH is higher (1) [2] (c) (i) Increase in the pH of the soil affects the half-cell reaction (1) Waterlogging reduces oxygen circulation (1) (ii) Fe3+ + e- ⇋ Fe2+ Eo = 0.77 V (1) In normal soil the Eo drops from 1.23 V to 0.83 V, any further drop takes it below that in the half-equation above (1) [4]

Page 4 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 (d) (i) Extreme reducing conditions produce hydrogen sulphide (1) SO4 2- + 10H+ + 8e- ⇋ H2S + 4H2O (1) (ii) Hydrogen sulphide will gradually kill plants as it reacts with iron (1) [3] [Total: 10] Phase Equilibria 5 (a) (i) The mass of gas which dissolves in a given volume of solvent at a particular temperature, is proportional to the pressure of the gas (1) (ii) 24 dm3 of oxygen weighs 32 g Hence 0.2 dm3 of oxygen weighs 0.2 x 32 = 0.267 g (1) 24 (iii) Volume of oxygen = 0.031 x 103 = 31 cm3 Thus the mass of oxygen = 31 x 32 = 0.041(3) g (1) [3] 24000 (b) Henry’s Law only holds at a given temp and when the same (molecular) species are present in both gas and liquid phases (1) The blood will not be at the same temperature as the atmosphere (1) In blood the oxygen is present as O2- haemoglobin complex (1) CO2 reacts with blood (1) [4] (c) (i) Mass of O2 = 5 x 5 x 0.0413 = 1.03 g (1) (ii) Oxygen will not form bubbles as it combines with haemoglobin, (1) hence the gas is nitrogen (1) CO2 reacts with blood/forms H2CO3/forms H+ and HCO3 - (1) [4] [Total: 10]

Page 5 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 6 (a) axes (1) points and lines (1) labels of 3 areas (1) [3] (b) (i) 140 °C/eutectic temperature (1) (ii) 41% Cd (eutectic) (1) [2] (c) The liquid is 66 + 2% Cd (1) Hence the composition by mass is Bi 40g and Cd 80g (1) The solid is cadmium, and there is 80 g of it (1) [3] (d) Two valid explanations e.g. The metals have different atomic radii Different electronic arrangement giving different colour The lattice structure of the alloy is different/disrupted 2 x (1) [2] [Total: 10]

Page 6 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 Spectroscopy 7 (a) Addition of ligands causes splitting of d-orbitals (1) Electron(s) are promoted from lower to higher energy orbitals (1) Energy is absorbed (1) This is in the visible region (1) [4] (b) Green/turquoise/cyan (1) Minimum energy absorbed is at 400 nm and above 600 nm (Accept in blue and red parts of spectrum) or colour is compliment of energy absorbed (1) [2] (c) (i) n → σ* (1) (ii) π → π * (1) (iii) π → π *, n → σ*, n → π * 3 → 2, 2 → 1, 1 → 0 (2) [4] [Total: 10]

Page 7 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 From mass spectrum 8 Mr of Y is 210 M : M + 1 = 0.65 : 0.11 No of carbons present = 0.11 × 100 = 15 1. 1 65 . 0 × (1) From nmr spectrum There are only two types of proton present (1) Since Mr of Y is 210, this suggests C15H14O (1) Absorption at 7.2 δ suggests C6H5- groups (1) This leaves -CH2- groups (1) C=O is central/between CH2 groups (1) From ir spectrum Strong absorption at 1720 cm-1 suggests C=O (1) There is no characteristic -OH absorption (1) There is no characteristic -C-O absorption (1) Y is likely to be (1) Additional possible marks from mass spectrum 91 - (1) 119 - (1) 28 - C+ = O (1) [Total: max 10]

Page 8 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 Transition Elements 9 (a) occurs as cobalamine/vitamin B12 (1) which is needed to prevent pernicious anaemia or used to synthesise amino acids or carbon-carbon bonds etc. (1) [2] (b) (i) Eo for Co3+/Co2+ is + 1.82V Eo for O2/OH- is -0.40V (1) O2 is not strong enough to oxidise Co2+(aq), but is more positive than Eo([Co(NH3)6]3+/[Co(NH3)6]2+), so oxidation occurs. (1) (ii) Eo for Co3+/Co2+ is + 1.82V Eo for Cr2O7 2-/Cr3+ is + 1.33V (1) so oxidation from green (Cr3+) to orange (Cr2O7 2-) will occur (1) 6Co3+ + 2Cr3+ + 7H2O → 6Co2+ + Cr2O7 2- + 14H+ (1) [5] (c) To make stainless steel/chromium plating/nichrome wire (1) [1] (d) (NH4)2Cr2O7 → N2 + 4H2O + Cr2O3 (1) gases are N2 + steam (1) [2] [Total: 10] 10 (a) both zinc and copper dissolve at the anode: (1) Cu - 2e- → Cu2+(aq) Zn - 2e- → Zn2+(aq) (both) (1) copper is preferentially discharged at the cathode or Cu2+ + 2e- → Cu(s) (1) Eo(Cu2+/Cu) = +0.34V Eo(Zn2+/Zn) = -0.76V hence zinc remains in solution (1) [4] (b) aldehydes reduce Cu(II) to Cu(I) not Cu (1) RCHO + 2Cu2+ + 5OH- → RCO2 - + Cu2O + 3H2O (1) or 2Cu2+ + 2OH- + 2e- → Cu2O + H2O Cu2O forms a (brick) red ppt. (1) [3]

Page 9 Mark Scheme Syllabus Paper A LEVEL – June 2005 9701 6 © University of Cambridge International Examinations 2005 (c) (i) CuI = 63.5 + 127 = 190.5 moles CuI = 1.16/190.5 = 0.00609 (1) mass of Cu = 0.00609 x 63.5 = 0.3867g % of Cu = 100 x 0.3867/0.5 = 77.3% (1) (ii) zinc (1) [3] [Total: 10]