14   6 Amino acids can be separated using chromatography. (a) State how chromatography separates the components of a mixture. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) A sample of a tripeptide was hydrolysed and then placed on an ‘X’ at the bottom right‑hand corner of a piece of chromatography paper. A simplified diagram of a developed chromatogram is shown. baseline 1 solvent front 1 (i) Give a possible reason for the presence of only two spots for the tripeptide other than two amino acids have almost identical Rf values. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 15   (ii) Some amino acid mixtures cannot be effectively separated in one chromatography ‘run’. The chromatography paper from the first run is dried but not developed. The chromatography paper is then rotated clockwise by 90° and placed in a different solvent. Complete the simplified diagram of the developed chromatography paper after a second ‘run’ for a tripeptide of alanine, glycine and valine by adding labelled spots for each amino acid. Amino acid Rf in solvent 1 Rf in solvent 2 alanine 0.38 0.43 glycine 0.33 0.26 valine 0.39 0.58 (3) baseline 1 baseline 2 solvent front 2 solvent front 1 16   (iii) Name a reagent that locates colourless amino acids by producing a coloured compound. (1) .................................................................................................................................................................................................................................................................................... (c) State the technique that is used in conjunction with gas chromatography (GC) when carrying out forensic testing. (1) .................................................................................................................................................................................................................................................................................... (Total for Question 6 = 7 marks) Turn over 17   7 Hardness in water is measured in terms of the concentration of dissolved calcium compounds. Titration experiments can be carried out to determine the hardness of a water sample. (a) A pipette is used to measure a 50.0 cm3 water sample for titration. (i) Describe how to remove an air bubble from the tip of the pipette. 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(ii) Calculate the maximum volume that would be obtained by using a 25.0 cm3 pipette twice to measure a total volume of 50.0 cm3. The uncertainty in each 25.0 cm3 pipette measurement is ±0.04 cm3. (1) (iii) Compare the percentage uncertainty in using a 25.0 cm3 pipette twice with using a 50.0 cm3 pipette once to measure 50.0 cm3 of water. The uncertainty in the 50.0 cm3 pipette measurement is ±0.05 cm3. (2) 18   (b) About 2 cm3 of a pH 10 buffer is added to each 50.0 cm3 water sample. (i) State whether or not a 100 cm3 measuring cylinder is suitable to measure this volume of buffer solution. Justify your answer. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) The pH 10 buffer can be made by adding solid ammonium chloride to an aqueous solution of ammonia of concentration 18.1 mol dm–3. The relevant equation is NH4 +  NH3 + H+ Ka = 5.62 × 10–10 mol dm–3 Calculate the mass of ammonium chloride that must be added to 100 cm3 of ammonia solution to make the pH 10 buffer. Assume that there is no change in the volume on the addition of ammonium chloride. (4) Turn over 19   (iii) State a necessary laboratory precaution, other than wearing a laboratory coat, gloves and goggles, that must be taken when using concentrated ammonia. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (c) The Eriochrome Black T indicator used in this titration forms an octahedral complex with the calcium ions in the water sample. The structure of Eriochrome Black T is shown with a calcium ion. (i) Complete the diagram to show how Eriochrome Black T forms three dative covalent or coordinate bonds with the calcium ion. (1) N O–Na+ N NO2 Ca2+ H S O O O H O (ii) State the number of water molecules needed to complete this complex. (1) .................................................................................................................................................................................................................................................................................... 20   (d) There are two types of water hardness: temporary hardness which is removed by boiling as a precipitate forms, permanent hardness which is unaffected by boiling. Levels of water hardness are expressed as the concentration of calcium ions in mg dm–3. A student carried out a series of experiments to determine the hardness of a sample of water. 50.0 cm3 samples of the water were titrated with EDTA. Further 50.0 cm3 samples of water were taken after boiling and then titrated with EDTA. (i) Name the process needed before titrating the sample of boiled water. (1) .................................................................................................................................................................................................................................................................................... (ii) The mean titre of 0.0100 mol dm–3 EDTA4– with a 50.0 cm3 water sample before boiling was 12.80 cm3. After boiling the mean titre was 5.15 cm3. There is a 1 : 1 ratio in the reaction between EDTA4– ions and Ca2+ ions. Calculate, in this water, the levels of permanent and temporary hardness in mg dm–3 of calcium ions. (6) (Total for Question 7 = 19 marks) Turn over 21   8 This question is about reaction kinetics and the Arrhenius equation. (a) Different iodine clock reactions are often used to investigate reaction kinetics. (i) The iodine clock reaction with hydrogen peroxide involves the reaction shown. H2O2 + 2I– + 2H+ → I2 + 2H2O Deduce two possible experimental techniques which could be used to monitor the progress of this reaction. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) The iodate(V) reaction has the rate determining step IO3 – + 3HSO3 – → I– + 3HSO4 – Give a possible reason why this is the slowest step. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iii) The chlorate(V) reaction has the rate determining step ClO3 – + 2H+ + I– → HIO + HClO2 Deduce the rate equation for this iodine clock reaction. (1) 22   (b) The diagram shows a sketch of the Maxwell‑Boltzmann curve for the distribution of molecular energies of a reaction mixture at temperature 298 K. Number of molecules with energy E Energy E (i) Add a curve to show the distribution at a temperature of 308 K. (1) (ii) Explain why a temperature rise from 298 K to 308 K results in a large increase in the rate of reaction. Refer to the Maxwell‑Boltzmann distribution in your answer. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 23   (c) The Arrhenius equation may be written in a logarithmic or an exponential form. ln k = – E RT a + ln A k = A e −E RT a A is a constant. (i) The rate constant, k, for the isomerisation of cyclopropane to propene was measured at various temperatures. The data obtained were used to draw the graph shown. 0 –2 –4 –6 –8 –10 1.1 × 10–3 1.2 × 10–3 1.3 × 10–3 1.4 × 10–3 ln k 1 1 T / K − Determine the activation energy, Ea , from the gradient of the graph. Include units in your answer. (3) 24   (ii) At a temperature T, the fraction of molecules with energy equal to or greater than the activation energy is given by the expression fraction of molecules = e −E RT a When a catalyst is added, the activation energy for a reaction is lowered. Explain, using calculations, why lowering the activation energy from 50 000 J mol–1 to 25 000 J mol–1 at 298 K results in a large increase in the rate of reaction. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 8 = 13 marks) Turn over 25   9 This is a question about isomers of C8H8O2 . (a) One of these isomers, methyl benzoate, is hydrolysed by alkali or by acid. (i) Hydrolysis with aqueous sodium hydroxide is followed by acidification to form benzoic acid. Give a reason why acidification is required after hydrolysis. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Write an equation, using structural formulae, for the acid hydrolysis of methyl benzoate. (1) 26   (b) Four other C8H8O2 isomers were investigated. • W and X are mono‑substituted aromatic compounds with the same functional group as methyl benzoate but only W is made from methanoic acid • Y is a mono‑substituted aromatic compound which reacts with sodium carbonate to give carbon dioxide • Z is a disubstituted aromatic compound with six peaks in its 13C NMR spectrum and forms a sweet‑smelling compound on reaction with ethanol Deduce the structures of isomers W, X, Y and Z. Justify your answers. (7) Turn over 27   .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 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.................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 28   (c) Piceol is found in the needles of Norway spruce trees. Its structure is shown. O H3C OH C (i) Piceol can be produced from the reaction of ethanoyl chloride and phenol. Assume the mechanism for the reaction with phenol is similar to that with benzene and involves the use of an aluminium chloride catalyst, which produces the electrophile [CH3C O]+. Complete the diagram, including curly arrows, to show the mechanism for this reaction to produce piceol. Include the regeneration of the catalyst. (4) O + H3C OH C (ii) Piceol can be distinguished from HOC6H4CH2CHO using simple chemical tests. Give the reagents for a chemical test, and the observation that would only be positive for piceol. 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(Total for Question 9 = 15 marks) Turn over 29   10 The Mond Process is an industrial method of purifying nickel. (a) The first step involves the reaction of nickel oxide with hydrogen gas at 473 K. NiO(s) + H2(g) → Ni(s) + H2O(g) The nickel is not pure because the impurities also react with the hydrogen gas. Complete the electronic configuration of the Ni2+ ion. (1) 1s2 . ........................................................................................................................................................................................................................................................................... (b) The second step involves passing carbon monoxide over impure nickel at 323 K. The impurities do not react. The nickel reaction is Ni(s) + 4CO(g) → Ni(CO)4(g) ΔrH d = –191 kJ mol–1 (i) Calculate the total entropy change, ΔS d total , for this reaction. Include a sign and units in your answer. Substance S d / J mol–1 K–1 Ni(s) +29.9 CO(g) +197.6 Ni(CO)4(g) +313.4 (5) 30   (ii) Predict the sign of the Gibbs Free Energy change, ΔG, for this reaction and justify your choice. No calculation is required. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iii) 50.0 mol of carbon monoxide is mixed with excess impure solid nickel at 323 K in an industrial reactor. At equilibrium, 0.750 mol of carbon monoxide remains. The pressure is maintained at 1.5 atm throughout. Calculate the value of Kp at 323 K. Include units with your answer. (6) 31   (c) The final stage of the Mond Process is the thermal decomposition of the nickel carbonyl gas, Ni(CO)4 , to give pure nickel and carbon monoxide. The reaction mixture is heated to 523 K. Explain, in qualitative terms, why the entropy change of the system, ΔS d system , for this decomposition reaction is positive. 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(Total for Question 10 = 15 marks) TOTAL FOR PAPER = 120 MARKS 32  