Name: Class: Form 7S Class Number: Total Mark:

Transcription

1 Paper II / 1 CCC HEEP WOH COLLEGE MOCK EXAMINATION F.7 AL CHEMISTRY PAPER II 1:30pm 4:30pm (3 hours) Question-Answer Book SETTER: MR. LAU C.K. Name: Class: Form 7S Class Number: Total Mark: 1. This paper must be answered in English. 2. There are TWO sections in this paper, Section A and Section B. 3. Section A carries 60 marks and Section B carries 40 marks. 4. All questions in Sections A and B are COMPULSORY. Answers are to be written in this Question-Answer Book. If you use supplementary answer sheet(s) for these two sections, write your Name and Class Number on each sheet and fasten them with string to this Question-Answer Book. 5. Some useful constants and a Periodic Table are printed below: Periodic Table / 100 Useful Constants Gas constant, R = 8.31 J K -1 mol -1 Speed of light in vacuum, c = 3.00 x 10 8 m s -1 Faraday constant, F = 9.65 x 10 4 C mol -1 Ionic product of water at 298K, Kw = 1.00 x mol 2 dm -6 Avogadro constant, L = 6.02 x mol -1 Specific heat capacity of water = 4.18 J g -1 K -1 Planck constant, h = 6.63 x J s Molar Volume of gas at r.t.p. = 24.0 dm 3 mol -1 Characteristic Infra-red Absorption Wavenumber Ranges (Stretching modes) Bond Compound type Wavenumber range / cm -1 C=C Alkenes 1610 to 1680 C=O Aldehydes, ketones, carboxylic acids, esters 1680 to 1750 C C Alkynes 2070 to 2250 C N Nitriles 2200 to 2280 O H Acids (hydrogen-bonded) 2500 to 3300 C H Alkanes, alkenes, arenes 2840 to 3095 O H Alcohols, phenols (hydrogen-bonded) 3230 to 3670 N H Amine 3350 to 3500

2 Section A: Answer ALL questions in this Section. Write your answers in the spaces provided. Paper II / 2 Question 1 [20 marks] (a) 40.0 cm 3 of ethanoic acid, CH3COOH(aq), was titrated with potassium hydroxide solution, KOH(aq) at 298K. The initial ph, before the addition of KOH(aq) was At the equivalence point of titration, the ph was (i) Explain the difference between the equivalence point and the end point of a titration. Given the dissociation constant of ethanoic acid at 298 K is mol dm 3, calculate (I) the initial concentration of ethanoic acid; (II) the volume of the KOH(aq) added to the equivalence point; (III) the concentration of the KOH(aq). (7 marks)

3 Paper II / 3 (b) Explain the following: (i) The dipole moment of PCl3 is 0.93D, whereas PCl5 has no net dipole moment. Sodium dichromate(vi) is more soluble in water than potassium dichromate(vi). (4 marks) (c) Aluminium(III) chloride forms an addition compound with triethylamine. (i) What is the likely composition of this addition compound? Draw a sketch to show its shape. What is the nature of bonding involved in this addition compound? (3 marks)

4 Paper II / 4 (d) (i) Using the concept of activation energy, together with an appropriate sketch of the Maxwell-Boltzmann distribution, explain why the addition of a suitable catalyst speeds up a chemical reaction. The decomposition of solutions of hydrogen peroxide is catalysed by certain transition element ions. 2H2O2(aq) 2H2O(l) + O2(g) Suggest a mechanism for the catalysis of this reaction by Fe 2+ ions. (You should refer to the standard electrode potentials given below) O2(g) + 2H + (aq) + 2e H2O2(aq) +0.68V Fe 3+ (aq) + e Fe 2+ (aq) +0.77V H2O2(aq) + 2H + (aq) + 2e 2H2O(l) +1.77V O2(g) + 4H + (aq) + 4e 2H2O(l) +1.23V Fe 2+ (aq) + 2e Fe(s) 0.44V 2H + (aq) + 2e H2(g) 0.00V (6 marks)

5 Paper II / 5 Question 2 [20 marks] (a) The electrolysis of a solution of potassium fluoride in a mixed solvent of hydrogen fluoride and water produces a colourless triatomic gas A at the anode. When g of A is introduced into an evacuated vessel of 1.00 dm 3 capacity at a temperature of 282 K, it produces a pressure of Pa. (i) Calculate the relative molecular mass of A. Suggest the molecular formula and the shape of A. (3 marks) (b) In acidic solution, bromide ion react with bromate(v) ion to form bromine and water. The process of the reaction can be followed by adding a fixed amount of phenol together with some methyl red indicator. The bromine produced during the reaction reacts very rapidly with phenol. When all the phenol is consumed, any further bromine produced bleaches the indicators immediately. So the time for the reaction to proceed to a given point may be determined. Experimental data: Experiment Temperature/ C Time for the disappearance of the red colour/ s (i) Write a balance ionic equation for the reaction between bromide ion and bromate(v) ion in acidic medium. Give an equation for the reaction between phenol and bromine.

6 Paper II / 6 (iii) Based on the Arrhenius equation, derive another equation so that time for the complete disappearance of the red colour(abbreviated as t ) can be related to Ea. (iv) Hence, or otherwise, determine Ea by plotting a suitable graph.

7 Paper II / 7 (v) Explain briefly the following in molecular terms: (I) temperature rise can increase reaction rate. (II) a reaction with low activation energy proceeds faster. (9 marks) (c) Hydrangeas are flowering plants which commonly have pink flowers. They produce blue flowers only in soils which contain high concentrations of Mg 2+ (aq). The ph of well-limed soil is 9.00, whereas the ph of peat-based soil is The numerical value of the solubility product, Ksp of magnesium hydroxide is mol 3 dm 9. (i) By calculating the theoretical maximum value of [Mg 2+ (aq)] in the two types of soil, deduce which type of soil favours good cultivation of blue hydrangeas. Horticulturalists use ammonium magnesium sulphate, (NH4)2Mg(SO4)2 6H2O, to provide Mg 2+ as well as nitrogen. Explain why NH4 + (aq) helps the uptake of Mg 2+ by the plant. (5 marks)

8 Paper II / 8 (d) The figure below can be used to represent the unit cell of either zinc blende (ZnS) or diamond: Making use of this figure, describe the similarities and differences of the crystal lattice present in zinc blende and diamond. (3 marks)

9 Paper II / 9 Question 3 [20 marks] (a) (i) State what is meant by standard electrode potential of a metal. The quantitative relationship showing reduction potential (E) of M n+ (aq) M(s) under non-standard condition is given by Nernst equation at 298 K: E = E + log [M n+ (aq)] n where n is the number of electrons transferred in the half-equation. Calculate the electrode potential of a zinc rod dipping into a solution containing mol dm -3 Zn 2+ at 298K. (iii) A cell is set up as below: (I) Prove that the e.m.f. of this cell at non-standard condition is [Zn (aq)] e.m.f = 1.56 log 2 2 [Ag (aq)]

10 Paper II / 10 (II) When this cell is discharged and reach equilibrium, what is the value of the e.m.f. of this cell at equilibrium? (III) Calculate the equilibrium constant, Kc, of the following reaction at 298 K. 2Ag + (aq) + Zn(s) Zn 2+ (aq) + 2Ag(s) Given: Zn 2+ (aq) + 2e Zn(s) 0.76 Ag + (aq) + e Ag(s) (6 marks) (b) For the reaction H2(g) + C2H4(g) C2H6(g) calculate the enthalpy of the above reaction by two different methods using (I) the enthalpy of formation at 298K H o f of C2H4 = kj mol 1 ; H o f of C2H6 = 84.6 kj mol 1 ; and

11 Paper II / 11 (II) the average bond energies at 298K H H +436 kj mol 1 ; C H +414 kj mol 1 ; C=C +620 kj mol 1 ; and C C +347 kj mol 1. Comment on the difference between the enthalpy of reaction obtained by the two methods. (7 marks) (c) A series of experiments was carried out on B, an ionic salt, to determine its identity. 1 Some solid B was tested with flame test and it gives lilac flame. 2 Some solid B was dissolved in distilled water. Aqueous silver nitrate(v) acidified with dilute nitric(v) acid was added and a cream precipitate was formed, which only slightly dissolved in dilute aqueous ammonia but was soluble in concentrated ammonia solution. 3 Some solid B was heated with concentrated sulphuric(vi) acid. The mixture of fumes produced was passed into water and the solution labelled C. The solution C was strongly acidic and coloured pale brown. 4 A sample of solution C was mixed with an excess of an aqueous solution of potassium iodide, then the colour of the solution turned to a very dark brown. The solution turned dark blue on adding one drop of starch solution but adding an excess of sodium thiosulphate solution removed the colour completely. (i) Identify B. Identify the component of the fumes which was responsible for the solution C being strongly acidic.

12 Paper II / 12 (iii) Account for the production of the pale brown colour in solution C. (iv) The starch solution turned dark blue. What was indicated by this change? (v) Write an ionic equation for the reaction which led to the disappearance of the dark blue colour on adding excess sodium thiosulphate solution. (d) Draw a labelled diagram to show the overlapping of orbitals of ethene. (5 marks) (2 marks) End of Section A

13 Paper II / 13 Section B: Answer ALL the questions on the Answer Book provided. Question 4 [20 marks] (a) Identify A, B, C, D and E. A is the reagent and condition needed to bring about the reaction. B, C, D and E are the major organic products of the reactions. (i) Br A A: KMnO 4, H 2 SO 4 (aq) heat B B: (iii) O 2 N C H 3 N NH O NaOH(aq) reflux C C: (iv) HO C OH COCl 2 polymer D D: (v) C H 3 N CH CH 2 C C O excess I E H 3 C CH 2 E: (5 marks)

14 Paper II / 14 (b) An organic iodide F is hydrolysed in dilute NaOH(aq) to give alcohol G at 298K. I OH CH 2 O CH 2 O F G (i) Outline a mechanism of the reaction. Draw a labelled energy profile of the reaction. (iii) Compared with iodide F, what change in rate would you expect when iodide H below is hydrolysed under the same condition? Explain your answer and draw a suitable resonance structure to illustrate the answer. I CH 2 O H (7 marks)

15 Paper II / 15 (c) The structure of the active ingredient in aspirin tablets is COOH O O As well as the active ingredient, aspirin tablets contain other substances. The aspirin content was determined by the method indicated below: Three aspirin tablets were crushed and then added to 25.0 cm 3 of 1.00 M sodium hydroxide solution and simmered for 30 minutes. When cooled the reaction mixture was diluted to cm 3 in a volumetric flask cm 3 sample solution were then back titrated with M sulphuric acid until concordant results were obtained. The average titre was cm 3. (i) Write an equation for the alkaline hydrolysis of aspirin. Calculate (I) the number of moles of excess sodium hydroxide in the volumetric flask. (II) the number of moles of sodium hydroxide which reacted with the aspirin. (III) the average mass of pure aspirin in each tablet. (iii) Why does the back titration technique have to be used to determine the mass of aspirin?

16 Paper II / 16 (iv) A sample of aspirin was prepared and then purified by crystallization. List the experimental procedure involved. (v) Describe a simple method to test whether the product you obtain from (iv) is pure and is the desired product. (8 marks)

17 Paper II / 17 Question 5 [20 marks] (a) Salbutamol is used for the relief of bronchospasm in conditions such as asthma. Its structure is given: OH H HO HO The clinical uses of Salbutamol are specifically indicated for the following conditions: acute asthma and symptom relief during maintenance therapy of asthma and it has a protection against exercise-induced asthma. However, the adverse effects of Salbutamol include tremor, palpitations and headache. Infrequent adverse effects include tachycardia, muscle cramps, agitation, and hyperactivity in children. (i) Draw the major structure of Salbutamol when it exists at (I) 1M HCl; N (II) 1M NaOH. The above structure can represent two stereoisomers. Draw the three-dimensional structures of the two stereoisomers and state a physical property which is different for the two stereoisomers. (iii) Would you expect both stereoisomers have the same medical effect? Explain briefly. (5 marks)

18 Paper II / 18 (b) Methylbenzene is a useful starting material for the synthesis of many intermediates used to make pharmaceuticals, dyes and agrochemicals. (i) Suggest the reagents and conditions needed for each of the reactions I, III and IV in the following scheme. CH 2 Cl D I methylbenzene II E III CO 2 H IV Cl COCl F I: III: IV: Suggest the mechanism of the conversion of methylbenzene to E. (iii) Describe and explain the relative ease of hydrolysis of the chloro compounds D, E and F. (9 marks)

19 Paper II / 19 (c) An organic compound G, contains carbon, hydrogen and oxygen, with relative molecular mass below 80. Upon complete combustion, 3.00 g of G gives 7.33 g of carbon dioxide and 3.00 g of water. (i) Deduce the molecular formula of G. A portion of the infra-red spectrum of G is shown below: What functional group(s) does compound G have? Explain your answer. (iii) Draw all possible structures for G. (iv) When G is treated with acidified potassium dichromate(vi) solution, no change can be observed. Suggest a structure for G and explain your suggestion. End of Paper (6 marks)