TJC H2 Chem 2013 Prelim P3 QP

9647/03/TJC Prelim 2013 [Turn Over 1 CHEMISTRY (H2) 9647/03 Paper 3 Free Response Friday 20th September 2013 2 hours Candidates answer on separate paper. Additional materials: Answer paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, Civics Group and Index number in the spaces provided on the cover page and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in bracke ts [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This question paper consists of 11 printed pages. 1370

9647/03/TJC Prelim 2013 [Turn Over 2 Answer any four questions. 1 (a) Describe and explain the trend observed in the ease of thermal decomposition of the nitrates of the Group II elements magnesium to barium. Write an equation for any reaction that occurs. [3] (b) An alloy of aluminium and magnesium is used in boat-building. A 1.75 g sample of the alloy was dissolved in the minimum volume of 4 mol dm -3 hydrochloric acid and the solution was then made alkaline by the addition of aqueous sodium hydroxide until no further reaction occurred. The resultant mixture was filtered and the residue, A, rinsed with distilled water, all washings being added to the filtrate, B. After drying, 0.18 g of A was obtained. Carbon dioxide was passed into B and a white solid, C, which contained aluminium, was collected. Heating C to constant mass gave a residue of mass 3.16 g. (i) Write ionic equations for the reactions that occur when sodium hydroxide is added to the solution as described above. (ii) Explain, using relevant equation(s), the formation of C when carbon dioxide is passed into B. (iii) Determine the percentage composition by mass of the alloy. [7] (c) Ozone mixed with oxygen is fairly stable but rapidly decomposes in the presence of a silver catalyst as shown in the equation below. 2O 3 3O 2 Silver was added to a 800 cm 3 sample of a mixture of oxygen and ozone. Below is a table showing the increase in volume of the mixture and volume of ozone at the respective time intervals. Time/s 0 200 440 900 1500 1800 Volume of mixture/cm3 800 805 810 815 818 819 Volume of ozone/cm 3 38 28 18 8 0 (i) Given that the bond angle of ozone is 116.8 0, draw the dot and cross diagram of the ozone molecule. (ii) Calculate the volume of ozone in the mixture at t = 1500 s. (iii) By means of a suitable graph, determine the order for the decomposition of ozone. (iv) Calculate a value for the rate constant and write the rate equation. (v) Use the concept of activation energy, together with an appropriate sketch of the Boltzmann distribution, to explain how silver affects the rate of decomposition. [10] [Total: 20] 1371

9647/03/TJC Prelim 2013 [Turn Over 3 2 (a) Haemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates. Structurally, haemoglobin consists of 4 polypeptide sub-units fitted together to form a compact, globular structure which is soluble in water. Each sub-unit is bonded to a haem group. Approximately 70% of each sub-unit is incorporated into - helices. The -sub-unit of haemoglobin is made up of 141 amino acids, 8 of which are listed below. Amino acid Formula of side chain (R in RCH(NH 2)CO2H) Number of amino acid residue per -sub-unit. leucine 18 valine 13 lysine 11 serine 11 histidine 10 phenylalanine 7 glutamic acid 4 cysteine 1 (i) Explain what is meant by the term primary structure of a protein, clearly stating the types of interactions involved. (ii) With the aid of a diagram, describe the secondary structure of the sub-units in haemoglobin, showing clearly how the structure is held in place. (iii) Explain why there is an absence of disulfide bonds within the -sub-unit. (iv) The affinity of haemoglobin for oxygen decreases when the pH of the solution is decreased. This is due to the presence of the histidine residue in the sub-units. Suggest a reason for this. 1372

9647/03/TJC Prelim 2013 [Turn Over 4 Haemoglobin binds to oxygen as shown in the equation below: Hb + 4O2 Hb(O2)4 (v) Using this equation, explain why haemoglobin binds to oxygen in the lungs and releases oxygen in the tissues in the body. [10] (b) Lysine is an amino acid found in haemoglobin. It is not synthesised by the human body and hence must be ingested as lysine or lysine-containing proteins. There are three pK a values associated with lysine: 2.15, 9.16 and 10.67. (i) Assign each of the pKa values to the specific functional groups in lysine, explaining the reasons for your choice. (ii) Given that the isoelectric point is the average of two relevant pK a values, calculate the isoelectric point for lysine and draw the structure of the major species present in a solution of lysine at that point. [5] (c) Carbon monoxide displaces oxygen from oxyhaemoglobin to produce carboxyhaemoglobin. The reaction of carbon monoxide with oxyhaemoglobin can be represented by the equation below. Hb(O 2)(aq) + CO(aq) Hb(CO)(aq) + O2(aq) The concentration of a gas A in an aqueous solution depends on the partial pressure of the gas in contact with the solution. This can be obtained from Henry’s Law given as: P A = kH x C Where P A = partial pressure of gas A kH = Henry’s Constant for gas A C = Concentration of gas A in water The Henry’s Constant for oxygen and carbon monoxide are given below Gas Henry’s Constant / dm3 atm mol-1 O2 769 CO 1052 To determine the equilibrium constant of the reaction between oxyhaemoglobin and carbon monoxide, a small amount of oxyhaem oglobin in water is exposed to air at 1 atmospheric pressure containing 3% carbon monoxide. After some time, the concentration of oxyhaemoglobin decreased to 24 1 of the original concentration. The concentration of carbon monoxide and oxygen in the air remains constant. 1373

9647/03/TJC Prelim 2013 [Turn Over 5 (i) Using Henry’s Law, calculate the concentrations of oxygen and carbon monoxide in the solution. (You may assume that air contains 20% oxygen) (ii) Write an expression for the equilibrium constant, K c, for the reaction between oxyhaemoglobin and carbon monoxide. Calculate the value of Kc. (iii) The toxic effects of carbon monoxide poisoning begin to show when 10% of the oxyhaemoglobin has been converted to carboxyhaemoglobin. Calculate the percentage of carbon monoxide in air at atmospheric pressure that will cause this to happen. [5] [Total: 20] 1374

9647/03/TJC Prelim 2013 [Turn Over 6 3 A fuel cell is an electrochemical cell that converts a fuel into an electrical current. The direct oxidation of alcohols in a fuel cell has been proven to be an efficient method of obtaining electrical energy. (a) The diagram below illustrates the functional parts of a direct-ethanol fuel cell (DEFC). (i) Write the ion-electron half-equations for the reactions which take place at the electrodes of the DEFC, and hence an overall equation for the cell reaction.