PJC H2 Chem 2013 Prelim P3 QP

© PJC 2013 9647/03/Prelim/13 [Turn over PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CHEMISTRY 9647/03 Paper 3 Free-Response 23 September 2013 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet Cover Page READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name on all 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 workings. 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 brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 13 printed pages. Name: Index No.: CT Group: 12 999

2 © PJC 2013 9647/03/Prelim/13 [Turn over Answer any four questions. 1 (a) With the aid of a diagram, describe how a polypeptide chain is held in the shape of an alpha helix. [3] (b) Haemoglobin is the oxygen-carrying protein in red blood cells. (i) With reference to the haemoglobin molecule, describe and explain what is meant by the term quaternary structure of proteins. In your answer, you should state the type of bonding or interaction involved. (ii) A typical polypeptide chain of haemoglobin consists of 141 amino acids. The R groups of 3 of the amino acids are given below. amino acid formula of side chain (R in RCH(NH 2)CO2H) glutamic acid CH2CH2CH2CO2H valine CH(CH3)2 lysine CH2CH2CH2 CH2NH2 Use all three amino acids above to construct the displayed formula of a possible section of the polypeptide. (iii) Uncoiling of the protei n structure is favoured w hen temperature is higher than physiological conditions. With the aid of the equation, ∆G = ∆H – T∆S suggest why this process is favoured at higher temperatures. [ 8 ] (c) One molecule of haemoglobin can bi nd up to four molecules of oxygen, according to the following equation. Hb(aq) + 4O 2(aq) Hb(O 2)4(aq) Kc = 3 x 1020 mol-4 dm12 (i) Write an expression for K c for this reaction. (ii) Calculate the percentage of Hb(O 2)4 in an Hb-Hb(O 2)4 mixture when [O2] = 7.6 x 106 mol dm3. [3] 1000

3 © PJC 2013 9647/03/Prelim/13 [Turn over (d) (i) Carbon monoxide, CO, mainly causes adverse effects in humans by combining with haemoglobin to form carboxyhaemoglobin in the blood. This prevents haemoglobin from rel easing oxygen in tissues, effectively reducing the oxygen-carrying capacity of the blood. The equations below show the reac tions of haemoglobin with oxygen and carbon monoxide respectively. Fe N N N N O N H2 Fe N N N N O2 N + O2 + H2O Equilibrium 1 haemogloblobi n oxyhaemoglobin Fe N N N N O N protein H2 Fe N N N N CO N protein + CO + H2O Equilibrium 2 haemoglobin carboxyhaemoglobin CO is found to have 245 times mo re affinity for haemoglobin than O 2. Explain how excessive inhalation of CO will give rise to poisoning in the blood. (ii) Suggest a method for the treatment of carbon monoxide poisoning. [3] (e) Oxygen can be liquefied by pressure al one if its temperat ure is below the 'critical temperature' of -118°C. Above this critical temper ature no amount of pressure will liquefy oxygen. Suggest a suitable value for the critical temperature of ammonia. Give your reasoning. [3] [Total: 20] 1001

4 © PJC 2013 9647/03/Prelim/13 [Turn over 2 (a) The diagram below shows the reactions of a salt, A, in aqueous medium. orange solution B brown solution C colourless solution colourless solution cream precipitate yellow precipitate colourless A(aq) AgNO3(aq)AgNO3(aq) A has the following composition by mass: K, 41.1%; S, 33.7%; O, 25.2%. The relative formula mass, M r, of A is 190.4. One formula unit of A contains only one type of anion. (i) Determine the formula of the salt A . (ii) Suggest the identities of B and C. (iii) By means of two ionic equations, s how how the yellow precipitate is formed from C. (iv) For the reaction of A and B, a titration was carried out to determine if the sulfur-containing product is S 4O6 2or SO4 2. It was found that 25.0 cm 3 of 0.200 mol dm -3 of B required 10.0 cm 3 of 0.125 mol dm-3 of A for complete reaction. Use the data to determine the final oxidation state of sulfur in the product. Hence, write a balanced equation for the reaction of A and B. (v) Construct a balanced equat ion for the reaction of B with hot aqueous potassium hydroxide. [8] 1002

5 © PJC 2013 9647/03/Prelim/13 [Turn over (b) Pseudohalogens are a family of inorganic compounds which possess chemical properties similar to the halogens. Two examples of pseudohalogens are cyanogen (CN)2 and thiocyanogen (SCN)2. The equations below repres ent the standard reduction potentials of cyanogen and thiocyanogen. (CN) 2 + 2H+ + 2e- 2HCN EӨ = +0.37 V (SCN)2 + 2e- 2SCN- EӨ = +0.77 V (i) Use the Data Booklet to predict what you woul d expect to observe when the following solutions are mixed. Write balanced equations for any reaction that occurs. I (CN) 2(aq) and Br2(aq) II KSCN(aq) and acidified KMnO 4(aq) (ii) A reaction occurs when an unknown metal, D, is added to (SCN) 2. The standard cell potential of the reaction is determined to be +0.43 V. Use the Data Booklet to deduce the identity of metal D, showing your working clearly. [5] (c) NanoPutians are a family of organic molecules whose structural formulae resemble human forms. These com pounds were designed and created by Dr James Tour from Rice Universi ty in 2003 as a part of a sequence of chemical education for young students. One such molecule, named the NanoKid, is shown below. O O 1003

6 © PJC 2013 9647/03/Prelim/13 [Turn over Inspired by their discovery of the NanoPutians and a motivation to create a digital mascot for the school, a team of year two chemistry students from Pioneer Junior College devis ed the following reaction sch eme to form “Pion-mon”. OO OHO H OO N H2 NH2 "Pion-mon " compound Estep I step II compound F step III step IV step V HCN, trace NaCN 15 oC compound G (i) Suggest reagents and conditions for steps I, II, IV and V. (ii) Draw the structural formulae of compounds E, F and G. [7] [Total: 20] 1004

7 © PJC 2013 9647/03/Prelim/13 [Turn over 3 (a) (i) Describe what you would see when separate samples of sodium and sulfur are burnt in air. (ii) The oxides of sodium and sulfur resulting from the reactions in (i) both react with water. Write equations fo r these two reactions and describe the effect of the resulting solutions on Universal Indicator solution. [4] (b) Sulfurous acid is an aqueous solu tion of sulfur dioxide. Sulfurous acid, H2SO3, is a weak dibasic acid with pK1 = 2.15 and pK2 = 7.21. (i) Calculate the pH of a 0.80 mol dm 3 solution of sulfurous acid (ignore the effect of pK2 on the pH). (ii) An amphiprotic species is one that r eacts with an acid and a base. The pH of a soluti