RVHS H2 CHEM P3 QP RV Prelim

River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II RIVER VALLEY HIGH SCHOOL YEAR 6 PRELIMINARY EXAMINATION (II) CANDIDATE NAME CLASS 6 CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9647/03 Paper 3 Free Response 23 September 2014 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Graph Paper Cover Page Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class, centre number and index number on all the work you hand in. Write in dark blue or black pen on both sides of 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. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. 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, with the cover page on top. This document consists of 16 printed pages.

2 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II Answer any four questions. 1 (a) TRIS is an organic compound with the formula (HOCH 2)3CNH2. It is extensively used in biochemistry and molecular biology as a component of buffer solutions. TRIS The table below summarises key inform ation about TRIS and other amines. Compound m.p./C b.p./C pKb Odour CH3NH2 93 6 3.43 Fishy CH3CH2NH2 80 20 3.19 Fishy C6H5NH2 6 184 9.37 Fishy TRIS 176 219 5.93 Characteristic (i) What is meant by the term buffer? (ii) 25.0 cm 3 of 0.10 mol dm –3 TRIS is titrated against 0.20 mol dm –3 hydrochloric acid. Calculate the pH of the solution when the following volumes of hydrochloric acid are added. 1. 0 cm3 2. To reach equivalence point 3. 30 cm3 (iii) Apart from its relatively low cost, suggest why TRIS is suitable for use in biochemistry and molecular biology experiments. [6]

3 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II (b) C6H5NH2, also known as aniline, played a key role in the proliferation of synthetic dyes. Many of these compounds involve azo dyes, which contain two aromatic fragments connected by a N=N double bond. Azo dyes are prepared in a two -step reaction, the first being the synthesis of an aromatic diazonium ion from an aniline derivative (note: alkyl diazonium ions are very unstable). The next step is the coupling of the diazonium salt with an aromatic compound. The reaction scheme is shown below. (i) A student who was interested in studying the rate of reaction of the diazotisation step found the following mechanism on the internet. HONO ⇌ H+ + ONO

4 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II Based on the mechanism, state the rate equation for the reaction between C6H5NH2 and HONO. (ii) State the units for the rate constant for this reaction. [2] (c) Compound G has the molecular formula C 12H15NO3. G does not react with PCl5 to give white steamy fumes. On boiling G with aqueous sodium hydroxide, H and a salt of compound I are formed. Compound H has a fishy smell and does not have a stable reaction with HONO. Compound I, C 11H12O4, rotates plane -polarised light and produces a yellow solid on warming with alkaline aqueous iodine. I gives a yellow precipitate with 2,4 -dinitrophenylhydrazine. On addition of aqueous bromine, compound J is formed with the formula, C11H11O4Br. Suggest structures for compounds G to J and explain the observations. [9] (d) By means of equations, describe two reactions in which iron or one of its compounds behaves as a catalyst, and state whether the catalyst is homogenous or heterogenous. [3] [Total: 20]

5 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II 2 Nitrogen is an element that is essential to life on earth. Nitrogen is found in nucleic acids such as DNA and RNA which store genetic information . It is also found in proteins. In spite of nitrogen’s abundance in the atmosphere, the quantity of nitrogen containing compounds that were available for human use was limited. Prior to the 20th century, nitrogenous compounds were either obtained from biological sources such as manure and guano (seabird or bat droppings) or by mining nitrate deposits. Nitrate deposits were very rare and the main deposit that was exploited was in the Atacama Desert, Chile. The Haber process for the manufacture of ammonia and the Ostwald process for the conversion of ammonia to nitric acid were developed in the early 20 th century. The First World War began one hundred years ago in July 1914. Almost all explosives contain nitrogen and nitric acid is an essential reagent in their manufacture. The ability to convert atmospheric nitrogen to ammonia and hence other nitrogenous compounds fre ed Germany and her allies from depending on the Chilean nitrate deposits which had to be transported across the Atlantic by ship. Historians estimate that the First World War would have ended in 1916 instead of 1918 as Germany would have run out of sources of nitrogen. (a) Ammonium salts and nitrates are sources of nitrogen. Suggest two reasons why deposits of ammonium and nitrate compounds are rare and found only in deserts. [2] (b) Ammonia is described as a ‘real’ gas. State and explain the conditions under which ammonia deviates the most from ideal gas behaviour. Relate your answer to the bonding in ammonia. [3] (c) In an industrial accident, a large q uantity of ammonia was released . Ammonia is lethal if the concentration of the gas exceeds 5000 ppm. (1 ppm is equivalent to 1 mg dm3) Assuming that the pressure of ammonia was 1 atm and that ammonia behaves as an ideal gas, calculate the density of ammonia at 30 C. Hence, determine if the concentration of the ammonia was lethal. [3]

6 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II (d) The Van’t Hoff equation relates the equilibrium constant of a reaction with temperature. The linear form of the Van’t Hoff equation is given below. ln c  H T where R is 8.31 J mol1 K1 The following table gives equilibrium constant values measured at different temperatures for the Haber process. 2 1 N2(g) + 2 3 H2(g) ⇌ NH3(g) T / K Kc / mol1 dm3 ln Kc T 1 / K1 300 4.34  103 5.44 3.33  103 400 1.64  104 8.72 2.50  103 (i) Determine the enthalpy change of the reaction. (ii) Determine the entropy change of the reaction. (iii) Hence, determine the temperature at which the reaction becomes spontaneous. [6] (e) Proteins are an important class of biological molecules. (i) State three functions of proteins. (ii) Secondary structures are repeated arrangements of amino acids that can be found in proteins. One such structure is termed the -pleated sheet. Describe the -pleated sheet structure of protein. Your answer should include a suitable diagram which shows the bonding between the strands and at least two amino acid residues per strand.

7 River Valley High School 9647/03/PRELIM/14 2014 Preliminary Examination II (iii) Globular proteins are a class of water -soluble proteins. Many techniques have been developed to extract proteins from natural sources. One simple technique is termed ‘salting -out’. In salting out an inorganic salt such as ammonium sulfate is gradually added to a solution containing proteins. Differen t proteins will be selectively precipitated out as concentration of the salt in the solution increases. Using your knowledge of the tertiary structure of proteins, explain why