NYJC H2 Chemistry P3 Prelim

H2 Chemistry 9647/03 NYJC J2/14 PX [Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 24 September 2014 2 hours Candidates answer Section A on the Question Paper Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Section B Answer all questions on separate answer paper. For Examiner’s Use Section A 1 2 3 4 5 Total This document consists of 12 printed pages and 0 blank page.

2 H2 Chemistry 9647/03 NYJC J2/14 PX Answer any four questions. 1 In 1939 the German chemist Gerhard Dom agk was awarded the Nobel Prize in Physiology or Medicine for the discovery of the antibacterial effects of Prontosil. It was found that sulfanilamide was the active component of Prontosil. (a) Benzene can be used as the starting reagent for the synthesis of sulfanilamide by the following route. (i) Suggest reagents and conditions for steps I and II, and the structure of compounds A and B. (ii) Step III involves the reaction of concentrated H 2SO4 together with SO 3 to generate the required electrophile. Write a balanced equation for the generation of the electrophile in Step III. (iii) State the types of reaction in steps II and V.   (iv) Explain why sulfanilamide is soluble in water. A derivative of sulfanilamide, compound C, is shown below. (v) The amino group of sulfanilamide has a p K b value of 12.0. Suggest a p Kb value for the amino group of compound C. Explain your answer. [10] C sulfanilamide (pKb = 12.0) sulfanilamide

3 H2 Chemistry 9647/03 NYJC J2/14 PX (b) Describe and explain the trend in thermal decomposition of Group II nitrates, writing an equation for any reaction that occurs. [3] (c) Ammonium nitrate is widely used as an ex plosive. It can easily decompose when ignited to give nitrogen gas, oxygen gas and steam. (i) Write a balanced equation, with stat e symbols, which represents the enthalpy change of decomposition of 1 mole of ammonium nitrate. (ii) Define the term enthalpy change of atomisation of ammonium nitrate. (iii) The standard enthalpy change of atomisati on of ammonium nitrate is +2928 kJ mol 1. Using suitable data from the Data Booklet , construct an energy cycle to calculate the ent halpy change of decomposition of ammonium nitrate. (iv) In a laboratory experiment, the decomposition of 1.0 g of ammonium nitrate is able to increase the temperature of 100 g of water by 4 C. Calculate the heat evolved from t he decomposition of one mole of ammonium nitrate from this experiment. Comment on the difference between the calculated values from (iii) and (iv). [7] [Total: 20]

4 H2 Chemistry 9647/03 NYJC J2/14 PX 2 Phosphorus is the most abundan t element of Group V, acco unting for 0.10% of the mass of the Earth’s crust. Phosphorus exists in two common allotropic forms: white phosphorus and red phosphorus. White phosphor us contains discrete tetrahedral P4 molecules. Red phosphorus, by contrast, has a polymeric structure. (a) Compounds A and B contain phosphorus in ox idation states of 3 and +5 respectively. The relative molecular masses of A and B are 34.0 and 208.5 respectively. The molecular shape of A is the same as that of ammo nia and it burns easily in air to form phosphoric acid only. B is an off-white solid that melts at 167 oC and gives white fumes and phosphoric acid on contact with moist air. Suggest the identities and draw the shapes of compounds A and B, writing equations to illustrate all the reactions mentioned. [4] (b) When phosphorus burns in oxyg en, it yields either P 4O6 or P4O10 depending on the amount of oxygen present. Both P 4O6 and P 4O10 are acidic oxides, and they react with water to form aqueous solutions of phosphorous acid and phosphoric acid, respectively. Phosphorous acid is diprotic , while phosphoric acid is triprotic and their successive dissociation constants are as follows: P O HO O H H P O OO O H H H Phosphorous acid, H3PO3 Phosphoric acid, H 3PO4 Ka1 = 1.0 x 102 mol dm3 Ka1 = 7.5 x 103 mol dm3 Ka2 = 2.6 x 107 mol dm3 Ka2 = 6.2 x 108 mol dm3 Ka3 = 4.8 x 1013 mol dm3 A 5.00 g sample of white phosphorus was burned in oxygen. The oxide produced was dissolved in enough water to make 250 cm3 of solution. When the solution was treated with an excess of aqueous Ca(NO 3)2, 19.4 g of a white precipitate was obtai ned. The white precipitat e could be either CaHPO 3 or Ca3(PO4)2 depending on the oxide t hat was formed when white phosphorus burned in oxygen. (i) Write equations for the reactions of phos phorus with oxygen to yield either P 4O6 or P4O10 and for the subsequent reactions of both oxides with water. (ii) Deduce the identity of th e oxide and hence state the identity of the white precipitate obtained. (iii) Calculate the pH of the 250 cm 3 solution. [11]

5 H2 Chemistry 9647/03 NYJC J2/14 PX (c) Phosphoric acid is sometimes called orth ophosphoric acid to distinguish it from diphosphoric acid, H4P2O7 that is obtained from the condensation of two phosphoric acid molecules. Draw the full struct ural formula of H 4P2O7. [ 1 ] (d) Nitrogen is another element of Group V. It is a col ourless, odourless and tasteless gas that makes up 78% of the Earth’s at mosphere by volume. The most important use of nitrogen is in the Haber process fo r the manufacture of ammonia, used in nitrogen fertilisers. Apart from the Haber process, another possible process for the synthesis of ammonia proposes that N 2 and H 2 molecules are catalytically dissociated into atoms in separate reaction vessels as shown by the equations below: N2(g) • 2N(g) log Kp =  43.10 H2(g)  2H(g) log Kp = 17.30 (i) Suggest and explain whether the positio n of equilibrium of both the above catalytic dissociation lies more to the right or to the left. (ii) Find the partial pressure of N at 1000 K and 200 atm. (iii) Calculate the number of N atoms present per dm3. (iv) Based on your answer in (iii), explain which of the following is a more likely step after the catalytic dissociation. Equation (1): N(g) + H(g)  NH(g) Equation (2): N 2(g) + H(g)  NH(g) + N(g) [4] [Total: 20]

6 H2 Chemistry 9647/03 NYJC J2/14 PX 3 Methanol is an important intermediate used to produce chemicals like methanal and ethanoic acid in industry and can also be used as a fuel. Wo rldwide, about 65 million tonnes of methanol are produced each year. (a) In 2012, German scientists came up with a new method for producing methanol using carbon dioxide and hydrogen as shown below. Carbon dioxide is a greenhouse gas and causes global warming. CO 2(g) + 3H2(g)  CH3OH(l) + H2O(l) (i) What is meant by the standard enthalpy c hange of formation of a compound? (ii) Calculate the enthalpy change for this reaction, by using the following data : ∆H f(CH3OH) = 238 kJ mol1 ∆Hc(C) = 394 kJ mol1 ∆Hc(H2) = 286 kJ mol1 (iii) State the significance of the sign of ∆Hf(CH3OH). [4] (b) Carbonylations are reactions that produce organic carbonyls, i.e. compounds that contain the C=O functional group such as aldehy