NJC H2 Chem 2013 Prelim P3 QP

NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Tues 17 Sep 2013 2 hours READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. 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 behind the cover page. This paper consists of 11 printed pages and 0 blank page. 133

NJC/H2 Chem/03/2013 [Turn over 2 Answer any four questions. 1(a) Dinitrogen difluoride is a chemical compound with the formula N 2F2. It is a gas at room temperature. N 2F2 was first identified as the thermal decomposition product of fluorine azide, N3F, in 1952. (i) Draw a dot- and-cross diagram for N2F2. Predict the shape around the central atom using the Valence Shell Electron Pair Repulsion theory. (ii) N 2F2 is able to exist as a pair of isomers at low temperature. State the type of isomerism exhibited by N 2F2. Hence, draw the structure of the more stable isomer. [4] (b) Nitrogen reacts with fluorine to give the compound N 2F2 in the following equation: N2 (g) + F2 (g) N2F2 (g) A 1 : 2 mixture of N 2 and F 2 is allowed to reach dynamic equilibrium at 500 K at an equilibrium pressure of 1.12 × 105 Pa in a 500 cm 3 flask. The mass of the equilibrium mixture is found to be 0.578 g. (i) Explain what is meant by the term dynamic equilibrium and write the expression for the equilibrium constant, Kp , for this reaction, stating its units. (ii) Calculate the average M r of the gaseous mixture. Hence, calculate the mole fraction of N2F2 in the equilibrium mixture. (iii) Using your answer to (b)(ii), calculate the partial pressure of N2F2. (iv) Predict the sign of ΔS for the reaction, showing your reasoning. (v) The value of ΔG for the above reaction is negative. Use this information, together with your answer to (b)(iv), to predict the sign of ΔH for the reaction. Explain your reasoning. [10] 134

NJC/H2 Chem/03/2013 [Turn over 3 (c) When phosphorus( III) oxide is heated in air, another oxide A which contains 49% of phosphorus by mass is obtained. On hydrolysis, 1 mole of A forms a mixture containing 1 mole each of H 3PO3 and H3PO4. Deduce the identity of A. [2] (d) Industrially, 1,2dichloroethane, CH 2ClCH2Cl is used to make chloroethene, CH2=CHCl, the monomer which the