CHEMISTRY a UNIVERSITY ENTRANCE EXAMINATION. Duration : 2 hours. Please read the following instructions carefully.

Transcription

1 UNIVERSITY ENTRANCE EXAMINATION CHEMISTRY a Duration : 2 hours Please read the following instructions carefully 1 This paper has THREE (3) sections A, B and C, and comprises FIVE (5) printed pages 2 Attempt all sections Do not write on the question paper 3 Answer all questions in Section A, B and C Indicate your answers on the answer sheets provided Write the question number clearly 4 At the end of the examination, complete the information required on the cover page Place the cover page on top of your answer sheets and tie them together using the string provided 5 Do not take any paper, including the question paper or unused answer sheets, out of the examination hall 1

2 SECTION A (35 Marks) 1 Compound A has many isomers For each of the following categories of isomers, draw one isomer of A: A: Skeletal isomer B Position isomer C Functional group isomer D Stereoisomer E (10 marks) 2 Draw skeletal structures for compounds F~I 3 Propose a synthesis of the following compound, starting from benzene Indicate the reaction conditions and products of each step clearly DO NOT draw the mechanism (13 marks) 2

3 SECTION B (33 Marks) 1 (i) Balance the redox reaction below H2O2 + MnO4 - + H + Mn 2+ + O2 + H2O Which species in the redox reaction given in (i) acts as oxidizing agent? Explain (iii) Give the electron configuration of Mn 2+ (1 mark) 2 (i) Suggest a dot-and-cross diagram to show the bonding in H2O2 Use your diagram in (i) to explain if H2O2 is planar 3 Chlorine acts as a non-metal oxidizing agent in the reaction with ammonia to give compound A and gaseous compound B Ammonia is one of the products formed when compound A reacts with compound C which is also called quicklime Compound B contains a triple bond only and it is one of the precursors in the manufacture of ammonia Compound D is formed when compound C is mixed with water Compound E and water are produced upon the reaction between compound D and gaseous compound F Once compound E is heated, both compound C and gaseous compound F are given only (i) Give the chemical formulas of the compounds A - F Write the balanced equations for the two reactions, the formation of ammonia from compounds A and C, and the formation of compound E from compounds D and F (8 marks) 3

4 SECTION C (32 Marks) 1 The following table gives the values of the ionic product of water, Kw, at different temperatures Temperature / o C K w / mol 2 dm x x x (i) Calculate the ph of water at 40 o C (2 marks) Is water neutral at 40 o C? Explain your answer (iii) Explain whether the dissociation of water is endothermic or exothermic (iv) An acid, HA (K a = 10-4 at 40 o C), is dissolved in water to give an initial concentration of mol dm -3 Determine the ph of the solution of HA at 40 o C (2 marks) 2 H2O2 reacts with I - to give I2 and water H2O2 (aq) + 2I - (aq) + 2H + (aq) --> I2 (aq) + 2H2O (l) A series of experiments was carried out at 298 K and the following results were obtained In each case, [H + ] was kept constant by using a buffer solution Expt number ph of buffer [H 2O 2]/mol dm -3 [I - ]/mol dm -3 Initial rate / mol dm -3 s x x x x 10-4 Find the order of reaction with respect to [H + ], [H2O2] and [I - ] respectively Write a rate equation for the reaction over the ph range of 4-5 and evaluate the rate constant 4

5 3 NO is formed when nitrosyl chloride, NOCl, dissociates according to the following equation 2NOCl (g) <---> 2NO (g) + Cl2 (g) Different amounts of the three gases were placed in a closed container and allowed to come to equilibrium at 230 C The equilibrium concentrations of the three gases at each temperature are given in the table below Equilibrium concentration / mol dm -3 NOCl NO Cl x x x 10-2 (i) Write the expression for the equilibrium constant, K c, for this reaction, giving its units Calculate the value of K c at 230 o C Given that the forward reaction is endothermic, what will be the effect on the equilibrium concentration of NOCl when the following changes are carried out? In each case, explain your answer (a) The pressure of the system is halved at constant temperature (b) The container is cooled to 100 o C (c) Neon (inert gas) is passed into the container under constant volume (10 marks) END OF PAPER 5