JJC H2 CHEM P3 Qn

[Turn over JURONG JUNIOR COLLEGE 2015 JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS 15S CHEMISTRY 9647/03 Paper 3 Free Response 15 September 2015 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number 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. The use of an approved scientific calculator is expected, where appropriate. 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 10 printed pages.

2 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 Answer any four questions. 1 (a) Sodium amide (NaNH 2), commonly known as ‘sodamide’, can be prepared by reacting metallic sodium with ammonia gas. reaction 1 2Na(s) + 2NH3(g)  2NaNH2(s) + H2(g) (i) Use suitable bond energies data from the Data Booklet to calculate the enthalpy change of formation of NH3(g). [2] (ii) The enthalpy change of formation of ‘sodamide’ is 119 kJ mol1. Use the above information and your answer in (a)(i) to calculate the enthalpy change for reaction 1. [2] (iii) Use oxidation states to explain why reaction 1 is a redox reaction. [2] When dissolved in liquid amm onia, ‘sodamide’ produces NH2  ion which acts as a strong base in organic reactions. (iv) Draw a ‘dot-and-cross’ diagram for a NH 2  ion. In your diagram, use a third symbol ‘’ for any additional electrons responsible for the overall negative charge. [1] (v) Write an equation to show how NH 2  ion acts as a Brønsted-Lowry base when it reacts with methanol, CH3OH. [1] (b) ‘Tris’, (CH2OH)3CNH2, and tert-butylamine, (CH 3)3CNH2, are weak monoprotic organic bases. (i) Predict and explain whether ‘Tris’ has a smaller or larger Kb value as compared to tert-butylamine. [2] ‘Tris’ (Mr = 121) is widely used as a component of buffers. A buffer solution A of pH 7.40 was prepared by mixing solid ‘Tris’ with 100 cm 3 of 0.500 mol dm3 hydrochloric acid and making it up to 1.00 dm 3 with deionised water at 25 °C. The Kb of ‘Tris’ is 1.20  106 mol dm3 at 25 °C. (ii) Write an equation for the reaction between ‘Tris’ and hydrochloric acid. [1] (iii) Calculate the [base] [salt] ratio in buffer A and hence the mass of ‘Tris’ to be used. [3]

3 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 [Turn over (c) Isocyanates, R NCO, are useful organic intermediates in the manufacture of polymers and pesticides. One method to prepare isocyanates is by reacting compound B with bromine and alkali. The reaction is thought to proceed through the following steps. (i) State the functional group in compound B. [1] (ii) What types of reaction are occurring in steps 1 and 2? [2] (iii) In step 3, anion C breaks down into an isocyanate and a Br ion. Draw ‘curly arrows’ on anion C to show the movement of electrons. [2] Carbaryl is an insecticide widely used in the United States to control insects in a variety of crops such as cotton and nuts. The reaction of isocyanate D with 1-naphthol to form carbaryl is shown below. (iv) Suggest the structure of isocyanate D. [1] [Total: 20]

4 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 2 Transition elements possess characteristic properties which are generally not found in the main group elements in the Periodic Table. (a) Explain what is meant by the term transition element. [1] (b) Iron and its compounds are often used to catalyse reactions. One example is the use of iron(III) bromide in the halogenation reaction of benzene. (i) With the aid of a Boltzmann distribution diagram, explain how FeBr 3 increases the rate of halogenation of benzene. [3] (ii) Name the mechanism and outline how FeBr 3 catalyses the halogenation of benzene, showing curly arrows, charges, dipoles and any relevant lone pairs. [3] However, phenylamine can be brominated without the use of FeBr3 as shown below. (iii) Explain why FeBr 3 is needed for the halogenation of benzene but not for phenylamine. [2]

5 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 [Turn over (c) Nickel ions can form many coloured complexes with ligands such as H2O and NH3. As shown in the table below, complex ions containing nickel of the same oxidation state of +2 but of different ligands will exhibit different colours. reaction Equilibrium G / kJ mol1 1 [Ni(H2O)6] 2+ + 6NH3 green  [Ni(NH3)6] 2+ + 6H2O blue  49.2 2 [Ni(H2O)6] 2+ + 3en  [Ni(en)3] 2+ + 6H2O violet 104.4 3 [Ni(H2O)6] 2+ + 4CN  [Ni(CN) 4] 2 + 6H2O orange where en represent ethane-1,2-diamine (NH2CH2CH2NH2). (i) Explain why nickel(II) complexes are coloured. [3] (ii) Use G of reactions 1 and 2 given in the table to deduce and explain the relative ligand strength of H2O, NH3 and en. [3] (iii) When a solution of NaCN was added to Ni 2+ in en solution, it was observed that the solution changes from violet to orange. Explain the observation. [1] (iv) It was found that the enthalpy change of reaction, H, for both reactions 1 and 2 are similar. By considering the bonds broken and formed in reactions 1 and 2, suggest a reason why H for both reactions are similar. [1] (v) Explain why S of reaction 2 is more positive than that of reaction 1. [1] (vi) Chelate effect is the phenomenon when a central metal ion coordinates with polydentate ligands, forming a complex ion that has a greater stability than one which is formed with comparable monodentate ligands. Using the information given in (c)(iv) and (c)(v), explain whether the chelate effect is entropy or enthalpy driven. [2] [Total: 20]

6 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 3 (a) Borane, BH 3, is an inorganic reagent commonly used in organic reactions such as reduction and hydration of alkenes. When acting as a reducing agent, BH 3 is highly chemoselective as it will only reduce carboxylic acid in the presence of other reducible functional groups such as ketone. R O OH BH3 in (CH3)2S R OH R O R' BH3 in (CH3)2S no reaction (CH3)2S is a solvent and can be regarded as inert. (i) Write an equation for the reduction of ethanoic acid. Use [H] to represent the formula of the reducing agent. [1] (ii) Draw a fully displayed formula of a molecule of ethanoic acid. Show clearly any lone pairs present. [2] (iii) Describe and explain the shape of the BH3 molecule. [2] (iv) In the first step during the reduction of ethanoic acid, BH 3 and ethanoic acid react in a 1:1 ratio to give an addition product. Draw a diagram to show the bonding within a molecule of this addition product. Name and explain the type of bonding involved. [3] (v) Given that the first step during the reduction of carboxylic acids by BH 3 is rate-determining, explain why the rate of reduction of compound E by BH 3 is faster than ethanoic acid. O OH compound E [1] (vi) Predict the outcome of the following transformations where the first stage involves the reduction of carboxylic acid using BH 3, drawing the structures of intermediate F and product G. G is a cyclic compound. [2]

7 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 [Turn over (b) BH3 is also widely used in the hydration of alkenes