2019 ASRJC Prelim H2 Chem P3 QP

ASRJC JC2 PRELIMS 2019 9729/03/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2019 JC 2 PRELIMINARY EXAMINATIONS CHEMISTRY 9729/03 Higher 2 18 September 2019 Paper 3 Free Response 2 hours Candidates answer on separate Answer Booklet. Additional Materials: Answer Booklet 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. You may use a pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 15 printed pages and 1 blank page.

2 ASRJC JC2 PRELIMS 2019 9729/03/H2 Section A Answer all the questions from this section. 1 (a) Iron(III) cation complexes with a thiocyanate anion (SCN –) to form the iron( III) thiocyanate complex, [Fe(SCN)]2+ at 298 K, according to the following equation. Fe3+(aq) + SCN–(aq) ⇌ [Fe(SCN)]2+(aq) H < 0 yellow colourless blood red (i) Write an expression for Kc for this reaction, stating its units. [2] (ii) The Kc value of this reaction is 4.99 x 102. Calculate the percentage of [Fe(SCN)]2+ in a Fe3+–[Fe(SCN)]2+ mixture when [SCN–] = 5.00 x 10–3 mol dm–3. [2] (iii) State and explain the difference in observations when the reaction is carried out at 500 K. [2] (b) Describe the variations in melting points of the elements sodium to chlorine and explain these variations in terms of their structures and bonding. [5] (c) Compounds B, C, D and E are oxides or chlorides of Period 3 elements. B and C have high melting points while D and E have low melting points. B is insoluble in water but soluble in hot concentrated sodium hydroxide. When water was added separately to 0.100 mol each of C, D and E, a neutral solution was obtained for C while an acidic solution was obtained for D and E. The resulting solution of D required 0.500 mol of silver nitrate for complete precipitation. 1 mol of E requires 2 mol of aqueous sodium hydroxide for complete neutralisation. Identify compounds B to E, and write equations for the reactions described above. [6] [Total: 17]

3 ASRJC JC2 PRELIMS 2019 9729/03/H2 [Turn over Question 2 starts on the next page.

4 ASRJC JC2 PRELIMS 2019 9729/03/H2 2 (a) Helium is used to fill party balloons. A 60.0 dm3 industrial tank of helium gas at 27 °C and 125 atm was used to fill up some balloons. After some balloons were filled, the pressure in the tank was reduced to 50 atm. (i) How many moles of helium was used to fill the balloons? [2] (ii) Assuming each filled balloon is a sphere with a radius, r, of 0.15 m and has an internal pressure of 1.2 atm, what is the maximum number of balloons that can be filled with the remaining gas in the tank at the same temperature? (volume of sphere = 4r3 3 ) [2] (iii) State and explain if helium gas behaves more ideally in the tank, or when present in balloons. [2] (b) (i) Using the thermochemical data shown in Table 2.1, draw a fully labelled energy cycle to calculate the standard enthalpy change of combustion of liquid propan–2–ol. Table 2.1 Standard enthalpy change of formation of liquid propan–2–ol –318 kJ mol–1 Standard enthalpy change of combustion of carbon –394 kJ mol–1 Standard enthalpy change of formation of water –286 kJ mol–1 Standard enthalpy change of vapourisation of water +41 kJ mol–1 [2] (ii) By using your answer to (b)(i), and relevant data from the Data Booklet and Table 2.1, calculate the standard enthalpy change of vapourisation of propan–2–ol. [2] (iii) Hence, explain why combustion of liquid propan–2–ol takes place at all temperatures. [2] (c) (i) State and explain how the acidities of 2 –methylpropan–2–ol, 2–chloro–2–methylpropanoic acid and 2,2 –dimethylpropanoic acid might compare with each other. (CH3)3COH (CH3)3CCO2H 2–methylpropan–2–ol 2,2–dimethylpropanoic acid (CH3)2CClCO2H 2–chloro–2–methylpropanoic acid [3] (ii) Outline how 2,2 –dimethylpropanoic acid may be produced from 2–chloro–2–methylpropane, (CH 3)3CCl. Include the structure of the intermediate compound in your answer. [3]

5 ASRJC JC2 PRELIMS 2019 9729/03/H2 [Turn over (iii) Grignard reagents, with different alkyl or aryl groups (R ’–MgBr), react with carbonyl compounds to yield alcohols. R'' C O R (or H) R'-MgBr followed by H3O+ R' C OH R'' R (or H) + Mg(OH)Br Use the above information to suggest a synthesis of 2 –methylpropan–2–ol from 2–chloropropane according to the scheme in Fig. 2.1. Include reagents and conditions for all steps, and the structures of the intermediate organic compounds, G and H, in your answer. G H Step 1 Step 2 Step 3 (CH3)3COH(CH3)2CHCl 2–chloropropane 2–methylpropan–2–ol Fig. 2.1 [5] [Total: 23]

6 ASRJC JC2 PRELIMS 2019 9729/03/H2 3 (a) Nitrate, NO 3–, and phosphate, PO 43–, are oxoanions of nitrogen and phosphorous respectively. (i) Draw dot–and–cross diagrams to show the bonding in NO3– and PO43–. Hence, suggest the shape of each of these ions. [3] (ii) The 2s and 2p orbitals of nitrogen atoms can hybridise in the same way as the 2s and 2p orbitals of carbon atoms. Suggest the hybridisation state of nitrogen in NO3–. [1] (iii) Table 3.1 shows the bond lengths of two nitrogen–oxygen bonds. Table 3.1 Bond N–O N=O Bond length (pm) 136 115 The experimental bond length of each nitrogen–oxygen bond in t