NJC Prelim P3 QP

1 NJC SH2 Prelim Exam 9729 / 03 / 18 [Turn Over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Data Booklet Answer Paper 9729/03 Monday 10 Sep 2018 2 hours READ THESE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use paper clips, highlighters, glue or correction fluid/tape. Section A Answers all questions. Section B Answers one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 11 printed pages.

2 NJC SH2 Prelim Exam 9729 / 03 / 18 [Turn Over Section A A n s w e r all the questions in this section. 1 (a) The melting points of some compounds are given Substance Octan-1-ol Iodine Fullerene Graphite Formula CH 3(CH2)7OH I2 C60 C Melting point/ K 277 286 873 >3000 (i) Explain why the melting points of octan-1-ol and iodine are comparable. [2] (ii) Explain why the melting point of graphite is higher than that of fullerene. [2] (b) In 1932, Harry Lister Riley and coworkers published their findings on the use of selenium dioxide, SeO 2, in the synthesis of aldehyde and ketone functional groups. One of such reactions is shown below. CH 3COCH3 + SeO2 CH3COCHO + Se + H2O reaction I Using the synthetic method above in one of your steps, devise a three–stage synthesis of compound B from compound A. O OH O OH Compound A Compound B [5] (c) The most common source of acidity in water is dissolved carbon dioxide. Carbon dioxide enters the water through equilibrium with the atmosphere. CO2(g) CO2(aq) KH = 3.3 x 10–4 mol dm–3 kPa–1 where K H is known as the Henry’s Law constant given by the equation: [CO2(aq)] = KH × ஼ைଶ Carbon dioxide can react with water to form carbonic acid as shown in the following reaction. CO 2(aq) + H2O H2CO3(aq) Kc = 1.3 × 10–3 (i) Calculate [CO2(aq)] at a pressure of 101.3 kPa, given that air contains 0.035% by volume of carbon dioxide. [2] (ii) Using your answer from (i), calculate [H2CO3]. [1]

3 NJC SH2 Prelim Exam 9729 / 03 / 18 [Turn Over Carbonic acid will further dissociate in the following reactions. H2CO3(aq) HCO3– (aq) + H+(aq) Ka1 = 2.5 × 10–4 HCO3–(aq) CO32– (aq) + H+(aq) Ka2 = 4.7 × 10–11 (iii) The pH of the carbonic acid is largely contributed by Ka1. Explain why. [1] (iv) Hence, calculate pH of carbonic acid. [1] (c) Many organic compounds that occur naturally have molecules that can show stereoisomerism, that is cis-trans or optical isomerism. Draw the structures of all the possible stereoisomers which have the following features. • They are acyclic. • They have molecular formula C 4H10N2. • No nitrogen atom is attached to any carbon atom which is involved in a double bond. • No carbon atom has more than one nitrogen atom joined to it. For each structure you draw, state the type of stereoisomerism it shows. [4] [Total:18]

4 NJC SH2 Prelim Exam 9729 / 03 / 18 [Turn Over 2 Electric or hybrid vehicles are expected to reach 27 million by 2027. Copper is used as a major component in the windings and copper rotors of electric vehicles. Crude copper was obtained when a particular copper ore was reduced. Crude copper contains cobalt and silver as minor impurities. It contained no other metal. In order to purify it, crude copper was made the anode of an electrolysis cell, with a pure copper cathode and aqueous CuSO 4 as electrolyte. (a) Explain, with reference to relevant E o values, what happens to the cobalt and silver impurities during this purification process. [3] An experiment was carried out to determine the percentage purity of the crude copper obtained from reduction of copper ore. A current of 2.15 A was passed through the cell described in (a) for 28.0 minutes, and the electrodes removed and weighed. It was found that the anode has lost 1.25 g. After filtering it off, the deposit underneath the anode weighed 0.07 g. On adding an excess of dimethylglyoxime to the el ectrolyte, the highly insoluble red complex with the formula Co(C4H7N2O2)2 was precipitated. Its mass was 0.55 g. (b) (i) Calculate the actual mass of copper removed from the crude copper. [2] (ii) Hence determine the percentage purity of the crude copper produced in (a), assuming that the crude copper is of uniform mixture. [1] (iii) Suggest how the procedure can be improved to increase the reliability of results. [1] (iv) Calculate the expected increase in mass of the cathode. [2] (c) Co(C4H7N2O2)2 has a solubility of 1.23 × 10−4 mol dm−3. Calculate its solubility product. [2]

5 NJC SH2 Prelim Exam 9729 / 03 / 18 [Turn Over Lithium is a scavenger for hydrogen, hence it is able to prevent the reaction between hydrogen and copper during pure copper casting. Reaction with hydrogen makes the copper brittle, causing it to fall apart under very light stress. Table 1 Enthalpy / kJ mol−1 Enthalpy change of formation of LiH(s) −90.5 Enthalpy change of formation of LiAlH4(l) −152.5 Enthalpy change of formation of Li3AlH6(s) −454 Enthalpy change of atomisation of Li(s) +159.5 Electron affinity of hydrogen atoms −73.0 (d) Heating lithium in a stream of hydrogen gas produces white, crystalline, ionic lithium hydride, LiH. (i) With the help of a suitable energy level diagram, calculate the lattice energy of LiH using relevant data from Table 1 and the Data Booklet. [3] (ii) By quoting relevant data from the Data Booklet, suggest and explain how the magnitude of the lattice energy of LiCl would compare to LiH. [2] (e) When lithium hydride is heated with anhydrous aluminium chloride, lithium aluminium hydride, LiAlH4 and lithium chloride are produced. 4 LiH + AlCl3 → LiAlH4 + 3 LiCl When 5 g each of AlCl3 and LiH are reacted in a bomb calorimeter, the temperature rise is 8.4 oC. Given that the heat capacity of the bomb calorimeter is 1.24 kJ K−1, determine the enthalpy change of this reaction per mole of LiAlH4. [3] (f) Just above its melting point, LiAlH4 decomposes according to the following equation. 3 LiAlH4 (l) → Li3AlH6(s) + 2Al(s) + 3H2(g) (i) Use relevant data in Table 1, calculate the standard enthalpy change of this reaction. [1] (ii) Given that ΔGo = −27.7 kJ mol −1, calculate ΔSo for this reaction at 298K, and comment on its sign with respect to the equation for this reaction. [2] (iii) Hence calculate the melti