2019 HCI Prelim H2 Chem P3 QP

This document consists of 11 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 18S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate answer booklet. Additional Materials: 12-Page Answer Booklet Data Booklet 9729/03 20 September 2019 2 hours READ THESE INSTRUCTIONS FIRST Write your name, CT group, centre number and index number on all the work you hand in. Write in dark blue or black pen. You may use a HB 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. Begin each question on a new page of writing paper. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the paper, submit only the 12-Page Answer Booklet and any additional booklets you have used.

2 © Hwa Chong Institution 2019 9729 / 03 / C2 Prelim 2019 Section A Answer all the questions in this section. 1 (a) (i) Write an equation for the thermal decomposition of MgCO3. [1] (ii) Explain how the magnitude of the lattice energy of MgCO3 differs from that of the solid product formed in (a)(i). [2] (iii) The thermal decomposition of MgCO3 is an endothermic process. Explain whether the decomposition of MgCO3 is spontaneous at high or low temperature. [2] (iv) State whether MgCO 3 or BaCO 3 has the lower decomposition temperature. Explain your answer. [3] (b) The Strecker synthesis is a method used to prepare amino acids. Alanine, 2-aminopropanoic acid, can be prepared from ethanal via Strecker synthesis as shown below. (i) Suggest the type of reaction occurring in steps 1 and 2. [2] (ii) Draw the fully displayed structure of compound E. [1] (iii) Suggest why step 3 produces an equimolar mixture of two stereoisomers. [2] (iv) Another amino acid, isoleucine, can be prepared from aldehyde F via Strecker synthesis. isoleucine Draw the structure of aldehyde F. [1]

3 © Hwa Chong Institution 2019 9729 / 03 / C2 Prelim 2019 (c) In the Petasis reaction, carbonyl compounds, amines and suitable boronic acids can react to form substituted amines. The general structure of a boronic acid is B(OH)2R where R is an alkyl or aryl group. An example of the Petasis reaction is given below. Consider the following Petasis reaction: (i) Draw the structure of compound G. [1] (ii) The Petasis reaction can be used in step 4 of the following synthesis of compound K from butanal. Suggest reagents and conditions for steps 1 to 4, and the structure of compound J. [5] [Total: 20]

4 © Hwa Chong Institution 2019 9729 / 03 / C2 Prelim 2019 2 When an onion is cut, the damaged tissues release the lachrymatory factor molecule, LF, that makes eyes water. The diagram below shows how LF is produced in the cut onion, starting from compound P. (a) In the first step, P reacts with a water molecule, in the presence of the enzyme alliinase, to give Q, R and ammonia. (i) Find the molecular formula of compound R. [1] (ii) Compound R reacts in a 1:1 ratio with PCl5 and gives a yellow precipitate with alkaline aqueous iodine. Draw the structure of compound R. [1] (b) State the isomeric relationship between Q and LF. [1] (c) LF is very soluble in water. Explain why LF dissolves well in water. [1] (d) LF is also volatile. For years, the conversion of Q to LF was thought to be direct, but in 2002, scientists discovered the pathway requires an enzyme LF synthase. Two methods, found on the internet, claim to prevent ey es from watering while cutting onions. Suggest how each of these methods could work, considering the volatility of LF, and the pathway that produces LF. (i) “Cool the onion before cutting it.” [1] (ii) “Heat the onion before cutting it.” [1] (e) In 2016, scientists developed “tearless” onions by irradiating seeds with heavy -ion beams to reduce LF synthase levels. This mutation causes increased thiosulfinate production from Q via the alternate pathway above. (i) In this alternate pathway, two molecules of Q react to form thiosulfinate, losing a water molecule. Name the type of reaction that occurs. [1]

5 © Hwa Chong Institution 2019 9729 / 03 / C2 Prelim 2019 Increased thiosulfinate means higher amounts of compounds which give onions flavor and antibacterial properties. One of these compounds is cepaene. The percentage of sulfur in an organic compound, such as cepaene, can be determined via the elemental analysis set-up in Fig. 2.1: Fig. 2.1 The organic sample is first combusted in a gas mixture of 40% O 2 and 60% He, to form CO2, H2O and oxides of sulfur: SO2 and SO3. The resultant gases then flow over a vessel containing heated copper metal to convert any SO3 into SO2. The heated copper also removes any unreacted O2. CO2, H2O and SO 2 gases are cooled in a separating tube immersed in liquid nitrogen at −196 oC, and then released one by one, by increasing the temperature, to the detector. In one analysis, 5.0 mg of cepaene (C9H16OS3, Mr = 236.3) is introduced into the combustion zone. (ii) Write an equation for the combustion of cepaene in the combustion zone to form CO2, H2O and SO3 only. [1] (iii) Hence, calculate the minimum volume of the O 2/He gas mixture at r.t.p. (in cm 3) required for the combustion of 5.0 mg of cepaene. [2] (iv) State the role of the heated copper metal, and hence write an equation to show how the unreacted O2 is removed by the heated copper. [2] (v) If the temperature of the heated copper metal is not high enough or insufficient copper is used, a sulfur-containing compound could be formed in the vessel as blue crystals. Identify the blue crystals and suggest how they may be formed. [1] (vi) Predict the sequence in which the three compounds in the separating tube are released to the detector. Explain your answer fully. [3] [Total: 16]

6 © Hwa Chong Institution 2019 9729 / 03 / C2 Prelim 2019 3 Cobalt is a transition element that can be found in the blue pigments used for jewel lery and paints. (a) Explain what is meant by the term transition element. [1] (b) A cobalt-chrome alloy is a metal alloy of cobalt and chromium. A sample of this alloy was dissolved in dilute HC l to form a solution containing 1.23 × 10–2 mol dm–3 of Cr3+(aq) and 5.77 × 10–3 mol dm–3 of Co2+(aq). Aqueous NaOH was then added slowly to 1 cm3 of the solution. (i) Using the Ksp information below, show that the addition of NaOH will not allow for the separation of the Cr3+ and Co2+ metal cation ions in the above solution. compound Ksp Cr(OH)3 1.60 × 10–20 Co(OH)2 5.92 × 10–15 [3] (ii) A better way to separate the two aqueous ions is to ad d excess NaOH into the solution as Cr(OH)3 dissolves. With the aid of equations, explain why the precipitate of Cr(OH)3 dissolves in excess NaOH using Le Chatelier’s Principle. [3] (c) When cobalt(II) chloride is dissolved in water, it forms a pink solution. Upon addition of excess aqueous potassium thiocyanate, KSCN, the solution turns blue. (i) Draw a fully labelled diagram to account for the splitting of the degenerate d orbitals into two energy levels in an octahedral ligand field. Explain how you arrive