HCI 2016 C1 Promotional Exam Paper 3

This document consists of 8 printed pages. HWA CHONG INSTITUTION C1 Promotional Examination Higher 2 NAME CT GROUP 16S CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper, Graph Paper, Cover Page and Data Booklet. 9729/03 30 September 2016 1 hour 20 minutes 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 an HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a new piece of paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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, together with the cover page.

2 2016 HCI C1 H2 Chemistry Promotional Exam / Paper 3 Section A Answer all the questions in this section. 1 (a) When 2-methylbutane is reacted with chlorine gas in the presence of sunlight, several mono‑chlorinated products are formed. (i) Draw the structures of all possible mono-chlorinated products. [2] (ii) Explain which of the products you have drawn in (i) was formed via the most stable radical. [1] (iii) Hence, describe the mechanism for the formation of the product you identified in (ii). [3] (b) A student has forgotten to properly label bottles known to contain the following two organic compounds: Compound A Cyclopenta-1,3-diene B 2-methylbut-2-ene Describe a simple chemical test that would allow the student to distinguish between A and B. [2]

3 2016 HCI C1 H2 Chemistry Promotional Exam / Paper 3 (c) A 2.00 g impure sample of yttrium(III) oxide, Y2O3 (Mr = 225.8), was known to be contaminated with a little hydrated yttrium(III) carbonate, Y2(CO3)3•3H2O and other inert material. After the sample was left exposed to moist air for several days, it was reweighed and found to have increased in mass. The change in mass was due to the conversion of some of the oxide to form compound Q. The resulting sample was completely dissolved in 35.00 cm3 of 2.00 mol dm−3 hydrochloric acid, releasing 16.40 cm3 of gas (measured at standard temperature and pressure) in the process. After all the gas was released, the resulting solution required 19.65 cm3 of 1.00 mol dm−3 sodium hydroxide solution for complete neutralisation. The following information may be relevant:  Balanced equation for reaction of yttrium(III) oxide with hydrochloric acid: Y2O3 + 6HCl → 2YCl3 + 3H2O  Q is made up of yttrium, oxygen and hydrogen only.  When separately reacted with a mineral acid, Q and yttrium(III) oxide form the same products.  The conversion of the oxide to form Q does not result in any change of oxidation state. (i) Calculate the amount of acid that reacted with the sample. [1] (ii) Calculate the amount of acid that reacted with the yttrium(III) carbonate. [1] (iii) Q consists of 63.5% yttrium and 2.14% hydrogen by mass. Identify Q and write an equation for its formation from the oxide. [2] (iv) Hence, or otherwise, determine the percentage purity of the original sample of yttrium(III) oxide. [3] [Total: 15]

4 2016 HCI C1 H2 Chemistry Promotional Exam / Paper 3 2 (a) Potassium manganate(VII) and hydrogen peroxide react to give oxygen gas as follows: 2MnO4−(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 5O2(g) + 8H2O(l) A thermometric titration between potassium manganate( VII) and hydrogen peroxide was carried out. The temperature of the reaction mixture was followed as acidified potassium manganate(VII) was added in 4.00 cm3 aliquots to 25.0 cm3 of 1.00 mol dm−3 hydrogen peroxide. The results of the titration are shown in Table 2.1. Table 2.1 Volume of KMnO4 added / cm3 Highest Temperature obtained / °C 0.00 31.0 4.00 37.0 8.00 44.0 12.00 50.0 16.00 54.0 20.00 49.0 24.00 44.0 28.00 38.0 32.00 33.0 (i) Use the data given in Table 2.1 to plot a graph of temperature / °C against volume of KMnO4 / cm3. [2] (ii) From the graph, determine the maximum temperature obtained and the volume of KMnO4 at which the maximum temperature obtained occurs. Show your working on your graph. [2] (iii) Using your answer to (ii), determine the enthalpy change of reaction per mole of hydrogen peroxide. Give your answer to the nearest whole number in kJ mol−1. [3] (b) Describe and explain the trend of the first ionisation energies of the elements in Period 3 as fully as you can. [4] (c) Describe the bonding and structure in the substances given below and their effects on the melting points, making use of the data given in Table 2.2. Table 2.2 Substance Formula Melting point / °C Argon Ar −189 Hydrogen peroxide H2O2 −0.43 Diamond C 3350 [4] [Total: 15]

5 2016 HCI C1 H2 Chemistry Promotional Exam / Paper 3 Section B Answer one question from this section. 3 Phosgene, COCl2, carbon monoxide and chlorine are all toxic gases. Despite their toxicities, these compounds are important starting materials in the synthesis of pharmaceutical and other organic compounds. (a) (i) Draw a ‘dot-and-cross’ diagram to show the bonding in phosgene. [1] (ii) Sketch a diagram showing all the hybrid orbitals around the carbon atom in phosgene. [1] (b) Phosgene can dissociate to form carbon monoxide and chlorine. COCl2(g) ⇌ CO(g) + Cl2(g) 0.200 mol of phosgene was placed into a rigid reaction vessel of volume 12.5 dm3 and allowed to dissociate at a temperature of 770 K. The total amount of gas at equilibrium was found to be 0.319 mol. (i) Calculate the equilibrium concentrations of COCl2, CO and Cl2, and the percentage of COCl2 dissociated at 770 K. [4] (ii) Write the expression for the equilibrium constant, Kc, for the above reaction and calculate its value at 770 K, showing its units clearly. [3] (iii) The standard Gibbs free energy change of reaction, Gr  , for the above dissociation is +67.7 × 103 J mol−1. The relationship between the equilibrium constant and the standard Gibbs free energy change of this reaction can be expressed as: Gr  = −RT ln K where K in the above expression is the numerical value of Kc. Calculate the value of K for the dissociation of COCl2 at 298 K. [1] (iv) Using your answers to (ii) and (iii), deduce whether the forward or backward reaction will be favoured when the temperature of the equilibrium mixture is decreased from 770 K to 298 K. Hence, deduce the sign of the enthalpy change of reaction, H, for the dissociation of phosgene. [2]

6 2016 HCI C1 H2 Chemistry Promotional Exam / Paper 3 (c) Benzaldehyde, C6H5CHO, is an aromatic compound with a characteristic almond-like odour, and hence has found use as a flavouring agent. Benzaldehyde may be synthesised by the Gatterman–Koch reaction, a Friedel–Crafts acylation involving the reaction between carbon monoxide and benzene in the presence of hydrogen chloride and aluminium chloride as catalysts. The mechanism for the reaction may be described as follows:  Generation of the electrophile, HCO+.  Reaction between the electrophile and benzene to form a carbocation intermediate in the rate-determining step.  Loss of a proton from the carbocation intermediate to regenerate HCl and AlCl3. (i) The electrophile, HCO+, is generated from the reaction between carbon monoxide, hydrogen chloride and aluminium chloride: Using the theories of acids and bases, state the roles of CO and AlCl3 in the above reaction. [2] (ii) Describe the mechanism of the reaction b