NYJC Prelim CHEM P3 ans

1 [Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 21 September 2016 2 hours Candidates answer Section A on the Question Paper Additional Materials: Writing Paper 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 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. You are reminded of the need for good English and clear presentation in your answ ers. 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. Section B Answer all questions on separate answer paper. For Examiner’s Use Section A 1 2 3 4 5 Total This document consists of 13 printed pages and 1 blank page.

2 H2 Chemistry 9647/03 NYJC J2/16 PX [Turn Over Answer any four questions. 1(a) Copper(I) sulfate, Cu 2SO4, can be made from copper(I) oxide under non -aqueous conditions. On adding this salt to water, it immediately undergoes a disproportionation reaction. (i) Suggest, with a reason, the colour of copper(I) sulfate. [2] (ii) Using suitable data from the Data Booklet , explain why the disproportionation reaction occurs, and write an equation for it. [3] (b) Palladium(II) salts can form square planar complexes. Successive addition of ammonia and hydrogen chloride to an aqueous palladium(II) salt produces, under different conditions, three compounds with empirical formula PdN 2H6Cl2. Two of these, A and B, are non-ionic, with Mr = 211. A has a dipole moment, whereas B has none. The third compound, C, is ionic, having Mr = 422, and contains palladium in both its cation and anion. For each A, B and C, deduce a structure that fits the above data , explaining your reasons fully. [6] (c) Benzene ring is often represented as a structure that has a ring within the hexagon. Alternatively, chemists have also represented the structure of benzene in the following forms, known as resonance structures. Benzene (two resonance forms) The resonance relationship is indicated by the double headed arrow between them. The only difference between resonance forms is the placement of the pi electrons and non-bonding electrons. Most aromatic compounds undergo electrophilic substitution. However aryl halides undergo a limited number of substitution reactions with strong nucleophiles. An example of a reaction is as follows: XA NuA Nu + X where A is an electron withdrawing group and X is a halogen The mechanism of this reaction has two steps:  addition of the nucleophile  elimination of the halogen leaving group

3 H2 Chemistry 9647/03 NYJC J2/16 PX [Turn Over Step 1 involves the addition of the nucleophile (Nu −). The Nu − attacks the carbon atom bonded to a halogen, causing the pi bond to break. A resonance stabilised carbanion with a new C−Nu bond is formed. The aromatic ring is destroyed in this step. X A Nu X A Nu  Step 2 involves the loss of the halogen X, reforming the aromatic ring. X A Nu  A Nu X Two other resonance structures of the intermediate in Step 1 are shown below: X A Nu  X A Nu  I II Z (i) Copy the above diagram and draw the resonance structure, Z. In your answer, show any relevant charges, lone pair s of electrons and movement of electrons in forming Z. [2] The reaction below shows the synthesis of compound, D. O2N Cl + OH N(CH3)2 O2N O N(CH3)2 D + HClpotassium metal (ii) Suggest the role of potassium metal in the reaction. [1] (iii) Use the information given above to draw out the full mechanism for the reaction that forms D, labelling the slow and fast steps. In your answer, show ing any relevant charges, lone pair of electrons and movement of electrons. [3] (d) Describe and explain the relative ease of hydrolysis of the following three chloro-compounds. [3] Cl COCl CH2Cl [Total: 20]

4 H2 Chemistry 9647/03 NYJC J2/16 PX [Turn Over 2 Sulfuric acid, H 2SO4, can behave as an acid, an oxidising agent or as a dehydrating agent in various reactions. (a) Draw a diagram to illustrate the shape of pure sulfuric acid and indicate the bond angle about the sulfur atom. [2] (b) The Contact Process is used for the manufacture of sulfuric acid. One of the reactions that takes place is the following reversible reaction: 2SO2(g) + O2(g) Ý 2SO3(g) H = –197 kJ mol–1 Sulfur dioxide and oxygen in a 2:1 molar ratio at a total initial pressure of 3 atm is passed over a catalyst in a fixed volume vessel at 400 oC. When equilibrium is established, the percentage of sulfur trioxide in the mixture of gases is found to be 30%. (i) Write an expression for the equilibrium constant, Kp, of the reaction. [1] (ii) Calculate the value of Kp at 400 oC, stating its units. [3] (iii) How would the percentage conversion of SO 2 into SO 3 be affected when the pressure is raised? Explain. [2] (c) Dilute sulfuric acid takes part in typical acid-base reactions and it can be used to distinguish the following solids: MgO, BaO and SiO2 . State the observations, if any, to indicate the differences in their reaction when water is added to each solid followed by dilute sulfuric acid. [4] (d) Sulfur dioxide is a major pollutant from sulfuric acid plants. The SO 2 emitted into the atmosphere is oxidised in the air, which then reacts with water to form sulfuric acid, hence causing acid rain: 2SO2(g) + O2(g) + 2H2O(l)  2H2SO4(l) H1 Using the data below and data from (b), construct an energy cycle to calculate (i) the enthalpy change of formation of SO2(g), and hence (ii) the enthalpy change of reaction, H1 for the above reaction. Enthalpy change of formation of H2O(l) = –286 kJ mol–1 Enthalpy change of formation of H2SO4(l) = –811 kJ mol–1 Enthalpy change of formation of SO3(g) = –493 kJ mol–1 [4]

5 H2 Chemistry 9647/03 NYJC J2/16 PX [Turn Over (e) Alcohols react with concentrated sulfuric acid at high temperatures to form alkenes. A common side reaction that can happen is the formation of ethers, which is also catalysed by concentrated sulfuric acid. C OH R R R2 C O R R R C R R Rconc H2SO4 heat + H2O The mechanism occurs via 3 steps: Step 1: An acid base reaction in which H+ from H2SO4 protonates the oxygen atom in alcohol. This step is very fast and reversible. Step 2: A second alcohol molecule functions as the nucleophile and attacks the product from step 1. The C–O bond is cleaved and a water molecule leaves the molecule. This creates an oxonium ion O + HR' R intermediate. Step 3: Another acid base reaction in which the proton in the oxonium ion is removed by a suitable base (in this case a water molecule) to give the ether product. This step is very fast and reversible. (i) Draw the ether formed when cyclopentanol undergoes the above reaction. OH cyclopentanol [1] (ii) Draw out the full mechanism for the reaction between two cyclopentanol molecules to form an ether. In your answer, show any relevant charges, lone pairs of electrons and movement of electrons. [3] [Total: 20]

6 H2 Chemistry 9647/03 NYJC J2/16 PX [Turn Over 3 Garlic contains many amino acids, minerals and enzymes. Garlic