2019 SAJC H2 Chem Prelim P3 QP

1 NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination H2 Chemistry (9729) Paper 3 Free Response 18 September 2019 2 hours Additional Materials: Data Booklet, Answer Booklet READ THESE INSTRUCTIONS: Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Marks [60] Section B Answer one question. Marks [20] 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 14 printed pages.

2 Section A Answer all the questions in this section. 1 (a) Use of the Data Booklet is relevant to this question. The Hoffmann voltameter is used to electrolyse concentrated iron ( III) chloride solution. The setup of the Hoffmann voltameter is shown below. (i) Describe the observations at the cathode and anode. Explain your answer with the aid of suitable ion-electron equations. [2] (ii) State the difference in the observations at the anode when dilute aqueous iron ( III) chloride solution is us ed instead. Explain your answer using relevant E0 data. [2] (b) The electrolysis of a hot aqueous solution of Na X yielded a reddish brown gas, X2, at the anode and hydrogen gas at the cathode. The red litmus paper turned blue when dipped into the solution at the cathode. (i) Suggest the identity of the reddish brown gas X2. [1] (ii) Write an ion-electron equation for the reaction at the cathode. [1] (iii) Some of the X2 dissolved in the alkaline medium to form X– and XO3– ions. Write a balanced equation to represent the reaction of X2 in alkaline medium. [1] Pt electrode Pt electrode + – electrolyte

3 (iv) Describe a chemical test, which does not involve silver nitrate, that could distinguish between sodium chloride and sodium iodide. [2] (c) Describe and explain how the thermal stability of the hydrogen halides varies down Group 17. [2] (d) When methylcyclopentane is treated with a small quantity of chlorine in the presence of ultraviolet light, 4 mono -chlorinated constitutional isomers are produced. Draw the structures of the isomers and state the ratio in which they are formed. [3] (e) SiCl4 reacts readily with water to form a solution of pH 2. However, SiO2 does not react with water and only reacts with hot concentrated alkali such as NaOH. (i) Explain why SiCl4 reacts readily with water, but SiO2 only reacts with hot concentrated NaOH. Use relevant equations to explain your answer. [3] (ii) Using Cl2(g) as an example, define the term bond energy. [1] (iii) Construct an energy level diagram to calculate the enthalpy change of formation of SiCl4. Your diagram should include relevant data from the Data Booklet together with the following data: Enthalpy change of atomisation of Si = +338 kJmol-1 Enthalpy change of vapourisation of SiCl4 = +29 kJmol-1 [3] [Total: 21 marks]

4 2 (a) Phosphorus-containing compounds can react in various ways: as an acid, as a base, as an electrophile, as a nucleophile, as an oxidising agent and as a reducing agent. Study the following reactions and decide in which way the phosphorus-containing compound is reacting in each case. Explain your answers fully. (i) H3PO4 + H2O → H2PO4− + H3O+ [1] (ii) [1] (iii) [1] (b) 2-chloropropanoyl chloride, W, can be obtained from propanoic acid by heating it with Cl2 and PCl5. Reaction 1 : CH3CH2CO2H(l) + Cl2(g) + PCl5(s) → CH3CHClCOCl(l) + 2HCl(g) + POCl3(l) W H > 0 (i) Explain the different reactivities of the two chlorine atoms in W towards water. [2] (ii) Explain fully why the pH of an aqueous solution of 2 -chloropropanoyl chloride is 1.0 while that of an aqueous solution of propanoic acid of the same concentration is greater than 1.0. [2]

5 (iii) W can be used to produce sweet -smelling Z, C 6H8O4, via compounds X and Y, as shown in Fig. 2.1 Fig. 2.1 Suggest structures for compounds X, Y and Z in Fig. 2.1. [3] (iv) W can undergo the following reaction scheme. State the reagents and conditions for Steps 1 and 2. [2] (v) V is an isomer of W. V rotates the plane of polarised light. It gives a positive test with 2,4 - dinitrophenylhydrazine. Draw the displayed formula of V, identifying the chiral carbon. [1] (vi) Use the table of characteristic values for infra-red absorption frequencies in the Data Booklet to answer this question. Infra-red adsorption frequencies can be used to identify functional groups in organic compounds. For example, propanol shows absorptions at 970– 1250cm–1 and 3580–3620 cm–1. Use the table to suggest how infra-red absorption frequencies can be used to distinguish 2-chloropropanoyl chloride from propanoic acid. [1] (vii) Given that reaction 1 is endothermic, explain why the reaction will only take place with heating. [2] [Total : 16 marks]

6 3 There are 2 isomers of 2-methylbutanoic acid, CH3CH2CH(CH3)COOH (Mr 102.0) ; one of the isomers is sweet-smelling and the other has a stinky odour. (a) (i) State the stereoisomerism exhibited by 2–methylbutanoic acid and how it arises. [2] (ii) Hence, explain why the isomers have different smells. [1] (b) Suggest a 2 stage synthesis of 2–methylbutanoic acid from 2–bromobutane. State reagents and conditions needed for each step, and show clearly the structure of any intermediate compounds. [3] (c) Safety regulations set by many countries state that a factory would be fined, if stinky smelling compounds reached unsafe levels. The factory would have to stop production and undergo cleansing. When 2–methylbutanoic acid was synthesised using 2–bromobutane in a factory, both the sweet -smelling and the stinky isomers were obtained in equal proportions. (i) The Mr of 2–methylbutanoic acid in the liquid phase is 204.0. With the aid of a diagram, explain the above. [2]

7 (ii) 2–methylbutanoic acid has a relatively low boiling point of 176 oC at 1 atm, and hence vaporises easily. This poses a problem when adhering to safety regulations. In a factory, during the synthesis of 2–methylbutanoic acid, the workers did not seal the chemical container properly. Given that the enthalpy change of vaporisation of 2 –methylbutanoic acid is +59.1 kJ mol –1, use the fol lowing equation (Clausius –Clapeyron Equation) to determine the vapour pressure of 2 –methylbutanoic acid in the factory at night when the temperature was 30.2 oC. ln P = ∆Hvaporisation R ( 1 Tb.p. – 1 T ) P = vapour pressure in atm at temperature T K ΔHvaporisation = enthalpy change of vaporisation of the substance in J mol–1 R = universal gas constant Tb.p. = boiling point of the substance in K at 1 atm [2] (iii) Safety protocols dictated that stinky smelling substances above 300 ppm (parts per million) is considered a safety hazard and likely to be fined. Based on your answers in (a)(i) and (c)(ii), and given that the pressure in the factory is 1 atm, determine whether the factory would be fined. [1 x 10-6 atm in 1 atm = 1 ppm] [2] (d) Ethyl 2–methylbutanoate is an ester which is commonly found in citrus fruits, such as pineapple and oranges. It is commonly used as the starting mate