2019 TJC H2 Chem Prelim P3 QP

1 9729 / TJC Prelim / 2019 [Turn Over PRELIMINARY EXAMINATIONS HIGHER 2 CHEMISTRY 9729/03 Paper 3 Free Response 16 September 2019 2 hours Candidates answer on separate booklet. Additional Materials: 12-Page Answer Booklet Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group, Centre number, index number and name 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. 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 and 2 blank pages.

2 9729 / TJC Prelim / 2019 [Turn Over Section A Answer all the questions from this section. 1 The term chelates, originates from the Greek word chele for “claw”. It refers to compounds containing ligands bonded to a central metal atom or ion at two or more points. Ligand exchange occurs when ammonia and ethylenediamine (en), H2NCH2CH2NH2, are added separately to copper sulfate solution. (a) (i) Explain all the changes observed when aqueous ammonia is added dropwise to Cu2+(aq) till in excess. Suggest the formulae of all relevant copper compounds formed and write equations for all reactions. [4] (ii) Suggest why ethylenediamine can function as a chelating ligand. [1] (iii) When ethylenediamine is added to Cu2+(aq), the following occurs. [Cu(H2O)6]2+ + 2en  [Cu(en)2(H2O)2]2+ + 4H2O Predict whether ethylenediamine or ammonia gives a more positive entropy change when added to Cu2+(aq). [2] (iv) Suggest the coordination number and shape of [Cu(en)2(H2O)2]2+. [1] (v) The complex ion [Cu(en)2(H2O)2]2+ can exist as three stereoisomers, A, B and C. Isomers A and B rotate plane-polarised light but C does not. The structure of A is represented below. where represents Isomer A ethylenediamine (en) Draw the structure of C. [1] (b) Photochromic glass used for sunglasses contains AgCl crystals which are added while the glass is in molten state. The glass darkens when exposed to bright light and the following reaction is involved. Ag+ + Cl ⇌ Ag + Cl (i) Suggest why the glass darkens when exposed to bright light. [1] The glass darkens significantly within about a minute of exposure to bright light but takes a longer time to clear when there is less light. Small amount of CuCl crystals are often added to the glass to speed up the reverse process. No elemental copper is involved in the process. Ag + Cl  Ag+ + Cl

3 9729 / TJC Prelim / 2019 [Turn Over (ii) Explain why the copper(I) ions can be described as a homogeneous catalyst. [2] (iii) State the property, typical of transition metals, which allows copper(l) ions to behave as a homogenous catalyst in the reverse reaction. Include relevant chemical equations to support your answer. [3] (c) Microorganisms synthesise and secrete organic molecules called siderophores to increase the total concentration of available iron in the surr ounding medium. Ferrichrome is a siderophore produced by fungi. Ferrichrome (i) Ferrichrome binds to iron(III) ions via its oxygen atoms. This process facilitates the transportation of iron(III) ions into the interior of a cell. Suggest what bonds are formed during this process and why iron(III) does not bind to nitrogen atoms in ferrichrome. [2] (ii) Draw the structures of the products formed when ferrichrome is heated with aqueous sodium hydroxide. [3] [Total: 20]

4 9729 / TJC Prelim / 2019 [Turn Over 2 Ibuprofen is a nonsteroidal anti-inflammatory drug. It works by reducing hormones that cause inflammation and pain in the body. Ibuprofen is used to reduce fever and treat pain or inflammation. The conventional synthesis of ibuprofen from isobutylbenzene is shown below. Step 4 of the synthesis resembles the reaction between a carbonyl compound and Brady’s reagent. (a) Name the types of reaction that are occurring during steps 4 and 5. [2] (b) Suggest the reagents and conditions for steps 1, 4 and 6. [3] (c) Draw the structures of the stereoisomers of the product formed from step 4 and state the type of stereoisomerism exhibited. [2] (d) Describe a simple chemical test to distinguish between the two compounds, X and Y obtained from synthesis above. [2] Compound Y Compound X

5 9729 / TJC Prelim / 2019 [Turn Over (e) Potassium chloride and potassium iodide can be distinguished by treating the compounds separately with concentrated sulfuric acid. (i) Potassium chloride and concentrated sulfuric acid reacts in an equimolar ratio to produce white fumes of hydrogen chloride. Write an equation for the reaction of potassium chloride with concentrated sulfuric acid. [1] (ii) Potassium iodide reacts with concentrated sulfuric acid in a similar manner. However, the white fumes of hydrogen iodide would further react with concentrated sulfuric acid to produce violet fumes and hydrogen sulfide gas. Suggest an explanation for the difference in reactions, and write equations for the above observations. [2] (f) When aqueous potassium salts of a dicarboxylic acid are electrolysed using inert platinum electrodes, alkenes are formed at the anode. (i) The electrolysis of aqueous potassium succinate gives ethene at the anode. potassium succinate Write an ion-electron half equation for the oxidation of succinate at the anode. [1] (ii) Write an equation for the reaction at the cathode and for the overall reaction. [2] (iii) The mechanism of the reaction at the anode involves three steps: Step 1: There is an initial loss of 2 electrons on succinate ion to form Step 2: This is followed by decarboxylation which involves the homolytic breaking of two C–C bonds, giving a radical intermediate •CH2CH2• Step 3: The third step involves forming a covalent bond, producing ethene as the product. Use the information given above to draw the mechanism for Steps 2 and 3. You are advised to use structural formulae for all species, such as , so that it is clear which bonds are broken and which are formed. Represent with a half arrow ( ) for movement of a single unpaired electron and indicate any unpaired electrons by a dot (•). [2] Potassium Succinate

6 9729 / TJC Prelim / 2019 [Turn Over (g) The strength of a carboxylic acid depends on its structure. An example of this is the comparison of succinic acid and its derivatives. Acid pKa1 4.21 - Explain if pKa of chlorosuccinic acid is lower or higher than the pKa of succinic acid. [1] [Total: 18]

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8 9729 / TJC Prelim / 2019 [Turn Over 3 On 28 November 2018, an explosion erupted near the Hebei Shenghua Chemical Industrial plant, killing at least 23 people and injuring many more. It was reported that the explosion originated from a truck transporting ethyne gas, C2H2, which then set off a chain reaction that engulfed at least 50 other vehicles. When mixed appropriately with air, C2H2 explodes upon ignition as follows. C2H2 (g) + O2 (g) 2CO2 (g) + H2O (g) This explosion may have generated an estimated 20.8 million kilojoules of energy. (a) Describe the hybridisation of the orbitals in, and the bonds between, the carbon atoms within an ethyne molecule. [3] (b) (i) Using the bond energies in the Data Booklet, calculate the enthalpy chan