2021 TJC H2Chemistry P3 Ans Prelim

1 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2021 CANDIDATE NAME CIVICS GROUP / CENTER NUMBER S INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 13 September 2021 2 hours Candidates answer on the Question Paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group, Centre number, index number and name in all the work you hand in. Write in dark blue or black pen. You may use a 2B pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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. For Examiner’s Use Paper 3 (Section A) 1 2 3 Paper 3 (Section B) 4 5 Total This document consists of 23 printed pages and 1 blank page. PRELIMINARY EXAMINATIONS HIGHER 2

2 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2021 SECTION A Answer all the questions in this section. 1 (a) The Contact Process is an industrial process for manufacturing sulfuric acid. The key stage in this process is the reaction between sulfur dioxide and oxygen. 2SO2(g) + O2(g) ⇌ 2SO3(g) DH = –197 kJ mol–1 Vanadium(V) oxide, V2O5, is used as a heterogeneous catalyst for this process. With the aid of an energy profile diagram, explain using collision theory why the addition of the catalyst speeds up the reaction. [3] • Well-labelled diagram The √ catalyst provides an alternative pathway, one which involves a lower Ea than that of an uncatalysed reaction. √ Number of molecules having energy greater than or equal to Ea’ increases significantly. The √ frequency of effective collisions increases, √ rate constant, k increases and thus rate increase. 2√: 1m (b) In an experiment to determine the Kp of the Contact Process in (a), a mixture containing 0.200 mol of SO2 and 0.100 mol of O2 was heated in a closed flask and allowed to reach equilibrium at 550 oC and 3.5 atm. The flask was then rapidly cooled to liquefy the SO3 so that it can be separated from the gaseous SO2 and O2. Excess water was carefully added to the liquid SO3, causing the following reaction to occur: SO3(l) + H2O(l) ® H2SO4(aq) The resulting solution was made up to 250 cm3 in a standard volumetric flask. 25.0 cm3 of this solution was titrated with 1.00 mol dm−3 NaOH and required 36.00 cm3 for complete neutralisation. (i) Calculate the equilibrium amounts of SO2, O2 and SO3 at 550 oC. [3]

3 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2021 2NaOH ≡ H2SO4 ≡ SO3 Amt of SO3 at eqm = ½ × 36.00 / 1000 × 1.00 × (250/25) = • 0.180 mol 2SO2(g) O2(g) 2SO3(g) Initial / mol 0.200 0.100 0 Change / mol –0.180 –0.09 +0.180 Eqm / mol • 0.020 • 0.01 0.180 (ii) Write the expression for Kp and calculate the value of Kp at 550oC and 3.5 atm, including its units. [2] Total number of moles = 0.020 + 0.010 + 0.180 = 0.210 mol • Kp = PSO32PSO22×PO2 = !𝟎.𝟏𝟖𝟎𝟎.𝟐𝟏𝟎 × 𝟑.𝟓'𝟐 !𝟎.𝟎𝟐𝟎𝟎.𝟐𝟏𝟎 × 𝟑.𝟓'𝟐!0.0100.210 × 3.5' = • 486 atm-1 or 0.00480 Pa-1 (answer + correct units) (iii) In liquefying the SO3, it was mentioned that the cooling was performed rapidly. Explain how the titre volume would be affected if the cooling had been performed slowly. [2] 2SO2(g) + O2(g) ⇌ 2SO3(g), ∆H < 0 Temperature falls when cooling happens. • By Le Chatelier’s Principle, equilibrium position will shift to the right to release more heat to increase the temperature. • More SO3 is formed, and titre volume increases. (c) Precious metals like platinum and rhodium are fitted into car engines as catalytic converters to reduce the amount of pollutants in the exhaust. With the aid of equations, show how the pollutants hydrocarbons, CxHy, and oxides of nitrogen can be removed from the exhaust. [2] • CO + NO à CO2 + ½N2 OR 2CO + 2NO à 2CO2 + N2 OR 2xCO + 2NOx à 2xCO2 + N2 • CxHy + (x + y/4)O2 à xCO2 + y/2 H2O (d) A student was tasked to investigate the rate of hydrolysis of compounds A, B and C as shown below. Compound A Compound B Compound C

4 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2021 (i) State and explain the difference in reactivity of compounds A, B and C towards hydrolysis. [3] • Reactivity: C > B > A (least reactive) Compound A is the least reactive towards hydrolysis as the √C‒Cl bond is strengthened (stronger) by the √delocalization of the lone pair of electrons from Cl to the double bond (or overlapping of the p orbital of Cl with the π orbital of C=C bond). Compound C is more reactive than B as the √C–I bond is weaker than C–Cl bond, due to √I atom being larger than Cl atom, causing less effective orbital overlap, less energy is needed to break the C-I bond. 2√: 1m (ii) Compound B reacts with bromine monochloride, BrCl, in the presence of an inert organic solvent. Name and describe the mechanism of this reaction. Show relevant lone pairs and dipoles, and use curly arrows to indicate movement of electron pairs. [3] • Electrophilic addition Note: Award 1m (out of 2m) if minor product is drawn. • • (iii) Suggest the structure of another carbocation that can be formed from the mechanism drawn in (d)(ii). Hence, explain which carbocation is preferentially produced. [2] • •

5 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM / 2021 is preferentially produced as it is a tertiary carbocation which is more stable than the secondary carbocation intermediate. Tertiary carbocations are more stable due to the presence of more electron donating alkyl groups that disperse the positive charge and stabilise the electron deficient carbocation. [Total: 20] 2 (a) Organometallic compounds are compounds that contain carbon-metal covalent bonds. Organometallic compounds of lithium and magnesium are strong bases and are of great importance in organic synthesis. Organomagnesium halides are also known as Grignard reagents. The reaction between an organomagnesium halide and ethylene oxide is shown below. (i) Suggest the types of reaction for steps 1 and 2.