2024 JPJC Chem Prelim Paper 4 Ans slides

© Jurong Pioneer Junior College [Turn Over NAME Answer slides CLASS 23S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2024 CHEMISTRY 9729/04 Higher 2 Paper 4 Practical 13 August 2024 2 hours 30 minutes Candidates answer on the Question paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a HB pencil for any diagrams, graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 20 and 21. Shift At the end of the examination, fasten all your work securely together. Laboratory The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 14 2 / 18 3 / 11 4 / 12 Total / 55 This document consists of 21 printed pages.

2 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2024 Answer all the questions in the spaces provided. 1 Investigation of some inorganic and organic reactions You are provided with the following samples: FA 1 is an aqueous solution of an organic compound, Y. FA 2 is an aqueous solution of an organic compound, Z, which is an isomer of Y. FA 3 is a solid sample with molecular formula XOn, where X is a metal. You will perform tests to identify: • the structure of Y in FA 1 • the functional group of Z in FA 2 • the cation in FA 3 (a) (i) Perform the tests described in Table 1.1, and record your observations in the table. Table 1.1 tests observations 1 To 1 cm depth of FA 1 in a test-tube, add a small piece of sodium. No observable change. 2 To 1 cm depth of aqueous AgNO 3 in a test-tube, add 1 cm depth of aqueous NaOH. Then add aqueous NH3 until the precipitate just dissolves. To this mixture, add 1 cm depth of FA 1. Heat the mixture in the hot water- bath. Brown ppt. (✓1) dissolves to give a colourless solution. (✓2) Upon warming with FA 1, no silver mirror (or no grey ppt.) (or no observable change). (✓3*) 3 To 1 cm depth of FA 1 in a test-tube, add 10 drops of aqueous sodium hydroxide. Now add aqueous iodine dropwise, until a permanent yellow/orange colour is obtained. Warm the mixture in the hot water bath for 2 minutes. pale yellow ppt. (✓4*) formed. [2] Redox rxn. H2? Identify —OH in alcohol, phenol & RCOOH. No —OH group  alcohol, phenol & RCOOH absent. Preparing Tollens’ reagent, Ag(NH3)2+(aq). Brown Ag2O(s) dissolves in NH3(aq) to give colourless Ag(NH3)2+(aq) Tollens’ reagent. Oxidation. Silver mirror/grey ppt.? Identify aldehydes. Aldehyde absent. Iodoform Test. Pale yellow ppt.? Identify ethanal & ketones with –COCH3 and alcohols with –CH(OH)CH3. CHI3 ppt. formed.  ketone with —COCH3 present. 4(✓) [2]; 1(✓*) [1] (✓3*) or (✓4*)

3 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2024 [Turn Over (ii) The molecular formula of Y is C 3H6O and it has one functional group . Use your observations in Table 1.1 to deduce the structure of Y in FA 1. FA 1 is a ketone with with —COCH3 Structure of Y in FA 1: CH3COCH3. [1] [1] (iii) FA 2 contains an organic compound, Z, which is an isomer of Y. {M.F. C3H6O} Z contains one functional group. Devise one confirmatory test , other than those stated in Table 1.1 , using the bench reagents provided to identify the functional group present in Z. Carry out the test. Record details of the test performed and observations made in Table 1.2. Table 1.2 Confirmatory Test Observations To 1 cm depth of FA 2 in a test-tube, add 1 cm depth of *dilute sulfuric acid (or dilute H2SO4) and a few drops of *aqueous potassium manganate(VII) (or KMnO4(aq)). Heat (or Warm) the mixture in the hot water-bath. [1] Purple KMnO4 decolourised [1] upon heating. The functional group present in Z: aldehyde [1] with correct test & observations [3] Note: Z is isomeric with Y  M.F. of Z is C3H6O. An alkene or alcohol with C3H6O will have 2 functional groups, e.g. CH2=CHCH2OH. Since Z contains only one functional group, Z is an aldehyde. *State the name or correct formula of the reagent used according to the label on the reagent bottle.

4 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2024 (b) (i) Perform the tests described in Table 1.3, and record your observations in the table. Test and identify any gases evolved. If there is no observable change, write no observable change. Table 1.3 tests observations 1 Place 1 cm depth of sulfuric acid in a test-tube. Add a spatula of FA 3 to this test-tube, followed by another 2 cm depth of aqueous potassium iodide. Filter the mixture into a clean test -tube and collect the filtrate. To the filtrate , add aqueous sodium thiosulfate dropwise, with shaking, until the solution first becomes colourless. Divide the resulting solution into two portions. These will be used in tests 2 and 3 respectively. Brown/ orange / yellow solution obtained. (*✓1) Brown/orange/yellow filtrate (*✓1) Black/ dark brown residue. (✓2) Brown/orange/yellow solution turns colourless. 2 To the first portion, add aqueous sodium hydroxide slowly until no further change is seen. Leave the mixture on standing for 2 minutes. \\\ Off-white/light brown ppt. insoluble in excess NaOH. (✓3) ppt. turned darker brown on standing. (*✓4) [3] FA 3 is a black solid. Redox rxn? Brown I2 soln? I- oxidises to I2? FA 3 is reduced? Filtrate colour? Residue colour? I2 + 2S2O32- → S4O62- + 2I- Brown I2 reduces to colourless I-. ppt? ppt sol. in xs? colour change of ppt. on standing? I- oxidises to brown I2 while FA 3 is reduced. Mn2+ present in the filtrate.

5 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2024 [Turn Over 3 To the second portion, add aqueous ammonia slowly until no further change is seen. Off-white/light brown ppt. insoluble in excess NH3. (✓5) ppt. turned darker brown on standing. (*✓4) 4 Place 3 cm depth of aqueous H2O2 into a test-tube. Add a spatula of FA 3 to this test-tube. Brisk effervescence noted. (✓6) O2 (✓7) gas relights a glowing splint. (✓8) No observable change to FA 3. (ii) Identify the metal ion in the filtrate. metal ion: Mn2+ [1] [1] (iii) Deduce the oxidation state of X in FA 3. oxidation state of X in FA 3: +4 or +6. [1] [1] ppt? ppt sol. in xs? O2 gas bubbles? H2O2 oxidises to O2? H2O2 → O2 + 2H+ + 2e- or H2O2 undergoes catalytic decomposition to give O2? 2H2O2 → O2 + 2H2O Note: With H2O2, H2 gas will not form. 7-8(✓) [3]; 4-6(✓) [2]; 2-3(✓) [1] In Test 1, KI(aq) reduces FA 3 to Mn2+(aq) which is collected in the filtrate. Since FA 3 has M.F. of XOn (where X is a metal), X is Mn; and Mn in FA 3 should have higher oxidation state than +2.  FA 3 is MnO2 or MnO3 (not MnO).

6 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2024 (iv) Suggest the nature of FA 3 in the reaction occurring in Test 1. Give evidence from the observations in your experiment. nature of FA 3: oxidising or oxidising agent [1] evidence: • FA 3/MnOn oxidises I−/KI to brown/orange/yellow I2(aq) after adding KI(aq). [1] OR • FA 3/MnO n is reduced to colourless Mn 2+(aq) which gives a off-white/light brown Mn(OH) 2 ppt. ins