2019 JPJC Prelim H2 Chem P4 ANS

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over JURONG PIONEER JUNIOR COLLEGE 2019 JC2 H2 CHEMISTRY (9729) Preliminary Examination Paper 4 (Suggested Answers) 1 Determination of the identity of the halogen, X, in CH2XCO2H (a) (ii) Titration results Titration number 1 2 Final burette reading / cm3 24.90 34.95 Initial burette reading / cm3 0.00 10.00 Volume of FA3 used / cm3 24.90 24.95   (iii) average volume of FA 3 used , VFA3 = 24.90 24.95 2  = 24.93 cm3 (b) (i) n(H2SO4) in 10.0 cm3 of FA 2 = 2.00  10.0 1000 = 0.0200 mol = n(H2SO4) in 250 cm3 FA 3 [H2SO4] in FA 3 = 0.020  250 1000 = 0.0800 mol dm3 or Using c1V1 = c2V2, [H2SO4] in FA 3 = 2.00 10.0 250  = 0.0800 mol dm3 (ii) n(H2SO4) reacted in titration = 24.93 0.08001000  = 0.001996 mol Since 1H2SO4  2 NaOH, n(NaOH) in 25.0 cm3 of FA 1 = 0.001992  2 = 0.00399 mol (iii) n(NaOH) in 250 cm3 of FA 1 = 0.00399 250 25.0 = 0.0399 mol = n(NaOH) left unreacted after reaction with W n(NaOH) added to W = 0.40 = 0.100 mol n(NaOH) reacted with W = 0.100  0.0399 = 0.0601 mol

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 (iv) From equation 1 and 2, since 1W  2NaOH, n(W) in 4 g = ½  0.0601 = 0.03004 mol Mr of W = 4  0.03004 = 133.2  133 (no units) Ar of X = 133.2 – 59 = 74.2 (no units) X is bromine/ Br (c) (i) max. % error in volume of FA3 used = 2(±0.05) 10024.95  =  0.401 % (ii) Error : Mass measurement of W was not precise as the mass was given to nearest g. Modification : Use a more precise weighing balance that can measure to 3.d.p. or Error : Substitution of halogeno group may be incomplete. Modification : Heat W with NaOH for a longer period of time. or Error : Loss of product through heating. Modification : Heat the reaction mixture under reflux 2 An experiment to investigate the behaviour of acids and bases in aqueous solutions (a) Determination of the enthalpy change of reaction between FA 4 and FA 5 Total volume of FA 5 added/ cm3 Maximum temperature/ C 0.00 29.0 2.00 30.1 4.00 30.8 6.00 31.5 8.00 32.0 10.00 32.5 12.00 32.8 14.00 32.6 16.00 32.3 18.00 32.0 20.00 31.7 22.00 31.5 24.00 31.2 26.00 31.0 28.00 30.8 30.00 30.6

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over (b) (i) (ii) & (iii) correctly reads Tmax to ½ small square + correctly calculates Tmax + correctly reads Vequivalence to ½ small square. (iv) n(NaHCO3) used = 25.0 ×1.001000 = 0.0250 mol = n(NaOH) reacted [NaOH] in FA 5 = 0.0250  12.4 1000 = 2.02 mol dm3 (v) From the graph, Vequivalence = 12.4 cm3. heat evolved, q = (25.0 + 12.4)(4.18)(3.85)= 603 J H3 = (603  103)  0.0250 = 24.1 kJ mol1 (c) The reaction between FA 4 and FA 2 (d) (40.0 28.6) (15.0 28.9) (40.0 15.0) averageT     = 28.7 C heat absorbed, q = (40.0 + 15.0)(4.18)(28.7 – 28.2) = 115 J n(NaHCO3) used = 1.00  40.0 1000 = 0.0400 mol Since NaHCO3 is the limiting reagent, H4 = +(115  103)  0.0400 = +2.87 kJ mol1

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 (e) H+ + OH  H2O Hneutralisation = 57.0 HCO3 + OH  CO32 + H2O H3 = 24.1 H2O + CO2  HCO3 + H+ H4 = (+2.87) Overall: 2OH(aq) + CO2(g)  CO32(aq) + H2O(l) H5 H5 = (57.0) + (24.1) (+2.87) = 84.0 kJ mol1 (g) reaction 3 role of HCO3 ions Bronsted acid explanation HCO3 is a proton donor (or loses a proton). reaction 4 role of HCO3 ions Bronsted base explanation HCO3 is a proton acceptor (or gains a proton). 3 Investigation of some reactions involving transition element ions. tests observations (a) (i) Add one spatula of FA 6 to a hard-glass test-tube. Heat gently for about 10s and then strongly for about 20s. Leave the test -tube and residue to cool completely. Keep the residue for use in 3(a)(ii). While waiting, you can start on 3(d). O2 gas relights glowing splint. Black residue obtained. (ii) Add deionised water to a small beaker until it is about half full. Place the beaker on a white tile. Pour the cooled residue from 3(a)(i) slowly into the deionised water in the beaker. Observe the solution closely. Swirl the solution and filter a small portion of this solution into a clean test - tube. The filtrate is FA 7. Solid dissolves to form green solution which then turns purple. Purple filtrate obtained. Dark brown residue obtained. (iii) Add 1 cm depth of FA 7 to a test -tube. Add Fe 2+(aq), slowly with shaking, until no further change is seen. Purple FA 7 turns yellow/orange/ brown and then pale green/ colourless. (b) (i) The green solution obtained immediately when water is added to the residue is MnO42. E cell = E red – E ox = (+2.26) – (+0.56) = +1.70 V > 0 (energetically feasible) Green MnO42 is oxidised to purple MnO4 and reduced to brown MnO2 ppt. (ii) disproportionation

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over (c) 2KMnO4  K2MnO4 + MnO2 + O2 (d) tests observations (i) Add 2 cm depth of aqueous sodium hydroxide in a test-tube. Add 5 drops of FA 8 to this test-tube. Add a few drops of aqueous potassium manganate(VII). Solution turns green. Green solution then turns brown/brown ppt. formed. (ii) Add 2 cm depth of aqueous sulfuric acid in a test-tube. Add 5 drop of FA 8 to this test-tube. Add a few drops of aqueous potassium manganate(VII). Place the mixture in the hot water bath for a few minutes. Purple FA 9 decolourises. (iii) Add 5 cm depth of deionised water in a test-tube. Add 1 drop of FA 8 to this test-tube. Add aqueous bromine slowly, with shaking, until no further change is seen. Yellow aqueous bromine decolourises (e) (i) molecular formula of FA 8 C6H10 explanation FA 8 undergoes oxidation from alkene to give diol. (ii) FA 8 compound Z

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 4 Planning (a) 1. Use a burette to transfer 50.00 cm3 of KMnO4 into a conical flask. 2. Using a 50.0 cm 3 measuring cylinder, transfer 50.0 cm 3 of H2SO4 into the same conical flask. Using a dropper, add 1 cm3 of MnSO4(aq) to the conical flask. 3. Using a 50.0 cm3 measuring cylinder, measure 50.0 cm3 of Na2C2O4. 4. Transfer the Na2C2O4 to the conical flask and quickly insert the bung into the conical flask. Immediately start the stopwatch and swirl the conical flask. 5. Record time taken using stopwatch when 40 cm3 of gas is collected in the gas syringe. 6. Repeat steps 1 to 5 for 4 other experiments, using the volumes as shown in the table below. Measure deionised water using a measuring cylinder. Expt Vol KMnO4 /cm3 Vol deionised water/cm3 Vol H2SO4 /cm3 Vol Na2C2O4 /cm3 Vol Mn2+ /cm3 t/s 1 50.00 0.0 50.0 50.0 1 2 40.00 10.0 50.0 50.0 1 3 30.00 20.0 50.0 50.0 1 4 20.00 30.0 50.0 50.0 1 5 10.00 40.0 50.0 50.0 1 (b) Rate = k’[KMnO4]a Since rate  1/t and [KMnO4]  VKMnO4 since total volume is kept constant, 1/t = k’(VKMnO4)a lg(1/t) = lg k’ + a lg(VKMnO4) lg(1/t) = a lg(VKMnO4) + lg k’ which is similar to a y = mx + c straight line graph

 Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over (c) Gradient of the line = order of reaction (d) (i) The reaction is slow as it has high activation energy due to the repulsion between both the negatively charged MnO4 and C2O42. (ii) Homogenous catalyst since Mn2+(aq) is in the same phase as the reactants. (iii)