RVHS_2022_H2_Chemistry_Prelim_P4_Ans

River Valley High School Pg 18 of 20 JC 2 H2 Chemistry 9729 2022 Preliminary Examination Paper 4 2022 H2 Chemistry Preliminary Examination Paper 4 Suggested Answers 1 Investigation of the oxidising ability of substances (a) (i) Table 1.1 Test Observations 1 To a 1 cm depth of FA 1 in a test-tube, add 1 cm depth of FA 4 , then gradually add FA 2 till 1 drop in excess. Purple FA 2 decolourised. Pale green/colourless FA 1 turned pale yellow and finally orange/ pink. 2 To a 1 cm depth of FA 3 in a test-tube, add 1 cm depth of FA 2. Purple KMnO4 decolourised. A brown solution/ppt formed. Effervescence/ bubbles of gas evolved rapidly. Colourless, odourless gas evolved relight a glowing splint. The gas is oxygen. 3 To a 1 cm depth of FA 1 in a test-tube, add about 1 cm depth of FA 4, followed by 1 cm depth of FA 3. To a portion of resulting solution, add aqueous sodium hydroxide till excess. Pale green /colourless FA 1 turned yellow. Red-brown ppt formed is insoluble in excess NaOH(aq). Effervescence/ bubbles of gas evolved (rapidly). (ii) Purple MnO4− oxidised (pale green) iron(II) to (yellow) iron(III) ions. Itself is reduced to colourless Mn2+. (reject pale Mn2+) (iii) Fe2+ (aq) → Fe3+(aq) + e− Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) (iv) Compound A / FA 3 (b) (i) Titration results Titration number 1 2 Final burette reading /cm3 24.20 24.25 Initial burette reading /cm3 0.00 1.00

River Valley High School Pg 19 of 20 JC 2 H2 Chemistry 9729 2022 Preliminary Examination Paper 4 Volume of FA 2 (added) /cm3 24.20 24.25 (ii) average volume of FA 2 used = 24.20 24.25 2 + = 24.23 cm3 (c) (i) [KMnO4] = 0.750 1000 39.1 54.9 (4 16.0) 250+ +  = 0.01899 mol dm−3 amount of MnO4− = FA2V0.01899 1000 mol = 0.000460 mol (ii) amount of Fe2+ in 25.0 cm3 = 5(c)(i) 1 mol [Fe2+] = 5 1000(c)(i) 1 25.0 mol dm−3 = 0.0920 mol dm−3 (d) Identify the cause: Chloride is oxidised by/ reacts with MnO4− And any one of the following modification : M16 • The titration needs to be carried out in the fumehood. Chloride is oxidised by MnO4− to give toxic chlorine gas. • Iron(II) chloride needs to be diluted prior to titration. The titre will exceed 50.00 cm3 if iron(II) chloride is not diluted. • Prepare higher concentration of MnO 4− for used. The titre will exceed 50.00 cm3 if FA 2 with original concentration is used. • Using a smaller pipette/ burette, measure a smaller volume of iron( II) chloride for titration, so that the titre will not exceed 50.00 cm3. (5FeCl2(aq) + 3MnO 4−(aq) + 24H +(aq) → 5Fe3+(aq) + 5C l2(g) + 3Mn 2+(aq) + 12H2O(l)) (e) (i) As V FA3 increases, more compound A/FA 3 was added to oxidise Fe 2+ in FA 1. This leaves less Fe2+ to be oxidised by MnO4− in FA 2. (ii) • Not an anomaly. • (Compound A in FA 3 is both an oxidising and reducing agent.) • I