2021 Paper 4 Ans

Upon addition of FA1, purple manganate (VII) decolourises On standing, solution remains colourless. Some effervescence observed Effervescence of gas that gives white ppt with limewater. The gas is CO2 Universal indicator paper turns dark red. The colour corresponds to pH 1. 2021 A level H2 Chem Paper 4 Answer

Green ppt formed is insoluble in excess NH3(aq) The green ppt turns brown upon standing. Fe2+ [Note: green ppt is Fe(OH)2, which is oxidized by air to brown ppt Fe(OH)3] [Note: reaction with acidified KMnO4 at room temperature: mild oxidation of alkene C=C Reaction with Na2CO3 and pH1: acid-carbonate reaction, presence of carboxylic acid −COOH] CCC HH COH O HO O

NH4+ SO42− To test for cation: To 1cm depth of FA2, add NaOH(aq) in excess. Filter the suspension and collect the filtrate for subsequent tests. [to isolate the 2nd cation in the filtrate] Green residue that turns brown on standing. Filtrate is colourless. To test for Ba2+/Ca2+: To 1cm depth of filtrate, add excess H2SO4 (aq) [to check for ppt of BaSO4 or CaSO4] No ppt observed. To test for Al3+/Zn2+: To 1cm depth of filtrate, add HNO3 (aq) dropwise until no further change [to check for ppt of Al(OH)3 or Zn(OH)2] No ppt observed. [If [Al(OH)4]− or [Zn(OH)4]2− is in the filtrate, slow addition of H+ will remove the excess OH− from the complex ion and produce white ppt of Al(OH)3 or Zn(OH)2] To test for NH4+: Add 1cm depth of filtrate into a boiling tube, warm gently using the Bunsen burner [the filtrate already contains NaOH] Gas produced turned moist litmus paper blue. The gas is NH3 [NH4+ + OH− ⎯→ NH3 + H2O] To test for anion: To test for CO32−: To 1cm depth of FA2, add HNO3(aq) No effervescence observed. To test for SO42−: To 1cm depth of FA2, add Ba(NO3)2(aq), followed by excess HNO3(aq) White ppt formed, insoluble in excess HNO3. [to confirm SO42−, we need to check that the ppt is insoluble in strong acid] To test for Cl−, Br−, I−: To 1cm depth of FA2, add AgNO3(aq), followed by excess NH3(aq) No white/yellow ppt observed. [Ag+ reacted with Fe2+ in FA2 to give a grey ppt via redox reaction Ag+ + Fe2+ ⎯→ Ag + Fe3+]

Mass of FA3 in capped container / g 9.429 Mass of empty capped container / g 5.272 Mass of FA3 used / g 4.157 Time/min Temp/ C Time/min Temp/ C 0.0 30.2 4.5 35.9 0.5 30.2 5.0 35.8 1.0 30.2 5.5 35.7 1.5 30.2 6.0 35.6 2.0 30.2 6.5 35.6 2.5 - 7.0 35.4 3.0 36.0 7.5 35.2 3.5 36.2 8.0 35.2 4.0 36.1

30.2 C 36.5 C +6.3 C

q = maqc∆T = 50  4.18  (+6.3) = 1316.7 J = 1320 J (3s.f.) Amount of Na2CO3 = 4.157 23.0  2 + 12.0 + 16.0  3 = 0.03922 mol ∆H2 = − q mol of limiting reagent = − 1316.7 0.03922 = −33572 J mol−1  −33.6 kJ mol−1 T is +ve, H is -ve Note: m is mass of aq solution ONLY. 50 cm3 aq solution = 50 g Amount of NaHCO3 = 4.00 23.0 + 1.0 + 12.0 + 16.0  3 = 0.04762 mol ∆H3 = − 𝑚𝑎𝑞c∆T mol of limiting reagent = − 50  4.18  (-6.6) 0.04762 = +28967 J mol−1  +29.0 kJ mol−1 T is −ve, H is +ve

By Hess’s Law, ∆H1 = 2  ∆H3 − ∆H2 = 2  (+29.0) – (−33.6) = +91.6 kJ mol−1 % error when measuring 50 cm3 of FA4 using measuring cylinder =  0.5 50  100 = 1.0% % error when measuring ∆T using 0.2 C thermometer =  2  0.1 6.3  100 = 3.17% (2 readings with 0.1 uncertainty to obtain T) Hence, the thermometer caused the greatest percentage error.

Titration 1 2 Final burette reading / cm3 19.20 38.50 Initial burette reading / cm3 0.00 19.20 Volume of FA6 used / cm3 19.20 19.30 Average vol. of FA6 used = 19.20 + 19.30 2 = 19.25 cm3

Amount of HCl used = 19.25 1000  0.100 = 1.925  10−3 mol Amount of Na2CO3 in 25.0 cm3 of FA5 = ½  1.925  10−3 = 9.625  10−4 mol Concentration of Na2CO3 in FA5 = 9.625  10-4 (25.01000⁄ ) = 0.0385 mol dm−3 Mr of Na2CO3•xH2O = 10.8 0.0385 = 280.5 (1d.p.) Mr of Na2CO3•xH2O = 280.5 23.0  2 + 12.0 + 16.0  3 + (18.0x) = 280.5 18.0x = 174.5 x = 9.70  10 (whole number)