2024 MI PU3 MYE P3 (Answers) with marker's comments

2024 Mid-Year Examination Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 5 July 2024 2 hours Section A 1 (a) (i) 1s2 2s2 2p6 3s2 3p6 3d5 4s2 Marker’s comment Many candidates did well for this question. (ii) 3𝑑𝑥𝑦 3𝑑𝑥2−𝑦2 3𝑑𝑧2 Marker’s comment Some candidates did not draw the 3dxy orbital between the x and y axis. (iii) forms ions of variable oxidation states / variable oxidation states in compounds forms coloured compounds high density Marker’s comment Candidates understand what the question wants. Weaker candidates wrote ‘more oxidation number’ instead of forms ions of ‘variable oxidation state’.

2 (b) MnO has giant ionic structure electrostatic forces of attraction between oppositely-charged ions Mn2O7 has simple molecular structure instantaneous dipole-induced dipole forces of attraction between molecules the stronger forces in MnO require more energy to overcome points: (1) identify structure and bonding in MnO (2) identify structure and bonding in Mn2O7 (3) identify MnO has stronger forces, compare energy [2] all 3 points [1] either (1) or (2) [0] just (3) Marker’s comment This question proved to be challenging to candidates. Many who attempted were unable to identify the correct structure and bonding in MnO and Mn 2O7. Some candidates explained why Mn 2O7 has a simple molecular structure, but did not continue to compare with the structure and bonding in MnO. (c) (i) Mn2O7 + H2O ⟶ 2HMnO4 Marker’s comment Many candidates did well for this question. (ii) hydrolysis Marker’s comment Some candidates did not attempt this question. Those who did, some wrote ‘hydration’ instead of hydrolysis. (iii) Marker’s comment Majority of the candidates were unable to do this question. (d) from Ti to Cu, relatively similar atomic radii nuclear charge increases shielding increases but due to electrons added to 3d 4s electrons experience similar effective nuclear charge [2] 5 / 5 [1] 3 / 5 Marker’s comment Many candidates confused this explanation with the trend of atomic radii across a period for the main group elements.

3 [Turn over 2 (a) (i) 2,4-dinitrophenylhydrazine / if orange precipitate forms, sample is O if no precipitate forms, sample is P Marker’s comment Some candidates gave tri-iodomethane test as the distinguishing test. For candidates who gave the correct distinguishing test, some gave ‘no visible reaction for P’, ‘P remains the same’ and ‘solution remains colourless for P’. (ii) information reaction inference L was heated with concentrated H2SO4 dehydration / elimination of water (1) L was an alcohol (3) M and N were alkenes (4) M and N were heated with KMnO4 / H+ oxidative cleavage / vigorous oxidation (2) for O, the carboxyl carbon and ketone carbon were once joined