Copy of AMKSs 2024 Chem P2 Answers

1 ANG MO KIO SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY [6092] ANSWER SCHEME Paper 2 Section A: Compulsory Structured Questions [70 Marks] 1a Substance X. X is a noble gas as it exists as monatomic atoms. Atoms of noble gases have completely filled outer shell and do not form bonds with other atoms as it is stable. 1 1 1b Substance Y. Each carbon atom in a molecule of carbon dioxide forms double covalent bond to each of the two oxygen atoms. 1 1 1c W is a giant covalent structure/giant molecule with strong covalent bonds between the atoms in the giant molecule. Large amount of energy is needed to overcome these strong bonds. 1 1 1d All the atoms in the four substances are non-metals / do not conduct electricity / (any appropriate answer) 1 Total 7 2a An amphoteric oxide reacts with both an acid and an alkali to form salt and water only. 1 2b nitric acid 1 2c Lead(II) sulfate is insoluble and will form a coating around lead(II) oxide. This prevent further reaction and will result in impure lead(II) sulfate salt formed. 1 1 Total 4 3a No of mole of MnO4− = 0.02 x 0.03 = 0.000600 mol 1 3b 2 moles of MnO4− reacts with 5 moles of SO32− 0.000600 mol of MnO4− = (5 ÷ 2) x 0.000600 = 0.00150mol of SO32− 1 1 3c The reducing agent is SO32−. The oxidation state of Mn decreases from +7 in MnO4− to +2 in Mn2+. Therefore, SO32− has reduced MnO4−. 1 1

2 Total 5 4a calcium 1 4bi 7 1 4bii 4 1 4c isotope number of protons number of neutrons number of electrons Fl−286 114 172 114 Fl−289 114 175 114 Each row correct = 1 2 4di High melting and boiling points / conductor of electricity / malleable / ductile / (any appropriate physical properties of metals) 1 4dii reacts with acid to form salt and water 1 Total 7 5a Oxygen in air would react with the iron wool. This would make the iron lose its function as the catalyst for the Haber Process. 1 5b Total energy taken in to break bonds = 3(436) + 945 = 2253 kJ Total energy given out to make bonds = 6(391) = 2346 kJ Energy change for the reaction = 2253 – 2346 = −93 kJ 1 1 5c Exothermic reaction. (must tally with their 5b ans) The total energy taken in during bond breaking is lower than the total energy given out during bond making. (base on whether they put it as endo or exo) 1 1 5d 3

3 5e (answer in 5d) 1 Total 9 6a 2 H2O2 (l) → 2 H2O (l) + O2 (g) (state symbols not required) 1 6b To prevent chemical spray / splash 1 6c Graph Y shows a slightly lower mass obtained / greater mass loss compared to Graph X. The actual mass loss in Graph X is less than theoretical mass loss in Graph Y is due to a small amount of oxygen formed dissolving in