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 the solution. 1 1 6d Heating the hydrogen peroxide solution / using smaller pieces of manganese(IV) oxide 1 6e Measure a fixed mass of manganese(IV) oxide and add to a solution of hydrogen peroxide. When the mass loss reaches a constant reading, filter the mixture to obtain manganese(IV) oxide as the residue. Dry the residue. Measure the mass of manganese(IV) oxide remaining. It is a catalyst if the mass remains the same. 1 1 1 Total 8 7a To remove any oxide layer formed on the metal surface so as to ensure better electrical conductivity. 1 7b X, W, Z, Cu, Y (first and last metals correct = 1; reactivity order correct = 1) 2 7c The further apart the metals are positioned in the reactivity series, the greater the magnitude of the voltage. 1 7d The voltmeter reading will be zero for all the four metals. Methylbenzene is a covalent compound and does not conduct electricity as there are no mobile electrons or ions present. 1 1 7ei A pink/brown/reddish-brown solid is deposited. The solution turns from blue to colourless/lighter blue. 1 1

4 7eii X (s) + Cu2+ (aq) → Cu (s) + X2+ (aq) (correct balanced equation = 1; correct state symbols = 1) 2 Total 10 8a Carbon monoxide is formed by incomplete combustion of carbon-based fuels. 1 8b There is zero percentage of carbon monoxide, oxides of nitrogen and unburnt hydrogen in the car exhaust gases of cars fitted with catalytic converters compared with trace amounts of these gases in cars without catalytic converters. The use of a catalytic converter helps to convert carbon monoxide and oxides of nitrogen into carbon dioxide and carbon dioxide. nitrogen 2 CO (g) + 2 NO (g) → 2 CO2 (g) + N2 (g) (state symbols not required) Unburnt hydrocarbons are also converted into carbon dioxide and water in a catalytic converter. (deduct ½ m if no equation is shown) 1 1 1 8ci carbon dioxide 1 8cii change in rainfall patterns / heat waves / tropical storms / ocean warming and acidification / glacial retreat and melting of the polar ice caps 1 8di During the day, trees take in carbon dioxide and give out oxygen during photosynthesis. This increases the percentage of oxygen and decreases the percentage of carbon dioxide in the day. 1 1 8dii (correct ratio of atoms and no. of electrons = 1; correct sharing of electrons = 1) 2 Total 10

5 9a Styrene has a simple molecular structure / is a simple molecule. Little amount of energy is needed to overcome the weak intermolecular attraction between the molecules. 1 1 9b (correct ratio of atoms and no. of electrons = 1; correct sharing of electrons = 1) 2 9c Enthalpy change is positive in value / heat taken in during the reaction. 1 9d The boiling points of styrene and ethylbenzene are too close / similar, with only 9 oC in difference. 1 9e Addition polymerisation. Carbon-carbon double bond on the side chain of styrene. 1 1 9f (correct addition polymerisation = 1; all three repeating units shown = 1) 2 Total 10 Section B: Free Response Questions [10 Marks] 10a electrode X: 2 H+ (aq) + 2 e− → H2 (g) electrode Y: 2 Cl− (aq) → Cl2 (g) + 2 e− 1 1 10b Colourless solution turns brown / a black solid is formed. 1

6 Cl2 + 2 I− → I2 + 2 Cl− (state symbols not required; full equation accepted) Chlorine is more reactive than iodine. Chlorine will displace iodine from potassium iodide. 1 1 10c Oxygen gas. Over time, the concentration of chlorine ions will decrease, and hydroxide ions become preferentially discharged to form oxygen gas. 1 1 10d Student A is correct. At the anode, copper(II) ions are added to the electrolyte when the copper electrode dissolves / reduces in size to form copper(II) ions. Cu (s) → Cu2+ (aq) + 2 e− At the cathode, copper(II) ions are removed from the electrolyte as they are preferentially discharged to form copper. Cu2+ (aq) + 2 e− → Cu (s) Therefore, the concentration of copper(II) ions remains unchanged in the electrolyte which remains blue in colour. 1 1 1 Total 10 11ai water 1 11aii sweet smelling 1 11aiii (correct ester bond = 1; correct structural formula = 1) 2 11b 2

7 (correct amide bond = 1; two repeating units shown = 1) 11c amide bond / linkage 1 11d unsaturated 1 11e Aqueous bromine becomes decolourised / changes from reddish-brown to colourless. Bromine is added to compound X through addition reaction as compound X is unsaturated. 1 1 Total 10