YCKSS Prelim Ans

YIO CHU KANG SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY 6092 ANSWER SCHEME PAPER 1 1 D 11 B 21 D 31 B 2 C 12 A 22 D 32 D 3 C 13 B 23 C 33 C 4 B 14 B 24 D 34 D 5 D 15 B 25 D 35 D 6 C 16 A 26 A 36 B 7 C 17 B 27 D 37 D 8 A 18 B 28 B 38 A 9 B 19 C 29 A 39 C 10 D 20 D 30 C 40 C

2 4E PRELIM EXAM 2024 6092 PAPER 2 SECTION A Qn Answers Marks A1 (a) Si [1] (b) Ar [1] (c) I [1] (d) N/ P/ As [1] (e) Ni [1] (f) S and O / N and O [1] Total : [6] A2 (a) No. of mol of sulfuric acid = 0.0005 mol ; No. of mol of sodium hydroxide = 0.001 mol ; Concentration of sodium hydroxide = 0.001/0.01; = 0.1 mol/dm3 ; ;;[1] Max [2] (bi) 10 cm 3 [1] (bii) ethanoic acid is monobasic while sulfuric acid is dibasic / ethanoic acid produces one mole of hydrogen ions while sulfuric acid produces 2 moles of hydrogen ions per mole of acid. To neutralise the same number of moles of NaOH, twice the volume of ethanoic acid is required compared to sulfuric acid given that the concentration of acid used is the same. [1] (c) A strong acid completely ionises in water while a weak acid partially ionises in water. [1] H2SO4 (aq)  2H+ (aq) + SO42- (aq) CH3COOH (aq) CH3COO- (aq) + H+ (aq) equations [1] ignore state symbols [2] Total : [6]

3 4E PRELIM EXAM 2024 6092 Qn Answers Marks A3 (a) Diamond has the highest resistivity as it does not conduct electricity. All its valence electrons are used in covalent bonding. No mobile/delocalised electrons present to conduct electricity. [1] The three precious metals have low resistivity as they are good conductors of electricity. They have a sea of delocalised/mobile electrons that act as charge carriers in their giant metallic lattice structure. [1] Accept : free electrons [2] (b) No. I do not agree with student’s comment. Graphite’s resistivity value will be closer to the metals because graphite has mobile/delocalised electrons in its structure that makes it a good electrical conductor. [1] (c) Pure metals are too soft to be sold commercially / alloys are harder. [1] In alloys, the different-sized atoms disrupt the regular arrangement of the atoms of pure metal. A larger force is required for the layers of atoms to slide over each other. [1] [2] (d) Resistivity of metals and graphite increases with increasing temperature [1] As temperature increases, metal ions/carbon atoms gain energy and vibrate faster / more vigorously. The increased vibrations (of ions / carbon atoms) interferes with and slows down electron movement. [1] [2] Total : [7] A4 (ai) (aii) general formula : CnH2n-2 any one of the following : They have the same functional group carbon-carbon (C≡C) triple bonds/ Each member differ from the next consecutive member by a -CH2 group/ There is a gradation in physical properties down the series (– melting and boiling point increases as the number of carbon atoms in the molecule increases.) [1] [1] (b) Name : propyne Full Structural Formula : H H – CC ≡ C – H H [1] [1] (ci) Carbon dioxide and water [1] (cii) Yes. Ethyne has a higher percentage of carbon by mass (92.3%) as compared to ethene (85.7%). Hence it is less flammable and gives a sootier flame. [1] Total: [6]

4 4E PRELIM EXAM 2024 6092 Qn Answers Marks A5 (a) P = activation energy/Ea [1] Q = enthalpy change (of reaction)/∆H [1] [2] (bi) energy change is positive/enthalpy change is positive/ energy of products (2NO) is higher than that of reactants (N2 and O2) implying that energy is absorbed. [1] (bii) bond breaking is endothermic/absorbs energy/takes in energy; bond making is exothermic/releases energy/gives out energy. [1] more energy is absorbed than released [1] [2] (ci) Provide alternative pathway with lower activation energy [1] (cii) 2.4/2 = 1.2 dm 3 [1] (ciii) By volume ratio, NO: N2 2 : 1 2.0/2.4 x 100% [1] = 83.3% [1] OR No. of moles of N2 = 1.0/24 = 0.04166 No. of moles of NO reacted= 2 x 0.04166 = 0.08333 [1] No. of moles of NO produced by engine = 2.4/24 = 0.1 % = 0.08333/0.1 = 83.3% [1] [2] Total: [9] A6 (ai) acts as a protective barrier / layer stops air/oxygen and water from getting to the surface (of the iron) [1] (a)(ii) with tin: - iron more reactive than tin, oxygen / water react more readily with iron than tin/iron reacts first (where it is scratched) ; [1] with zinc : - zinc more reactive than iron - zinc is sacrificial metal / idea of sacrificial protection i.e. zinc corrodes more readily than iron / zinc reacts first [1] [2] (bi) has layer of (aluminium) oxide that will not flake off / [1] layer of insoluble / unreactive (aluminium) oxide / layer of impermeable (aluminium) oxide / protective oxide layer / reject: oxide coating without further qualification reject: forms a protective layer with oxygen [1]

5 4E PRELIM EXAM 2024 6092 Qn Answers Marks A6 (bii) correct use ; [1] e.g. drink cans / car bodies / aircraft bodies / high voltage electricity cables / cooking foil / window frames / ladders / ALLOW: cooking utensils / mirrors (as does not corrode) reject: for cutlery correct explanation related to specific use stated ; [1] e.g. drinks cans → will not react with water / acids car bodies → will not corrode aircraft bodies → lightweight / low density electricity cables → lightweight / good conductor of electricity [2] Total: [6] A7 (ai) It reduces hydrogen peroxide to water by removal of oxygen/decreasing the oxidation state of oxygen from -1 to -2/ while itself loses electrons/increase in oxidation state from -1 to 0 and is oxidised to form iodine. [1] (ii) colourless KI turns brown [1] (b) potassium iodide : increase concentration increase rate [1] When concentration of sulfuric acid is kept constant in experiments 1 and 2, and concentration of KI doubles, the speed of reaction doubles. [1] Accept : comparison of data for experiment 1 and 4 / 2 and 4 sulfuric acid : no effect [1] In experiments 1, 3 and 5, when the concentration of KI is kept constant and the concentration of sulfuric acid doubled or tripled, the speed of reaction remains the same. [1] [4] (c) Reactant particles have less energy and move slower at lower temperature Number of collisions lesser and fewer particles have energy greater than activation energy. Frequency of effective collisions decrease, rate decreases. [1] [1] (d) number of protons 53 number of electrons 54 number of neutrons 78 All 3 correct [2] 1 or 2 correct [1] [2] Total: [10]

6 4E PRELIM EXAM 2024 6092 Qn Answers Marks A8 (a) No. of moles of AgCl = 0.287/143.5 = 0.002 Mole ratio from equation AgCl : Ag 1 : 1 No. of moles of Ag = 0.002 [1] Mass of Ag = 0.002 x 108 = 0.216 g [1] OR 143.5 g AgCl contains 108 g Ag [1] 0.287 g AgCl contains 0.216 g Ag [1] [2] (b) copper(I) i