Yuan Ching Prelim Ans

Yuan Ching Secondary School Preliminary Examination 2024 Secondary Four Express Chemistry (Revised) 6092 Paper 1 Answer Key 1 D 21 A 2 D 22 D 3 C 23 B 4 C 24 C 5 C 25 A 6 C 26 B 7 A 27 A 8 D 28 D 9 D 29 D 10 D 30 A 11 C 31 C 12 A 32 D 13 C 33 D 14 C 34 C 15 D 35 C 16 D 36 D 17 C 37 A 18 B 38 D 19 B 39 D 20 B 40 D

Chemistry (Revised) 6092 Paper 2 Marking Scheme Section A (70 marks) S/No. Answers Marks 1 (a) C 1 (b) A 1 (c) B 1 (d) D 1 2 (a) Methane, it has the smallest / lowest relative molecular mass than the rest of the gases 1 (b) Temperature affects the kinetic energy of the particles. At higher temperatures, particles move faster and further apart (ORA) Keeping the temperature at a constant ensures that particles have the same average kinetic energy. 1 1 (c) 132s 1 (d) Ammonia is soluble in water. Hence it will back into the way of the graduated cylinder. 1 3 (a) Y is the weak acid Reference to Universal indicator colour to pH (i.e orange indicator a higher pH than red) Reference to rate of gas produced in 10s (i.e. rate of reaction is slower as it produces less hydrogen in 10s compared to X) ALLOW: infer lower concentration of hydrogen ions is lower in acid Y as compared to acid X. [1m only] MUST quote data (“use information from table”) 1 1 1 (b) no. of mol of NaOH produced = 221000 x 0.100 = 0.0022 mol by mol ratio, H2A : NaOH 1 : 2 0.011: 0.0022 Concentration of tartaric acid = 0.0011 / 0.025 = 0.044 mol/ dm3 1 1 1

4 (a) halogen Colour and state at room temperature and pressure chlorine Yellow-green gas bromine Red-brown liquid iodine Purple-grey solid [1m] correct states [1m] correct colours [MAX 1m]: 2 correct states & colours 2 (b)(i) Chlorine is more reactive than bromine in Group 17, chlorine displaces bromide from aqueous sodium bromide solution, causing aqueous bromine to form which is brown in colour. 1 (b)(ii) When a jet of iodine gas is aimed at a filter paper soaked in a solution of sodium bromide. There is no displacement reaction as iodine is less reactive than bromine. Filter paper turns brown slowly as iodine gas dissolved in the aqueous solution to give a brown solution When a jet of bromine gas is aimed at a filter paper soaked in a solution of sodium iodide. Filter paper turns brown as iodine is less reactive than bromine hence bromine displaces iodine from sodium iodide, causing aqueous iodine to form which is brown in colour. [1m]: brown colour for iodine in aq. state in both cases [1m]: explain no displacement reaction for iodine gas with bromide salt [1m]: displacement reaction takes place for bromine gas with iodide salt 3 (c) Chlorine has a strong tendency to gain electrons from other elements to form a negatively charge ion. This causes itself to be reduced and hence an oxidising agent. ALLOW: reverse argument 1 5 (a) 1

(b) Graph a is steeper than graph d hence speed of reaction in a is faster than in d. In d, particle size is bigger, hence there is less surface area to volume ratio. Hence frequency of effective collision between reacting particles decreases, resulting in slower reaction. ALLOW: reverse explanation as well 1 1 1 (c) Experiment B 1 (d) 3 6 (a)(i) Hydrogen, H2 = 256 ÷ 2 = 128 kJ/ g 1 g of hydrogen releases 128 kJ of energy Methanol = 715 ÷ 32 = 22.3 kJ / g 1 g of methanol releases 22.3 kJ of energy 1 1 (a)(ii) 1. From the calculated results, 1 g of hydrogen produces/releases more energy (128kJ) than 1 g of methanol (22.3kJ). 2. From the melting and boiling point, hydrogen is a gas while methanol is a liquid at room temperature. Hence, for the same volume of fuel used, there are more particles per unit volume when burning ethanol than hydrogen. 3. Also being a fuel gas, hydrogen is highly flammable and difficult to store. Pressurised cylinder is needed to contain liquefied hydrogen while methanol is much easier to handle and store. 1 1 1

(b) [1m] enthalpy change, single arrow down with -715kJ/mol; [1m] activation energy, single arrow up from reactants. [1m] shape of profile diagram as exothermic REJECT: [-1m] no labels of axes and/or methanol, oxygen and products of this reaction 3 7 (a) It is described as substitution reaction because the hydrogen atoms in methane is replaced by halogen atoms. It is photochemical because it requires (ultraviolet) light to initiate the reaction. 1 1 (b) 1 (c) The ClO particle produced in step 1 reacts with an oxygen atom in step 2 to produce another Cl atom. This Cl atom is then able to start step 1 again. This is a chain reaction. 1 1 (d) Ozone absorbs out excessive ultraviolet radiation that could lead to increased incidence of skin cancer. 1 8 (a)(i) It is an ester (contains structure contains ester linkage), and esters are generally sweet-smelling ALLOW: has the lost relative molecular mass, it is the most volatile compound / diffuses fastest 1 (a)(ii) [1m] for correct structure from acid [1m] for correct structure from alcohol REJECT [-1m] not displayed formula 2

(b)(i) Y has a smaller molecular mass than Z and should be more volatile than Z. OR Y has a smaller molecular mass than Z hence it diffuses faster than Z. OR Y is an alcohol as it contains the hydroxyl group hence it is volatile. 1 (b)(ii) Heat each compound with acidified potassium manganate(VII). Y will turn acidified potassium manganate(VII) from purple to colourless, but Z will not. 1 1 9 (a)(i) Electronegativity decreases down Group 2 elements. As the atomic size increases, the attraction between the nucleus and valence electrons is weaker, making the atom less likely to attract electrons. 1 1 (a)(ii) Any value between 0.82 to 0.98 (actual 0.93) 1 (b) Helium, neon and argon have completely filled valence electron shells and they are unable to form bonds with other elements. ALLOW: these elements are noble gases and noble gases are unreactive [1m MAX] 1 1 (c) Σ of Al2O3 = 3.44 – 1.61 = 1.83 Σ value of Al2O3 does not support. Al2O3 is an ionic compound but its Σ is less than 2.0. REJECTΣ value of AlCl3 supports the bonding in AlCl3. 1 1 (d) Student 1 No. For example, Σ value (H-F) / bond energy of H-F = 1.78/562 = 0.00317 and Σ value (H-Cl) / bond energy of H-Cl = 0.96/431 = 0.00223 correct set of working to prove ( i.e. Σ value (H-F) / bond energy) [1]; do not give the same constant value. [1] ACCEPT: Other examples with single covalent bonds. Student 2 No. From table 9.1, there is only one Σ value for CO bond and it cannot be used to predict the bond strengths of C-O, C=O or C≡O (cannot tell the difference between the type of bonds in CO and hence the strengths). OR From table 9.1,Σ value for CO bond is zero, but the bond energy is 432 kJ/mol which is about the same bond energy as HF bond but it’s Σ value is higher at 1.78 2 1

(e)(i) Nitrogen is more electronegative than hydrogen and the bonding electrons should be closer to the nucleus of the nitrogen. OR Dot-and-cross diagram does not show that nitrogen atom having a higher tendency to attract the bonding pair of electrons to itself in the chemical bond. 1 (e)(ii) 1 10 (a) The electrodes and copper wires have free moving electrons [1m]; to conduct electricity whereas the electrolyte has free moving ions to conduct electricity. [1m] 2 (b) Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g) Or Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ACCEPT: If no state symbols are written. REJECT: If state symbols are written incorrectly. 1 (c)(i) Ag+