Punggol Prelim P2 Ans

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 1 Section A ( 70 marks) 1 Suggested solution accept reject (a) ZnCl2 [1] (b) PbO [1] (c) V2O5 [1] (d) (i) NH4Cl [1] (ii) NaOH + NH4Cl → NaCl + H2O + NH3 [1] 2 Suggested solution accept (a) Isotopes are atoms of the same element with the same number of protons but different number of neutrons. [1] (b) (i) particle number of protons number of electrons number of neutrons G1 16 16 16 G2 16 18 18 G3 16 16 20 [2]: 5 correct [1]: 1 to 4 correct (ii) Nuclide notation of atom G2: S16ଷସ [1]: chemical symbol, proton number and mass number. 3 Suggested solution (a) (i) gases time/s relative molecular mass carbon monoxide, CO 119 28 chlorine, Cl2 190 71 methane, CH4 90 16 oxygen, O2 127 32 [1] All correct (ii) Time taken = k x Mr For CO, k = 119 / 28 = 4.25

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 2 For Cl2, k = 190 / 71 = 2.68 For CH4, k = 90 / 16 = 5.63 For O2, k = 127 / 32 = 3.97 The measurements do not support the conclusion. [1] The constant, k, is not the same for the gases. [1] Award 2nd mark if student compares at least two k values. (b) Accept any time between 127 seconds and 190 seconds. [1] Sulfur dioxide is soluble in water so the water level rises faster than pre- dicted. [1] 4 Suggested solution (a) Cu2O is oxidised to CuSO4 because the oxidation state of copper increases from +1 in Cu2O to +2 in CuSO4. [1] Cu2O is reduced to Cu because the oxidation state of copper decreases from +1 in Cu2O to 0 in Cu. [1] This is disproportionation as Cu2O is oxidised and reduced simultaneously. [1] (b) (i) Element Cu O % by Mass 84.1 15.9 Number of moles/ mol ଼ସ.ଵ ଺ସ = 1.3141 ଵହ.ଽ ଵ଺ = 0.99375 Mole ratio (divide by the smallest number) ଵ.ଷଵସଵ ଴.ଽଽଷ଻ହ = 1.3224 ଴.ଽଽଷ଻ହ ଴.ଽଽଷ଻ହ = 1 X3 X3 Simplest ratio 4 3 Empirical formula is Cu4O3 [1] Working + Empirical formula Let the molecular formula of paramelaconite be (Cu4O3)n. (4 x 64 + 3 x 16)n = 304 n = 1 Working to prove that the molecular formula of paramelaconite is Cu4O3. [1] (ii) Cu4O3 + 2H2SO4 → Cu2O + 2CuSO4 + 2H2O [1] 5 Suggested solution (a) Atoms of the different metals have different sizes. The regular layers of atoms are disrupted. [1] The layers of atoms cannot slide over each other easily when a force is applied. [1]

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 3 (b) (i) Insert a lighted splint into the gas. Lighted splint extinguishes with a ‘pop’ sound. [1] (ii) Chromium sulfate is insoluble in water. [1] A layer of insoluble chromium sulfate will form over the surface of chromium metal and prevent the reactants from reacting further. [1] (iii) Add aqueous chromium nitrate to sulfuric acid. Filter to collect chromium sulfate precipitate. Wash the precipitate with a small amount of cold water and dry between filter paper. [1] Two soluble reactants that form chromium sulfate [1] Filter + Wash + Dry (c) Layer of chromium(III) oxide prevents oxygen and water from coming into contact with iron in stainless steel. [1] (d) (i) Chromium reacts faster with acid than iron. [1] Chromium is more reactive than iron. [1] (ii) Iron will not rust as chromium is more reactive than iron, chromium will corrode instead of iron. [1] 6 Suggested solution (a) (i) Iron(III) chloride is used up in step 1 and regenerated in step 2. [1] (ii) Iron(III) chloride provides an alternative pathway with a lower activation energy. [1] As a result, there are more particles with energy equal to or greater than the activation energy. The frequency of effective collisions increases. Thus, the rate of the reaction increases. [1] (b) (i) Both electrons come from nitrogen atom instead of one electron from each of the bonded atom. [1] (ii) Simple molecular structure [1] (i) [1]

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 4 7 Suggested solution (a) The energy absorbed to break 10 C-H, 2 C-C, 2 C-O and 7 O=O bonds is smaller than the energy released to form 8 C=O and 12 O-H bonds. [1] List the bonds correctly [1] List the magnitude of energy correctly (e.g. smaller) (b) (i) enthalpy change for the reaction for one mole of ethanol = − (2736 /2) = − 1368 kJ/mol [1] (ii) [1] Shape of curve + Label ∆H and Ea correctly [1] Label reactants and products correctly (c) (i) Sugarcane absorbs carbon dioxide during photosynthesis which offsets the carbon dioxide produced when bioethanol is burnt. [1] (ii) Ethene is obtained from crude oil and is finite. Ethene cannot be replaced quickly enough to keep up with our rate of use. [1] Sugarcane can be replaced relatively quickly after use. [1] (iii) Large areas of land are needed to grow sugarcane for the extraction of bioethanol. This may compete with the use of land for growing other food crops. [1] Or Some processes used in growing sugarcane such as use of machinery may require use of fossil fuels. More carbon dioxide may also be produced in these processes. Accept other reasonable answer. 8 Suggested solution (a) (i) X : Respiration Y: Photosynthesis [1] Both X and Y are correct (ii) The balance between the processes that produce carbon dioxide such as respiration and the processes that remove carbon dioxide such as photosynthesis regulates the amount of carbon dioxide in the atmosphere. [1] Process such as respiration add carbon dioxide into the atmosphere and process such as photosynthesis removes carbon dioxide from the atmosphere [1] Idea of a balance between respiration with photosynthesis (b) Carbon dioxide contributes to global warming, which causes melting of polar ice cap which leads to rising sea levels which leads to flooding of low land. [1]

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 5 9 Suggested solution (a) (i) This minimises wastage and reduces operating costs. [1] Must state one reason for recycling to award the mark. (ii) Fractional distillation [1] (b) (i) CH2=CH2 (g) + H2O (g) → CH3CH2OH(l) Number of moles of C2H4 = (10 x 1000) / (2 x 12 + 1 x 4) = 357.14 mol [1] Number of moles of CH3CH2OH = 357.14 mol Theoretical yield of CH3CH2OH = 357.14 x (12 x 2 + 1 x 6 + 16) = 16428.6 g = 16.4286 kg % yield = (8.4 / 16.4286) x 100 = 51.1% [1] (ii) There are impurities present in ethene used to produce ethanol. [1] (c) (i) Substitution [1] (ii) [1] (iii) Both polymers have macromolecular structure with intermolecular forces of attraction between the polymer chains. [1] The size of poly(dichloroethene) is bigger than poly(ethene) and more energy is needed to overcome the stronger intermolecular forces of attraction between the polymer chains. [1] (iv) Cracking forms shorter chain alkanes which can be used as fuel. Fuels are more useful and in higher demand. [1]

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 6 10 Suggested solution (a) Concentration of Ca2+ ions in mol/dm3 = 62.1 / 40 = 1.55 mol/dm3 The hardness classification is soft and is not ideal for aquarium water. [1] working and conclusion (b) (i) Cl2 + 2S2O32  2Cl + S4O62 [1] (ii) Volume of water = 360 / 2 = 180 dm3 Mol of Cl2 = (0.023 / 71) x 180 = 0.058310 mol Cl2 : S2O32 1 : 2 Mol of S2O32 needed to react with Cl2 = 2 x 0.058310 = 0.11662 mol [1] Mol of NH2Cl = (0.020 / (14 + 2 + 35.5)) x 180 = 0.069903 mol NH2Cl : S2O32 1 : 2 Mol of S2O32 needed to react with NH2Cl = 2 x 0.069903 = 0.13981 mol [1] Total number of mol of S2O32 = 0.11662 + 0.13981 = 0.25643 mol Total volume of S2O32needed = 0.25643 / 0.010 = 25.6