VS 2023 O Level Pure Chemistry P1+P2 Answer Scheme

2023 Answer1 Victoria School2023 Chemistry O Levels / 6092Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20BADBADBDCBABDCCACBACQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40DACDBDBDCCDACBDBCABAPAPER 2 (SECTION A)QuestionAnswerMarkA1aY: 2, Li+Z: VI, Se2- 22bF-.Since number of protons = number of electrons and the mass number is given by total number of neutrons + total number protons. Thus, the mass number of X (i.e. 10+9 = 19) is the same as the relative atomic mass shown on the Periodic Table.11cHeliumdNo observable changeBromine is less reactive than fluorine, thus bromine unable to displace fluorine from its salt solution. [reject if fluoride is used in place of fluorine]11

2023 Answer2 A2aNegative electrode: silverPositive electrode: oxygen gas11bWhen passing through platinum electrodes (metal): there are mobile electrons, hence platinum remains chemically unchanged. When passing through the solution (ionic compound): mobile ions in the solution act as mobile charge carriers. (Thus, at the cathode (negative electrode), Ag+ is selectively discharged to form silver metal. While at the anode (positive electrode), OH- is selectively discharged to form oxygen gas / The electrolyte decomposes.)11ccompound: nitric acid/HNO3 [Reject: H+ ions]explanation: Since Ag+ and OH- were selectively discharged, only H+ and NO3-ions are left remaining in the solution.11diHydrogen gasdiiAll the Ag+ has been preferentially discharged at the start of the electrolysis (gain electrons more readily) and leaving behind only H+ ions in the solution. Hence, H+ions are discharged and hydrogen gas is formed.Reject: silver ions less reactive than hydrogen ions.A3ai4CuFeS2 + 11 O2  4Cu + 2Fe2O3 + 8SO21m for correct reactants and products1m for balanced equation2aiiMass of Cu in 1 tonne of chalcopyrite= = 347.8 kg = 348 kgMass of Cu in tonnes = 0.348 tonnes (3sf)1 for working1biAcid: silicon dioxideBase: iron(III) oxide1biiBasic compound used in the extraction of iron is calcium oxide which is obtained from the thermal decomposition of calcium carbonate, whereas the basic compound used in the extraction of copper is iron(III) oxide.The molten slag formed in the extraction of iron is calcium silicate, instead of iron silicate. [not in syllabus]11cCO2 and H2O [1m each] Reject: hydrogen gasExplanation: 4CuO + CH4  4Cu + CO2 + 2H2O 2

2023 Answer3 A4aThe ratio of atoms of metal to atoms of oxygen decreases [1] from 2 :1 in Li2O to Na2O, to 1 : 1 ratio in Na2O2 and K2O2, to 1 : 2 ratio in KO2 [1]. 2b 1 m for bonded electrons and correct charge1 m for ‘extra’ electron added to each ion2ciO2- 1ciiOne of the oxygen atoms in the superoxide ion does not have a fully filled valence electron shell1diGroup 1 metal hydroxides dissociate in water to form OH- so the solution turns blue.Hydrogen peroxide is a bleach, which is a substance that removes colour/bleaches, so the solution turns colourless.Oxygen is produced (from decomposition of hydrogen peroxide), so bubbles are formed.2m for 3 pointsdii2KO2 + 2H2O 2KOH + H2O2 + O21m for correct reactants and products1m for balanced equation2A5aPhotosynthesis1bA 1cProcess B. (Must state but no marks)

2023 Answer4 Process B consists of a reversible arrow, / reaction can proceed forward or backward./ CO2 (g) dissolves in water to form CO2 (aq) and reversibly to form gaseous CO2 again. 1dCarbon dioxide gas dissolves in water to form aqueous carbon dioxide. Dissolved carbon dioxide vapourises into the atmosphere to form carbon dioxide gas. 1eexothermicendothermicA√C√D√ 1fThe amount of dissolved carbon dioxide liberated as gaseous carbon dioxide through Process B is also the same amount taken in from the atmosphere by phytoplankton through process A which is photosynthesis (carbon cycle) . Hence, there is no net change in the concentration of dissolved and liberated carbon dioxide gas. [1]11A6a O O || ||-N—(CH2)6—N—C—(CH2)4—C- | | H H1bSimilarity: Both monomers should contain a dicarboxylic acid.Difference: monomer 1 is a diamine whereas one of the monomers used to make the polymers in Fig 6.2 is a diol.11ciA macromolecule is a very large molecule that is made up of many small molecules. 1ciiIn condensation polymerisation, the monomers with carboxyl, hydroxyl and amine group combine to form a polymer, with the removal of a small molecule, such as water. In addition polymerisation, the monomers are unsaturated(carbon-carbon double bond) and they join together without losing any molecules or atoms / only single product.11

2023 Answer5 PAPER 2 (SECTION B)QuestionAnswerMarkB7aThe impurities are ethanoic acid and ethyl ethanoate. (state but no marks)Ethanoic acid is formed from the oxidation of ethanol with oxygen in the air in the presence of bacteria.Ethyl ethanoate is formed from the reaction between ethanol and ethanoic acid. 11bEthanol is carbon neutral as it comes from sugarcane. As sugar cane grows, it absorbs carbon dioxide during photosynthesis, which offsets the carbon dioxide produced when ethanol is burnt. It is also renewable as it can be regrown and replaced within a short period time.However, ethanol can be produced from the ethene which comes from non-renewable sources such as crude oil.Separation of ethanol via distillation method requires the burning of fossil fuels to provide heat to boil the liquid mixture. The burning produces carbon dioxide and hence not carbon neutral.11ciE70 contains 70% of ethanol by mass.% by mass of oxygen = 16/(2 x 12 + 6 +16) x 100% = 16/46 x 70% = 24.3%11iiIncreasing the amount of oxygen would result in more complete combustion of the fuel, thus producing less poisonous gas like carbon monoxide. 1dGGE of LPG = 34 200/26 500 = 1.3 [reject 1.29 as data shows 2 s.f.]Comment on storage space: LPG would require more storage space than petrol. 11eMolar mass of octane = 8 x 12 + 18 = 114 g/molMass density of petrol in g/dm3 = 0.75 x 1000 = 750 g/dm3No. of moles of octane in 1 dm3 = mass/molar mass = 750/114 = 6.579 mol1

2023 Answer6 Energy density of octane =5470 x 6.579 = 35 987 kJ ≈ 36 000 kJ/L11B8aIn experiment 1, sulfuric acid, a dibasic acid is used as compared to a monobasic acid(HCl) in experiment 2. Hence the concentration of H+ ions in experiment 1 is twice that of experiment 2. As there are more reactant particles per volume, the frequency of effective collisions increases, rate increases and hence the volume of gas collected is twice higher in experiment 1 than 2.In experiment 3, the HCl used is at a higher temperature than experiment 2. The reactant particles would have a higher kinetic energy hence more particles would have energy greater than or equal to the activation energy. The frequency of effective collisions increases, rate increases and hence the volume of gas collected is higher in experiment 3 than 2. 1111biZn + 2H+  Zn2+ + H2 1biiThe oxidation state of Zn increases from 0 in Zn to +2 in Zn2+. Oxidation occurred.The oxidation state of hydrogen increases from +1 in H+ to 0 in H2. Reduction occurred.Or Zinc loses electrons to form zinc ion(Zn2+). Oxidation occurred.H+ gains electrons to form H2. Reduction occurred.Wrong to say hydrogen is reduced, it should be hydrogen ions/H+ is reduced.1cAdd excess zinc to 25.0 cm3 of 1.0 mol/dm3 of HCl. Measure the volume of gas evolved after 30s.Repeat the experiment by adding 1g of copper added to the same acid. The experiment with copper added will produce a higher volume of gas in 30s.Note: zinc must be added in excess as reaction cannot proceed without zinc as copper cannot react with acidNo need to mention about weighing the mass of copper as it does not refer to the experimental results given in the table.11EITaiHydrogen gas and zinc oxide1

2023 Answer7 HERB9Reject: wateraiiMass of solid increases as zinc gained oxyge