VS 2021 O Level Pure Chemistry P1+P2 Answer Scheme

2020 Answer1 Victoria School2021 Chemistry O Levels / 6092Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20BBAABDCCCCDAADABDCDAQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40BDCBCCCADBBCBABCDACBPAPER 2 (SECTION A)QuestionAnswerMarkA1aipotassium and chlorine1aiichlorine1aiiicopper1aivaluminium1avpotassium and chlorine1bTrue, False, True, True3A2aPetroleum is heated and vapourised in a furnace before it enters the fractionating column.The vapours consist of different fractions, which have different boiling points.The fractionating column is cooler at the top than at the bottom. As the vapours rise up the column, they will cool and condense. Each fractions will condense at different temperatures.The smaller hydrocarbon, propane, has a lower boiling point will be distilled and collected at the top of the fractionating column while the bigger hydrocarbon, octane, has a higher boiling point and will be collected lower in fractionating column.111bPropane and butane are of similar sizes and have very similar boiling points. 1

2020 Answer2 ccompoundmolecular formulaempirical formulapropaneC3H8 C3H8octaneC8H18C4H9 2diOctane has a greater number of carbon and hydrogen atoms than butane. More carbon dioxide and water are formed in the combustion of octane than butane, hence more energy is released in forming of the bonds in water and carbon dioxide than absorbed in breaking the bonds in octane and oxygen.Energy is released in forming 8 more moles of carbon dioxide and 8 more moles of water in octane than butane. The This more than the energy absorbed in the breaking of bonds in the alkanes and oxygen. 11diinumber of moles of 50 kg octane = 50000 / 114 = 438.60 moltotal energy released from 50kg of octane = 438.60 x 5470 = 2399122 kJnumber of moles of butane required to produce energy = 2399122 / 2880 = 833.03 molmass of butane required = 833.03 x 58 = 48300 g = 48.3 kg111A3a1: hydrogen, oxygen2: hydrogen, chlorine4: copper, copper(II) ions2bIn experiments 1 and 2, carbon electrodes that were inert were used at the positive electrodes. This allowed anions in the solutions to be preferentially discharged and oxidised, forming substances like oxygen and chlorine.In experiment 4, copper electrodes that were not inert were used at the positive electrode. This caused the copper electrode to oxidise, forming copper(II) ions.11ciReddish brown solid is deposited on the negative electrode.1cii4OH-(aq)  O2(g) + 2H2O(l) + 4e- 1ciiiBlue solution turns colourless.The copper(II) ions were preferentially discharged at the negative electrode and concentration of copper(II) ions decreases. 11A4acalcium silicate1

2020 Answer3 CaSiO3biMaximum mass of iron = 112 / 160 x 14000 = 9800 tonnes3biiIron ore is a mixture containing iron(III) oxide mixed with impurities like sand and clay.Solid iron(III) oxide cannot be reduced completely with carbon monoxide.Iron could be oxidised back to iron(III) oxide due to presence of oxygen and high temperatures in the blast furnace.Any 2 of the above.11cNitrogen is present from the hot air used to provide oxygen to react with coke.Carbon dioxide is formed from coke reacting with oxygen in the hot air and decomposition of limestone.Carbon monoxide is formed from the reaction between carbon dioxide with coke. 111dPure iron is soft and adding carbon will form the alloy steel, which is stronger and harder. Carbon added is of a different atomic size from iron and it disrupts the orderly arrangement of the layers of iron atoms. It is more difficult for the layers of atoms to slide over one another.Amounts of carbon added is carefully controlled as higher amounts of carbon will make steel brittle. 11A5a OHHNCNHH 1m: bonding electrons between N and H1m: bonding electrons between N and C + C and O1m: non-bonding electrons3 biAt a high temperature, the aqueous solution of DEF will react with water vapour to form the gases, NH3 and CO2. As gases occupy a greater volume than liquids, this increases the pressure in the tank.1biiAmmonia produced from DEF can remove oxides of nitrogen, NO and NO2, from the exhaust gas, reducing acid rain formation which would otherwise harm aquatic life and plants.Removing oxides of nitrogen would (decrease photochemical smog and) also reduce respiratory difficulties. 11

2020 Answer4 biiiThe DEF does not remove carbon dioxide from the exhaust gas. Carbon dioxide produced traps excess heat in the atmosphere which causes global warming. Global warming causes ice bergs at the poles to melt, sea level to rise and leading to floods in lowlands. 11biv4(NH2)2CO + 6NO2  7N2 + 8H2O + 4CO2 2cnumber of moles of urea = 160 / 60 = 2.6667 molconcentration of urea = 2.6667 / 0.500 = 5.33 mol/dm3 11PAPER 2 (SECTION B)QuestionAnswerMarkB6aiHydrochloric acid and ethanoic acid fit the general formula, HA. As hydrogen ion has the formula of H+, the formula of the anion would be A-. Hence, Cl- and CH3COO- in hydrochloric acid and ethanoic acid can be represented by A-.Sulfuric acid does not fit the general formula as the sulfate ion has a formula of SO42- and the charge on A- is incorrect.11aii2HA + M2CO3  CO2 + H2O + 2MA1aiii2CH3COOH + Ca  (CH3COO)2Ca + H2As the positive charge on calcium is 2+, x = 2when x =2, Equation 2 will be 2HA + M  MA2 + H2 11biThe relationship shows that the initial concentration of the acid is inversely proportional to the pH, which is incorrect. It is expected for pH of different acids to be the same when initial concentrations of the acids are the same.Hydrochloric acid, sulfuric acid and ethanoic acid at the same concentration of 0.01 mol/dm3 have a pH of 2.0, 1.7 and 3.4 respectively, showing that there are other factors that affect pH of an acid.11biiThe strength of the acid affects the pH of the acid. Hydrochloric acid is a strong monobasic acid, while ethanoic acid is a weak monobasic acid. At acid concentration of 0.01 mol/dm3, the complete dissociation of hydrochloric acid produces a higher concentration of hydrogen ions (0.01 mol/dm3) than the partial dissociation of ethanoic acid (0.0004 mol/dm3) and a lower pH.The basicity of the acid also affects the pH of the acid. Sulfuric acid is a strong dibasic acid, while hydrochloric acid is a strong monobasic acid. At acid concentration of 0.01 mol/dm3, the complete dissociation of sulfuric acid (0.02 mol/dm3) produces double the concentration of hydrogen ions compared to the complete dissociation of hydrochloric acid (0.01 mol/dm3) and a lower pH.1m strength, 1m basicity, 1m higher concentration + lower pH3

2020 Answer5 biii0.04, 1.40.10, 1.011(CH2)4COOO(C6H4)COB7aiRelative molecular mass of repeating unit = 16 + 4(14) + 16 + 28 + 76 + 28 = 2202aiiOOHHCOCOCHHCHH 2biThere are ester linkages joining the monomers together.1biiBoth polymerisation processes involve only one type of monomer.Condensation polymerisation of molecule C eliminates a simple molecule like water while addition polymerisation of ethene does not eliminate any molecule.Condensation polymerisation of molecule C involves monomers with functional groups at each end of the molecule while addition polymerisation of ethene involves unsaturated monomers. 111EITHERB8aAluminium is the most reactive, followed by carbon and silicon.Carbon is more reactive than silicon as silicon dioxide can be reduced by carbon. Aluminium is more reactive than carbon as aluminium oxide cannot be reduced by carbon. 11bAluminium oxide has a giant ionic structure with strong ionic bonds between oppositely charged ions. A large amount of energy is needed to overcome the strong ionic bonds in aluminium oxide and it has a high melting and boiling point and exists as a solid with high density.In molten state, the giant ionic structure collapse and the ions are free to move to conduct electricity.Silicon dioxide has a giant molecular structure with an extensive network of strong covalent bonds between the atoms. A large amo