VS 2020 O Level Pure Chemistry P1+P2 Answer Scheme

2020 Answer1 Victoria School2020 Chemistry O Levels / 6092Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20DDAABCBAABAACDBCBDCCQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40CBBADABDDBDCDCDDABABPAPER 2 (SECTION A)QuestionAnswerMarkA1aicalcium hydroxide1aiihydrogen AND iron 1aiiigraphite1aivethanol1avgraphite1avihydrogen AND graphite 1bTrue, True, False, False1A2aUnder the conditions of the car engine, the nitrogen and oxygen molecules gain thermal energy and thus gain kinetic 1

2020 Answer2 energy and move faster. In addition, there are more reactant particles that have the minimum energy for reaction to occur. There are more collisions between the reactant particles, hence increasing the frequency of effective collisions between reacting particles. Thus, rate of reaction increases. 11biequation 1: 2NO + O2 → 2NO2equation 2: 3NO2 + H2O → 2HNO3 + NO11biisulfur dioxide1A3a electron anion (metal) cation cation1 for metal1 for ionicbMetals have a sea of mobile (delocalised valence) electrons that are able to move and carry charge.Ionic compounds are able to conduct electricity in molten and aqueous states as the giant ionic structure collapses and the ions are able to move and carry charge. 11ciIn pure iron, the iron atoms are of the same size and arranged in regular rows. Hence, the layers of iron atoms can slide over each other easily with lesser force, resulting in pure iron being malleable and ductile.In mild steel, the addition of different sized carbon atoms disrupt the regular/orderly arrangement of the metal lattice. Hence, the layers of iron atoms can no longer slide over each other as easily when a force is applied, resulting in iron alloy being stronger and more brittle.11ciiThe table shows that iron atoms have a larger atomic radius than the carbon atoms but are shown in the diagram to be smaller. 2

2020 Answer3 ORThe table shows that there are 3 types of elements in mild steel, but only iron and carbon are shown in the diagram.ORThe percentage by mass of carbon is the diagram is approximately 3.5% but it is stated in the table to be only 0.25%.CHOOSE ANY 2 ANSWERSA4aThe iron can should be made the cathode/connected to the negative terminal of the battery, while a piece of tin should be made the anode/connected to the positive terminal of the battery. Both items should be dipped into the electrolyte which is an aqueous solution containing Sn2+ ions/ aqueous tin(II) nitrate.111bSn(s) + 2H+(aq) → Sn2+(aq) + H2 (g) 1ciPoly(ethene) does not react with dilute acids found in food/is non-biodegradable.1ciiMacromolecule: A large molecule made up of many small molecules succesively joined together. Addition polymerisation: The successive additon of monomers that are joined together to form only one (large) product without any loss of atoms or molecules. 11A5aThe melting point of silicon dioxide is higher than carbon dioxide.Silicon dioxide has a high melting point as it has a giant molecular structure so a lot of energy is used to overcome the strong covalent bonds between the atoms. Carbon dioxide has a low melting point as it has a simple molecular structure so less energy is used to overcome the weak intermolecular forces of attraction.111bacidic oxide: CO2, NO2, P2O5, SiO2 (Choose any one)basic oxide: Li2O, BeO, Na2O, MgO (Choose any one)1ci 1 for cation1 for anion

2020 Answer4 Mg2+O2-1m – cation 1m – anionOO1m – bonding elections1m – non-bonding electrons1 for bonding 1 for non bondingciiIn magnesium oxide, the magnesium atom loses electrons to the oxygen atom forming a cation and an anion respectively. The oppositely charged ions are held together by strong electrostatic forces of attraction/ionic bonds.In oxygen, the 2 atoms share 2 pairs of electrons and are held together by strong electrostatic forces of attraction between the shared electrons and the nuclei of the 2 atoms. 11A6a2M + H2 → 2MH1biAmount of LiH = 8.0 / 8 = 1.00 mol1

2020 Answer5 Amount of NaH = 24.0 / 24 = 1.00 molAmount of KH = 40.0 / 40 = 1.00 molEach reaction produces 1 mole of metal hydride per gram of oxygen reacted.The information in the table matches the statement made by Beth, not Jean.1biiAmount of H2 = 1.0 / 2 = 0.500 mol0.500 mol H2 ≡ 1. 00 mol RbHMass of RbH = 1.00 x (85+1) = 86.0 g Therefore 1 gram of hydrogen will produce 86.0 g of rubidium hydride.As this is an increase of more than 16.0 g from the mass of potassium hydride produced, I do not agree with Ryan. 11cThe potassium would glow even brighter than sodium/burst into flame.Potassium is more reactive than sodium and thus loses electrons more readily. Hence it would give a more vigorous reaction than sodium. 11dThe oxidation state of oxygen decreases from 0 in O2 to -2 in water, hence this is reductionThe oxidation state of sodium increases from 0 in Na to +1 in NaH, hence this is oxidation. 11

2020 Answer6 PAPER 2 (SECTION B)QuestionAnswerMarkB7a30 g of NaCl dissolves in 100cm3 of water300 g of NaCl will dissolve in 1dm3 of waterAmount of NaCl = 300 / (23+35.5) = 5.13 molAny value between 5.1 and 6.8 mol1BSimilarity: Increasing the mass of both NaCl and CaCl2 initially caused the freezing point of water to drop. Difference: Once more than 20 g of salt is added, the CaCl2 continues to cause a drop in the freezing point while the freezing point in the solution with NaCl begins to increase. Difference: On addition of 40g of salt, the NaCl solution becomes saturated, but the CaCl2 can still dissolve. Difference: CaCl2 will eventually increase the freezing point above 0 °CDifference: CaCl2 gives a lower freezing point of -45 °C as compared to NaCl which only reduced the freezing point to -20 °C.(Pick any 2 differences)111cHygroscopic de-icers attract water vapour from the air that allows them to form a solution on the surface of the ice, allowing the de-icer to dissolve even when there is little liquid water available.De-icers that dissolve exothermically are able to release heat when they dissolve and melt the ice under the layer of water, allowing them to work deeper into the ice.2d 1 for both Ea1 for both ΔH1 for both general shape and equation

2020 Answer7 eUrea does not speed up the corrosion of metal.ORUrea is non-toxic to plants1fChloride (Cl-). All de-icers that are harmful to plants contain chloride ions.1B8aX,Y and Z all have the same empirical formula and the same Mr, meaning that they have the same molecular formula, but different structural formula as shown in the table.1bIsomer X Functional Group: carboxyl1 for each structure1 for both carboxyl 1 for ester

2020 Answer8 Isomer Y Functional Group: carboxylIsomer Z OR Functional Group: estercEffervescence of colourless gas observed due to formation of hydrogen gas.The rate of effervescence would be the same for both reactions as they produce solutions of the same pH at the same concentrations.11EITHERB9aTable 9.1 shows that all the bonds have different bond lengths, but in the ball and stick diagram, the bonds are all shown to be the same length.Table 9.2 shows that all the atoms have different atomic radii, but all the atoms shown in the ball and stick model are 1

2020 Answer9 shown to be the same size.In the dot and cross diagram the C-C and C-O bonds are shown to have the same bond length as do the C-H and O-H bonds. However, table 9.1 shows that these bonds have different bond lengths.11biC2H6O 1biiThe molecular formula of ethanol cannot be further simplified. 1ciC2H4 + H2O C2H5OH Amount of ethene = 1000 / 24 = 41.67 mol41.67 mol C2H4 ≡ 41.67 mol C2H5OHAmount of ethanol = 41.67 mol Mass of ethanol = 41.67 x (12x2 + 1x6 + 16) = 1916.82 gYield of ethanol = 1916.82 x 5/100 = 95.841 g ≈ 95.8 g1111ciiUnreacted ethene is pumped back into the reactor to be reacted again. 1ORB10aiAtom economy = (14x2)+(1x8)+32+(16x4)_______ (14x2)+(1x8)+12+(16x3)+40+32+