VS 2015 O Level Pure Chemistry P1+P2 Answer Scheme

2015 Answer1 Victoria School2015 Chemistry O Levels / 5073Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20BDDBCBAABDCCDCABADCBQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40DBDBBCDCAAADBABBCBCAPAPER 2 (SECTION A)QuestionAnswerMarkA1aA – non-metalB – metalD – metalE – non-metal 1biC 1biiA 1biiiB 1cThe elements in the table form different compounds by gaining or losing electrons, hence they have different oxidation states. For example, Element A has an oxidation state of -2 as it gains two electrons to form an ion of charge 2-. Since, Group 0 elements have a fully filled valence/outermost electron shell, they do not bond with any other elements to form compounds, hence do not have any oxidation states. 11

2015 Answer2 A2aiAs the reactivity of the metal decreases, the temperature needed for the reduction of its oxide decreases. 1aii700°CIron is less reactive than zinc but more reactive than lead.Hence the temperature needed to reduce iron oxide with carbon is lower than 900°C but higher than 400°C. 11bMetal: MagnesiumMetal oxide: Silver Oxide1cZinc oxide andHydrogen11A3aName of compoundStructural formulaProcess(es) used to manufacture the compound ethene Fractional distillation of crude oil / petroleum followed by catalytic crackingpoly(ethene) Addition polymerisation of etheneethanol Two separate processes can be used. Process 1: Catalytic addition of steam to ethene. Process 2: Fermentation of glucose 112

2015 Answer3 ethanoic acid Oxidation of ethanol1biBoth acids react with metal oxides to form salt and water only in a neutralisation reaction. 1biiDilute ethanoic acid is a weak acid while dilute hydrochloric acid is a strong acid. Hence, ethanoic acid undergoes partial dissociation when dissolved in water but hydrochloric acid undergoes full dissociation when dissolved in water. The concentration of hydrogen ions is lower in ethanoic acid than hydrochloric acid, thus lower frequency of effective collisions between the hydrogen ions and metal oxide particles so the rate of reaction in ethanoic acid is slower than in hydrochloric acid.11A4a600 – 700 °C1bExtraction of aluminium from bauxite requires the ionic solvent to be heated to 900 °C compared to recycling used cans which only requires heating to 700 °C, hence more heat energy is required for extraction from bauxite. Also, extraction of aluminium from bauxite makes use of electrolysis which requires electrical energy. Both processes increase the use of energy in extraction from bauxite. 11cConcentrated sodium hydroxide is very caustic/corrosive and causes harm to living tissues, such as chemical burns and skin irritation.Carbon dioxide is a greenhouse gas which traps excess heat in the atmosphere. This leads to global warming which will result in ice caps melting, sea levels rising and causing flooding in low lying areas. 11dAluminium is a finite resource and hence recycling helps to conserve natural resources / Land will be free for other purposes such as agriculture and housing with a decrease in mining of bauxite / Fewer landfills will be required for disposal of waste materials generated from the extraction of aluminium from bauxite. [Any one possible answer]1eiAlloy 1

2015 Answer4 eiiDuralumin is stronger and harder than pure aluminium so the aircraft will be able to withstand the high pressure. 1A5aAt the cathode: 2H+ + 2e  H2At the anode: 4OH-  2H2O + O2 + 4e11biFor every 4 moles of electrons that passes through the set-up,2 moles of hydrogen gas and 1 mole of oxygen gas are produced. Hence the theoretical ratio of hydrogen to oxygen is 2:1. 11biiOxygen is soluble in water. Hence some of the oxygen gas produced will dissolve in water, resulting in the volume of oxygen collected to be smaller than expected.The ratio of hydrogen to oxygen will become higher than 2:1.11biiiNo more oxygen can dissolve in water, hence the volume of oxygen collected increases and the difference from the expected ratio becomes smaller after some time. 1cThe concentration of sodium chloride increases. Hydrogen and hydroxide ions are being discharged, which is essentially the electrolysis of water. Hence, the volume of water decreases and concentration of sodium chloride increases. 1dSimilarity: Hydrogen gas is produced at the cathode for electrolysis of both dilute and concentrated sodium chloride. Difference: Oxygen is produced at the anode for electrolysis of dilute sodium chloride but chlorine gas is produced at the anode for the electrolysis of concentrated sodium chloride. 11ePlatinum is an unreactive metal and hence it is an inert electrode. It does not react with the products formed at the electrodes. 1A6aLimestone is insoluble in water but quicklime reacts exothermically with water to form an alkaline solution. Slaked lime is an alkali which is formed by adding water to quicklime.In the aqueous state, quicklime and slaked lime can dissociate into hydroxide ions to neutralise the acidity quickly. 11

2015 Answer5 biBlast furnace slag is a waste material and hence by using it to make products, it is more economical and incurs less cost. 1biiGroup IV and Group V oxides are acidic oxides as they are non-metal oxides, hence they do not react with acids to neutralise the acidity. 11cPercentage by mass of calcium in quicklime:= 40 / (40 + 16) x 100% = 71.4% (to 3 s.f.)Percentage by mass of calcium in limestone:= 40 / (40 + 12 + 3 x 16) x 100% = 40.0% (to 3 s.f.)Percentage by mass of calcium in slaked lime:= 40 / (40 + 2 x 16 + 2 x 1) x 100%= 54.1% (to 3 s.f.)Hence, quicklime has the highest percentage by mass of calcium.111dCaCO3  CaO + CO2No. of moles of limestone = (25 x 1000 x 1000) / (40 + 12 = 3 x 16) = 250000 molMole ratio of limestone : carbon dioxide = 1 : 1 No. of moles of carbon dioxide = 250000 molVolume of carbon dioxide = 250000 x 24 = 6000000 dm3 111PAPER 2 (SECTION B)QuestionAnswerMarkB7aAs the number of carbon atoms in each alkane molecule increases, the boiling point increases. As the number of carbon atoms in each alkane molecule increases, the melting point generally increases, except for the 1

2015 Answer6 melting point of propane (-188 °C). bPropane.The melting point of propane should be higher than -183°C but its melting point is -188°C, which is lower.1cMelting point: -80°C (between -76°C to -92°C)Boiling point: 98°C (between 96°C to 104°C)1dAs the number of carbon atoms in each alkane molecule increases, the boiling point and flashpoint increase. 1eAs the alkane becomes more branched, the flashpoint decreases / Isomerism decreases the flashpoint.The flashpoint of straight chain pentane is -49°C but in branched alkane 1, where there is one branch, the flashpoint decreases to -57°C. In branched alkane 2, where there are two branches, the flashpoint decreases further to -65°C.Similarly, straight chain octane has a flashpoint of 13°C but branched chain alkane 3 has a flashpoint of 4°C, which is lower. 1111fiPentane and branched alkane 2 have lower flashpoints than octane so they will decrease the flashpoint of the mixture. In cold countries, where the temperature is low, the mixture will burn more easily at low temperatures. Hence this mixture works better than pure octane alone. 11fiiIn hot weather, the temperature is high so the winter blend petrol will evaporate easily.Since the petrol tank is not airtight, the petrol vapour will escape faster than it can burn, resulting in increased loss of petrol.11B8aThe lean burn engine will result in a lower amount of carbon monoxide and nitrogen monoxide formed compared to a normal car engine. 1

2015 Answer7 With a higher amount of air in the mixture, there will be a larger amount of oxygen so there is more complete combustion. Hence, less carbon monoxide will be formed due to incomplete combustion. Nitrogen reacts with oxygen in the engine under high temperatures to form nitrogen monoxide. With a lower running te