VS 2017 O Level Pure Chemistry P1+P2 Answer Scheme

2017 Answer1 Victoria School2017 Chemistry O Levels / 5073Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20DCABCABDCCDCCADCCADAQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40ACADBDBBDDCCCACABACBPAPER 2 (SECTION A)QuestionAnswerMarkA1aiFe 1aiiH2 1aiiiCu 1aivO2 1avZn 1biCu, Fe, Zn1biiHigh melting and boiling pointsTransition metals and their compounds are good catalysts2

2017 Answer2 Forms coloured compoundsVariable oxidation states in their compounds[any 3 points – 2 marks, 2 points – 1 mark, 1 point – 0 marks]A2ai(aq), (aq), (s), (aq)[any 2 correct – 1 mark]2aiiFor reaction 1:The oxidation states of all the elements remain unchanged, hence this is not a redox reaction. For reaction 2, The oxidation state of X increases from -1 in AgX to 0 in X2. This is oxidation. The oxidation state of Ag decreases from +1 in AgX to 0 in Ag. This is reduction. Since oxidation and reduction occurs simultaneously, this is a redox reaction.11biyellow1biiAs the reactivity of the halogen increases, the rate of breakdown of the silver halide increases. 1A3aCombustion 1biphotosynthesis: 6CO2 + 6H2O  C6H12O6 + 6O2respiration: C6H12O6 + 6O2  6CO2 + 6H2O 11biiDuring photosynthesis, plants take in carbon dioxide from the atmosphere and decreases the amount of carbon dioxide in the atmosphere. During respiration, carbon dioxide is given out by plants and animals into the atmosphere and increases the amount of carbon dioxide in the atmosphere. 1cIn the long term, trees will die and undergo bacterial decay. The bacteria will break down the carbon containing compounds in the trees and release carbon dioxide into the atmosphere, which will contribute to the increase in the amount of carbon dioxide.11A4aiCatalytic cracking1

2017 Answer3 aiiThe heavier and longer chain hydrocarbons such as diesel and lubricating oil are in low demand but in large supplies. The lighter and shorter chain hydrocarbons such as naphtha and petrol are in high demand but in low supplies. Cracking helps to break down the heavier and longer chain hydrocarbons into lighter and shorter chain hydrocarbons to meet the demand for lighter and shorter chain hydrocarbons.11b [Any 2 of the above. 1 mark each] 2 creagentobservation for ethaneobservation for propeneaqueous bromineno visible reaction / solution remains reddish brownreddish-brown solution turns colourless / decolourises rapidly[3 correct – 2 marks, 2 correct – 1 mark, 1 correct – 0 marks]2diSimilarity: Both vegetable oils and alkenes contain carbon-carbon double bonds.Difference: Alkenes contain one carbon-carbon double bond but vegetable oils can contain more than one carbon-carbon double bond. 11

2017 Answer4 diiTo produce margarine, hydrogen is added to vegetable oils at a temperature of 200 °C and a nickel catalyst. [3 points – 2 marks, 2 points – 1 mark, 1 point – 0 marks]2A5aMost reactive: magnesium chromium cobaltLeast reactive: copper 1bmetal sulfatecolour of solutioncopper(II) sulfatebluecobalt(II) sulfatepinkchromium(III) sulfategreen[3 correct – 2 marks, 2 correct – 1 mark, 1 correct – 0 marks]2cichromium + cobalt(II) sulfatecobalt + chromium(III) sulfateH, enthalpy changeEa, activation energyenergy / kJprogress of reaction1m – correct shape of energy profile and label names of products 1m – Indicate and label activation energy1m – Indicate and label enthalpy change 3

2017 Answer5 cii2Cr(s) + 3Co2+(aq)  2Cr3+(aq) + 3Co(s)1m – balanced ionic equation1m – state symbols2dCalcium is a very reactive metal and it reacts with water in the salt solutions to form calcium hydroxide and hydrogen gas, causing the fizzing. 1A6ai F1m – bonding electrons 1m – non-bonding electrons 2 aiiCClFFF 1aiiiFreon-141

2017 Answer6 aivNo. [must state but no marks]CFCs are compounds containing the elements carbon, chlorine and fluorine. However, Freon-14 does not have any chlorine atom in the molecule. 1bThe energy absorbed to break the C-F bond is 485 kJ/mol, which is higher than the energy of 327 kJ/mol absorbed to break the C-Cl bond.Hence, more C-Cl bonds are broken than C-F bonds, resulting in more chlorine atoms in the ozone layer. 11ci2O3  3O2 2ciiThe chlorine atom is used up in the first equation when it reacts with O3 to form ClO but it is regenerated in the second equation when ClO reacts with more O3 to form O2. 1PAPER 2 (SECTION B)QuestionAnswerMarkB7aiFor iron:1st experiment: (28.6 - 20.0) / 20.0 = 0.430 (to 3 s.f)2nd experiment: (42.7 - 30.0) / 30.0 = 0.423 (to 3 s.f)For mercury:1st experiment: (27.0 - 25.0) / 25.0 = 0.0800 (to 3 s.f)2nd experiment: (41.6 - 40.0) / 40.0 = 0.0400 (to 3 s.f)Iron obey the Law of Constant Composition since the increase in mass was proportional to the mass of the metal used for both experiments. Mercury does not obey the Law of Constant Composition as the increase in mass was not proportional to the mass of the metal used for both experiments.111aiiPbO

2017 Answer7 Mass / g25.01.9Ar 20716No. of moles / mol25.0 / 207= 0.120771.9 / 16= 0.11875Simplest ratio11Empirical formula is PbO, hence lead(II) oxide was formed. 21biDalton’s formula for water shows one hydrogen atom and one oxygen atom, but the modern formula of water is H2O, which contains two hydrogen atoms and one oxygen atom. Dalton’s formula for hydrochloric acid shows one hydrogen atom and three oxygen atoms, but the modern formula of hydrochloric acid is HCl, which contains one hydrogen atom and one chlorine atom. Dalton’s formula for nitrogen monoxide shows one nitrogen atom and one oxygen atom, which is the same as the modern formula of nitrogen monoxide, NO. 1111biiname: sodium chloride reason: Since Dalton’s formula shows that the salt is formed from hydrochloric acid, and it contains the sodium ion, the anion inside the salt should be chloride, as the modern formula of the salt is NaCl. 11B8aMolar mass of 1 mol of octane = 8(12) + 18(1) = 114 g/molVolume of 1 mol of octane = 114 / 703= 0.162 dm3 (to 3 s.f.)Enthalpy change when 1 kg of octane is completely burnt= -5075 / 114 x 1000= -44500 kJ/kg (to 3 s.f.)Enthalpy change when 1 kg of hydrogen is completely burnt= -286 / 2 x 1000= -143000 kJ/kg (to 3 s.f.)11111bThe advantage is that when 1 kg of hydrogen and 1 kg of octane are burnt, more energy is given out for hydrogen. When 1 kg of hydrogen is burnt, it releases 143000 kJ of energy compared to octane which releases 44500 kJ of energy. 1

2017 Answer8 The disadvantage is that hydrogen requires more space for storage compared to octane. Hydrogen is a gas at room temperature and pressure but octane which is a liquid. 1 mole of hydrogen occupies a volume of 24 dm3 compared to 1 mole of octane which occupies 0.162 dm3.[Use of data to support answer]11EITHERB9aCu(NO3)2 1biExperiment 3. [must state but no marks]Comparing experiments 1, 3 and 4, where the acids are of the same concentration (0.5 mol/dm3), the time taken to collect 10 cm3 of gas is the same for experiments 1 and 4 (15 s) but shorter for experiment 3 (7 s). This is because the rate of reaction increases as temperature increases, hence the time taken is shorter for the acid at higher temperature. 11biiAt a higher temperature, the particles have more kinetic energy and hence move faster. There are more collisions between the hydrogen ions and copper(II) carbonate particles. The frequency of effective collisions increases and hence rate of reaction increases. 111cexperimentacidconcentration in mol/dm3time taken to collect 10 cm3 of gas / stotal volume of gas / cm35ethanoic0.5any value above 301506sulfuric0.57.5300For ethanoic acid:Ethanoic acid is a weak acid hence it will dissolve and dissociate in water partially to give a low concentration of hydrogen ions. Hence, the time taken to collect 10 cm3 of gas in experiment 5 is longer than in experiment 1 as the rate of reaction decreases with a lower concentration of hydrogen ions. 11

2017 Answer9 The total volume of gas formed in experiment 5