VS 2019 O Level Pure Chemistry P1+P2 Answer Scheme

2019 Answer1 Victoria School2019 Chemistry O Levels / 6092Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20ABCDDDCBACDCBCADDBBAQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40CACCABBAAADACABBBCDAPAPER 2 (SECTION A)QuestionAnswerMarkA1aD and G 1bA and E 1cC 1dG 1eA 1A2aIsotopes are atoms of the same element with the same number of protons but different number of neutrons. Isotopes 1, 2 and 3 have the same number of 16 protons but different number of neutrons, i.e. 13, 14 and 15 respectively. 11bThe data shows that all 3 isotopes have the same number of 6 outer shell electrons. The isotopes have the same chemical reactions as they are in the same group. 1

2019 Answer2 ciX: sulfurY: oxygenZ: zinc 1ciilowest melting point: Y (oxygen) X (sulfur)highest melting point: Z (zinc)Z (zinc) has the highest melting point as it has a giant metallic structure so a lot of energy is used to overcome the strong metallic bonds between the positive ions and sea of delocalised electrons. X (sulfur) and Y (oxygen) both have simple molecular structures. However, X has a higher relative molecular mass than Y, hence more energy is used to overcome the weak intermolecular forces of attraction in X, hence higher melting point than Y. 111ciiiZZ is a reactive metal and metals react with dilute acids to form salts and hydrogen gas. 11A3aLead is a reactive metal which can react with dilute nitric acid. Gold is a less reactive metal than lead and cannot react with dilute nitric acid. 11bLead compounds react with dilute nitric acid to form lead nitrates which are soluble in water.Lead compounds react with dilute hydrochloric acid and dilute sulfuric acid to form lead chlorides and lead sulfates respectively, which are insoluble in water. 11ciLead and aluminium ions react with aqueous ammonia to form a white precipitate which is insoluble in excess aqueous ammonia. Hence, it is not possible to distinguish between lead and aluminium ions. Zinc ions react with aqueous ammonia to form a white precipitate which dissolves in excess aqueous ammonia to form a colourless solution. Hence, only zinc ions can be identified.11ciiAdd aqueous sodium chloride (or any other solutions containing chloride ions) to the solutions of lead ions and aluminium ions. 1

2019 Answer3 There will be a white precipitate observed for the solution of lead ions but no visible reaction for the solution of aluminium ions.[Note: can also add any solutions containing iodide ions. Yellow precipitate will be formed for the solution of lead ions] ciiiAdd excess aqueous sodium hydroxide to the solution, followed by a piece of aluminium foil. Warm gently. Effervescence of colourless and pungent gas will be observed. Gas turns moist red litmus paper blue, so ammonia gas is given off. 111A4aNH3(g) + HCl(g) → NH4Cl(s)1m – balanced equation1m – state symbols2bidistance travelled / cmMrNH36017HCl4036.51biiNo [must state but not marks]As the Mr increases, the distance travelled by the gas decreases, which shows that the rate of diffusion has decreased. However, the rate of diffusion is not inversely proportional to the Mr as the product of distance travelled and Mr are not related by a constant value, i.e. for NH3, 60 x 17 = 1020, for HCl, 40 x 36.5 = 1460.[Note: y is inversely proportional to x when y = k/x, hence k = yx] 11cSimilarity: The white ring of solid ammonium chloride will be formed at the same position in both experiments. Difference: The time taken for the white ring to form in experiment B is longer than the time taken in experiment A. 11

2019 Answer4 A5aexperiment using sodium: Red litmus indicator turns blue. Sodium burns in oxygen to form sodium oxide, which is a basic oxide. Sodium oxide dissolves in the solution to form an alkali, which turns red litmus indicator blue. experiment using sulfur: Red litmus indicator remains red. Sulfur burns in oxygen to form sulfur dioxide, which is an acidic oxide. Sulfur dioxide dissolves in the solution to form an acid, hence red litmus indicator remains red. 1m – both observations correct1m – explanation for sodium1m – explanation for sulfur3 bNa+2O2-1m – cation 1m – anion2ci4Na + O2 → 2Na2ONo. of moles of Na = 0.50 / 23 = 0.021739 molNo. of moles of Na2O = 0.021739 / 4 x 2 = 0.01087 molTheoretical yield of Na2O = 0.01087 x (2 x 23 + 16) = 0.67394 g = 0.674 g (to 3 s.f.)11ciiMass of Na2O = 94.82 – 94.50 = 0.32 g1

2019 Answer5 Percentage yield = 0.32 / 0.67394 x 100% = 47.5% (to 3 s.f.) 1ciiiThe sodium sample was impure, such that some of the impurities reacted with oxygen to form gaseous oxides which escaped, resulting in a lower mass of sodium oxide. Some of the sodium oxide fumes could have escaped, resulting in a lower mass of sodium oxide. [any one of the above or any other possible answers] 1A6aMagnesium is more reactive than iron so it loses electrons more readily than iron. Hence it is oxidised more easily and corrodes in place of iron, protecting the pipe from rusting. Mg → Mg2+ + 2e 11biOil and paint acts as a protective layer and cover the surface of the pipe, preventing the iron from being exposed to oxygen and water. 11biiOil and paint are easily scratched off and damaged, which causes the pipes to rust as the pipes are exposed to oxygen and water. 1A7aDifferent metals have different reactivities, hence when two different metals are connected, the flow of electrons from the more reactive to less reactive metal produces a voltage. The larger the difference in the reactivity of the 2 metals, the larger the voltage produced. 11bmetal 1metal 2predicted voltage / Vcopperironany value between 0.61 – 1.09 silvermagnesiumany value between 1.57 – 4.00 11

2019 Answer6 PAPER 2 (SECTION B)QuestionAnswerMarkB8aThe chlorate(I) ion is negatively charged and contains an oxygen atom covalently bonded to a chlorine atom.1biNH2Cl + NaOCl → NHCl2 + NaOHNHCl2 + NaOCl → NCl3 + NaOH1biiOxidation state of nitrogen in NH2Cl is -1.Oxidation state of nitrogen in NHCl2 is +1.Oxidation state of nitrogen in NCl3 is +3.[2m – 3 correct, 1m – 2 correct, 0m – 1 or 0 correct]The oxidation state of nitrogen increases from -1 in NH2Cl to +1 in NHCl2 and +1 in NHCl2 to +3 in NCl3. This is oxidation, so sodium chlorate(I) acts as the oxidising agent. 21cClO- 1m – bonding electrons 1m – non-bonding electrons2dPercentage by mass of chlorate(I) in sodium chlorate(I) [NaOCl]= [(16 + 35.5) / (23 + 16 + 35.5)] x 100%= [51.5 / 74.5] x 100%= 69.1% (to 3 s.f.)Percentage by mass of chlorate(I) in calcium chlorate(I) [Ca(OCl)2]= [2(16 + 35.5) / (40 + 2(16) + 2(35.5))] x 100%= [103 / 143] x 100%= 72.0% (to 3 s.f.)12eCaO2 1

2019 Answer7 fHypochlorite bleaches reacts to form compounds of nitrogen with chlorine which are irritating and toxic to people. Hydrogen peroxide bleaches breaks down compounds and produces water which is harmless to people.1B9astage 2: Carbon monoxide gas molecules adsorb onto the surface of the catalyststage 3: Each carbon monoxide molecule forms bonds with an oxygen atom to form a carbon dioxide moleculestage 4: Carbon dioxide gas molecules are detached from the surface of the catalyst. 111bO2 + 2COHEaenergy progress of reactionreaction without catalyst2CO2 O2 + 2COHEa, catalyst energy progress of reactionreaction with catalyst2CO2 1m – shape of both energy profile diagrams + label reactants/products 1m – label both activation energies, where Ea, catalyst < Ea1m – label both enthalpy changes, H (same value) 3 cThe palladium catalyst provides an alternative pathway of lower activation energy for the reaction to occur. Hence, more particles have the minimum energy for reaction to occur. There are more collisions between oxygen and carbon monoxide molecules and the surface