NYGH 2018-S3EOY-IP Chem P1 & P2 Ans

Secondary 3 (IP) EOY Answers 2018 Sec 3 End-of-Year Exam Chemistry Answers Paper 1 1 B 2 D 3 B 4 A 5 B 6 D 7 D 8 B 9 D 10 B 11 B 12 A 13 B 14 D 15 D 16 B 17 D 18 C 19 A 20 C 21 D 22 D 23 C 24 D 25 A 26 D 27 C 28 C 29 A 30 B Explanations for Paper 1 MCQ Explanations 1 Substance B has boiling point between 0 to 25 °C None of the other substances has melting or boiling points between 0 to 25 °C 2 During melting, heat energy supplied (option A is not true) is used to overcome attractive forces between particles B: Particles will have kinetic energy at all temperatures except for 0 K C: No chemical reaction is occurring 3 Separating funnel: used to separate 2 or more immiscible liquids A: aqueous solution of NH4Cl and Pb(NO3)2 present B: aqueous KI will form immiscible layer with CHCl3 C: aqueous solution of NaNO3 and KNO3 present D: insoluble solid + liquid present 4 When cooled to room temperature, since lead(II) sulfate is insoluble at room temperature, it will crystallise à crystals can be separated from the mixture by filtration, obtaining crystals as residue 5 Since gas is collected by upward delivery à gas must be less dense than air à average Mr of gas must be less than average Mr of air (28.9) à gas must be H2 6 Best choice of indicators will be 1 indicator which changes colour just below pH of 4.5 (methyl orange), and 1 indicator which changes just above pH of 4.7 (methyl red) 7 A strong acid ionises completely in water to form H+ ions and anions 8 Ionic equations for reactions: (1) 2H+(aq) + MgO(s) à Mg2+(aq) + H2O(l) (2) 2H+(aq) + 2K(s) à 2K+(aq) + H2(g) (3) Ba2+(aq) + SO42–(aq) à BaSO4(s) Since H+ ions are not involved in reaction 3, H2SO4 is not behaving as an acid

9 Titration: used to prepare soluble group 1 and ammonium salts à only options A and D Precipitation: used to prepare insoluble salts à only options A, B and D Adding excess solid reactants: used to prepare soluble salts that are not group 1 or ammonium salts à only options B, C and D Only option D is correct for all 3 methods 10 Since metallic oxides are already expected to be basic, an equation showing that the metallic oxide is also acidic (ie a reaction with base/alkali) will show that metallic oxide is amphoteric à option B shows the metal oxide reacting with OH– C: Reaction of metal oxide with H+ just confirms the fact that it is basic 11 From topic of QA, Pb(OH)2 is insoluble in excess aqueous ammonia, while Cu(OH)2 is soluble in excess aqueous ammonia to form dark blue solution Therefore Pb(OH)2 can be obtained by adding aqueous ammonia and filtering D: Cu(OH)2 will be obtained instead of Pb(OH)2 12 Test with KI: Pb2+ present Test with aqueous NaOH, Al: NO3– present (Note that NH4+ could also be present but answer cannot be option D as SO42– cannot be present since R is a colourless solution) 13 Same observations if NH3(aq) or NaOH(aq) added Different observations if H2SO4(aq): white precipitate formed if Pb2+ is present, no observable change if Al3+ is present No observable change if Ba(NO3)2(aq) is added 14 KNO3 K2CO3 Aqueous NH3 No observable change No observable change Aqueous AgNO3 No observable change Yellow precipitate formed (colour of this precipitate is not required in syllabus) Aqueous HCl No observable change Effervescence of colourless odourless gas 15 Since there are 3Mg2+ and 2OH–, [Si4O10] has to have a charge of 4– for the sum of the charges to be 0 16 A: molar mass of D2O = 2 + 2 + 16 = 20 B: Since H2O turns anhydrous copper(II) sulfate blue, D2O will have the same property (isotopes have similar chemical properties so the compounds they form should also have similar chemical properties) C: Since H2O is a liquid, D2O should also be a liquid since the strength of their intermolecular forces will be similar D: H2O has a bent shape, so D2O also has a bent shape

17 Ion of X has 36 electrons and charge of 2+ à X has 38 protons Isotopes contain same number of protons (either options C or D), but different number of neutrons (only option D) 18 B: Carbon nanotube has a giant covalent structure so it is not expected to be soluble in water C: Only 3 out of 4 valence electrons of each carbon atom are used for covalent bonding, so carbon nanotube has mobile delocalised electrons to conduct electricity (similarity to graphite) D: Carbon nanotube has a giant covalent structure so it is expected to have high melting point By elimination, option A is only possible answer 19 A: True since electrostatic forces of attraction can act over long distances, just that the attraction will be weaker for ions that are further away B: Sharing of valence electrons is how covalent bonds are formed C: Cannot be true since option A is true D: NaCl forms a giant ionic structure, not a molecule 20 A: I2 contains covalent bonds between I atoms, and intermolecular forces between B: Since both I atoms have same electronegativity, I2 is non-polar C: Down the group, ionisation energy decreases as valence electrons are further away from the nucleus so they experience weaker electrostatic forces of attraction to the nucleus à 1st IE of iodine is lower than bromine D: I can gain electron to form I– 21 Moles of C atoms = 0.400 x 6 = 2.40 mol Number of C atoms = 2.40 x 6.02 x 1023 = 1.44 x 1023 (3sf) 22 Moles of HClO4 = !"!""" x 2.5 = 0.125 mol Moles of H+ = 0.125 mol [H+] = 0.125 ÷ !""!""" = 0.250 mol dm–3 23 (1) molecular formula = C2H4O2, empirical formula = CH2O (2) molecular formula = C4H8O2, empirical formula = C2H4O (3) molecular formula = C4H8O2, empirical formula = C2H4O 24 Moles of CaCO3 = 50.1 ÷ 100.1 = 0.50050 mol Moles of CaO = 0.50050 mol Theoretical mass of CaO = 0.50050 x 56.1 = 28.078 g % yield of CaO = !.!"!".!"# x 100% = 20.0% (3sf)

25 % by mass of C in C3O2 = !".! ! !!".! x 100% = 52.9% % by mass of C in CO = !".!!".! x 100% = 42.9% % by mass of C in CO2 = !".!!!.! x 100% = 27.3% 26 Moles of CO2 = 66.0 ÷ 44.0 = 1.50 mol Moles of H2O = 36.0 ÷ 18.0 = 2.00 mol Moles of CO2:H2O = 1.50:2.00 = 3:4 Moles of C:H = 3:8 27 H2SO4(aq) + CuO(s) à CuSO4(aq) + H2O(l) Since none of the reactants are gases, pressure of surrounding air will not affect the reaction 28 Since adding substance X (catalyst) decreases the activation energy, it will cause the rate of reaction to increase 29 (1) False. The decrease in rate of reaction is faster initially, as concentration of reactants decreases faster initially (due to faster rate of reaction) (2) True. For an effective collision to occur, molecules must collide in correct orientation and with energy equal to or greater than Ea (3) False. Fast reactions generally have lower Ea (4) True. Rate is fastest at the start as concentration of reactants is the highest 30 Reaction 2 has lower rate and higher yield Effect of changes on reaction: A: lower rate, same yield B: lower rate (as added H2O2 has lower concentration), higher yield (more moles of H2O2 in solution) C: lower rate, same yield D: different rate, same yield Paper 2 Question Answers Marks A1 (a) Number of sigma-bonds = seven / 7. Number of pi-bonds = one / 1. [1] for both correct answers. Do not award half-marks. [1] (b)(i) Tetrahedral [1]. [1] (b)(ii) Four bonding pairs of electrons and zero / no non-bonding or lone pairs of electrons [1]. [1] (c) Both aqueous ethanoic acid and sodium ethanoate contain mobile ions [1]. Accept mobile charge carrying particles. Do not accept mobile electrons. [1] (d) There are relatively weak London dispersion forces (between –[2]

Question Answers Marks CH3) or relatively weak hydrogen bonds (between –OH) between the molecules of ethanoic acid [1], but strong electrostatic forces of attraction between the oppositely charged ions of sodium ethanoate [1]. (e)(i) Titration [1]. [1] (e)(ii) Add calcium carbonate to ethanoic acid while warming and stirring [1]. Add until there is excess / unreacted calcium carbonate in the beaker [1]. Filter the mixture, the calcium ethanoate is the filtrate [1]. Three marking points to a maximum of two marks. [2]