NYGH 2017-S3EOY-IP Chem P1 and P2 Ans

Secondary 3 (IP) EOY Answers 2017 Sec 3 End-of-Year Exam Chemistry Answers Paper 1 1. B 2. D 3. D 4. C 5. A 6. B 7. B 8. C 9. B 10. C 11. C 12. B 13. C 14. D 15. B 16. A 17. B 18. C 19. A 20 B 21. B 22. B 23. A 24. D 25. C 26. D 27. B 28. A 29. A 30 B Paper 2 Question Answers Marks A1 (a) Spot A is more soluble in water than spot B while Spot C is not/least soluble in water. [1] (b) He would observe that the boiling occurred over a range of temperatures [1], because the colourless liquid was impure/a mixture [1]. Not accepted: The liquid contained several substances (vague)/the boiling point was higher (than the b.p. of which component? This is impossible to ascertain) [2] Total [3] A2 (a) protons: 16 valence electrons: 6 [2] (b)(i) Not penalized although not recommended: -Brackets for calcium/no brackets for sulfide -not pairing of “dot” with “cross” for transferred electrons [1] (b)(ii) No. Calcium sulfide has a giant ionic crystal lattice [1] at room temperature. Its ions are held in fixed positions/are not mobile [1]. Note: Penalty for any contradictory terms e.g. use of “atoms”/”molecules” interchangeably with “ions”. Not penalised although not recommended: Use of “mobile charged particles” instead of “ mobile ions”. [2] Total [5]

A3 Concentration of H+ ions for the solution = 0.001 mol dm-3 Maximum concentration of H+ ions from arsenic acid = 0.0005 × 3 = 0.0015 mol dm3 [1] Since the concentration of H+ ions is lower than the maximum H+ ion concentration [1], it shows that arsenic acid does not dissociate fully [1] and is a weak acid. Accept: 0.001/0.0005=2 [1] At pH 3, 2 moles of H+ ions are produced per mole of acid but H3AsO4 should be tribasic. [1] This shows that arsenic acid does not dissociate fully [1] and is a weak acid. [3] Total [3] A4 (a)(i) Gas syringe / eudiometer Accept: syringe [1] (a)(ii) C12H22O11 [1] (a)(iii) Assuming 100 g of compound, C H O mass / g 41.5 6.40 52.1 molar mass / g mol-1 12.0 1.0 16.0 no of moles [1] 3.458 6.40 3.256 simplest ratio 1.062 1.965 1 mole ratio 1 2 1 empirical formula = CH2O [1] [2] (a)(iv) When the number of atoms of some elements are very close together, there is a possibility that they appear to be the same number. Accept: The empirical formula method involves rounding off values to the nearest whole number, resulting in inaccuracies. Reject: It is inaccurate because the ratios are not the same. [1] (b)(i) Filtration [1] (b)(ii) Crystallisation [1] Total [7] A5 (a) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3 [1] (b)(i) The two isotopes have different numbers of neutrons / numbers of nucleons / nucleon mass / atomic mass. [1] (b)(ii) Let x be the percentage abundance of 73As 74.9 = [75(100 – x) + 73x] ÷ 100 [1] 2x = 10 x =5.00 % [1] [2]

(c) As arsenic molecules sublime, (arrangement) the molecules that were initially closely packed will move far apart [1]; (movement) molecules that were vibrating about fixed positions will move at high speeds [1] as a gas; (explanation) by gaining heat energy / gaining kinetic energy / overcoming the intermolecular forces of attraction [1]. Reject: gaseous molecules fly / float / move freely strong covalent bonds were broken [3] (d) The ease of losing electrons increases [1] down the group / the ease of gaining electrons decreases [1] down the group. [1] Total [8] A6 (a) W is copper(II) hydroxide / Cu(OH)2 Y is ammonia / NH3 No marks for missing oxidation state of copper [2] (b) NH4+ and I- [2] (c) It is iodine / I2 [1] 2NH4I(aq) + Cl2(g) à 2NH4Cl(aq) + I2(s) [1] OR It is astatine / At2 [1] 2NH4At(aq) + Cl2(g) à 2NH4Cl(aq) + At2(s) [1] [2] (d) Add acidified silver nitrate / acidified lead(II) nitrate. A yellow precipitate will form. (test for iodide / astatide ion by precipitation) OR Add aqueous bromine / Bubble fluorine gas. A black solid will form. (test for iodide / astatide ion by displacement) [1] (e) Calcium sulfate is sparingly soluble so some of the calcium sulfate may have remained dissolved / calcium ion concentration was too low to form enough precipitate. Reject: Calcium is less reactive than barium so it did not react. Ammonia was present (it would have reacted with the acid). Barium hydroxide / carbonate was formed (it would have reacted with the acid). [1] Total [8] A7 (a) A compound is made up of two or more elements that are chemically combined. Reject: Mixture of elements (the term mixture is different from compounds) Elements that are bonded together (need to be different elements) Mentioning “atoms” or “ions” (compounds can include both) Substances that are combined together (substances is vague) [1] (b) Transition metal compounds tend to be coloured / have high [1]

melting and boiling point. Reject: Lustrous, shiny, malleable, ductile, less reactive to air/water, have different oxidation states (these are properties of the transition metals, not their compounds) Soluble / Insoluble in water (they are neither all soluble nor insoluble) (c) CrO42-(aq) + Pb2+(aq) à PbCrO4(s) [1] (d) TiCl4 is likely to be simple or discrete molecules / have a simple molecular structure. [1] Its low melting and boiling points show that it has weak intermolecular forces of attraction [1] that require little energy to overcome. Its lack of electrical conductivity in any state shows that there are no mobile charged particles available [1]. Accept: Simple covalent structure Reject: Simple covalent bond (bonding is different from structure) Any contradiction in answers (e.g. it has a simple molecular structure with weak electrostatic forces of attraction between ions). [3] Total [6] B8 (a)(i) n MgCO3 = 4.03 g/84.3 g mol-1 = 0.047805 mol n H2SO4 = (250/1000) dm3 × 0.2 mol dm-3 = 0.05 mol [1] MgCO3 is the limiting reagent [1] (n MgCO3:n H2SO4 1:1) Mass of MgSO4 = 0.047805 mol × 120.4 g mol-1 = 5.76 g (3sf) [1] [3] (a)(ii) To increase the rate of reaction OR increase surface area OR ensure that reactants are evenly mixed Not accepted: dissolve faster, ensure complete reaction of reactants [1] (a)(iii) The hydrated crystals decompose at high temperature OR water of crystallisation is lost (as steam) OR the anhydrous salt. MgSO4, is formed. [1] (b) 126/246.4 × 100 = 51.1% [1] (c) Magnesium oxide or magnesium hydroxide or magnesium [1] Reaction of dilute acid with (an excess of an insoluble) base/metal. [1] Not accepted if formula is given Not accepted for method: neutralisation, titration, react acid with insoluble solid/reactant/substance [2] (d) Calcium sulfate is relatively insoluble [1] and will coat calcium carbonate and prevent further reaction with sulfuric acid [1] (Also accept low yield of calcium sulfate/ cannot separate calcium sulfate and calcium carbonate). Not accepted if stated that product obtained will be impure without explanation about difficulty in separating product and reactant [2]

B9 (a) An element with properties intermediate between the metals and non-metals/properties of both metals and non-metals. Not accepted references to compounds or alloys. [1] (b) [1] Gray arsenic [1] The presence of delocalized valence electrons [2] (c)(i) •• HוAs•×H ו H Not accepted without lone pair of electrons; ionic formulations not accepted. [1] (c)(ii) Arsine is a simple covalent compound with a low boiling point, hence it has weak intermolecular forces between the molecules [1] and little energy is needed to overcome these forces of attraction. [1] Arsine is insoluble in water due to an inability to form strong interactions/intermolecular forces with water molecules. Accept reference to weak interactions/weak intermolecular forces [1] (Also accept reference to inability to hydrogen bonding). Not accepted arsine molecules are non-polar. [3] (d) 3Cu2+ + 2AsO33- → Cu3(AsO3)2 [1] (e) Number of moles of As2O3 = 8870 g ÷ 197.8 g mol-1 =44.84 mol Number of moles of As = 89.69 mol [1] Mass of As = 6.717 kg % yield = 5.33 kg ÷ 6.717 kg × 100% = 79.4% [1] [2] EI