NYGH 2016-S3EOY-IP Chem P1 and P2 Ans

1 2016 Sec 3 End-of-Year Exam Chemistry Answers Paper 1 1. D 2. C/D 3. B 4. D 5. B 6. D 7. C 8. B 9. D 10. D 11. C 12. B 13. D 14. C 15. D 16. A 17. D 18. B 19. D 20 A 21. C 22. C 23. C 24. C 25. B 26. C 27. A 28. D 29. B 30 D Paper 2 Section A No Answers Marks A1(a) A: any carbonates except for silver carbonate and lead(II) carbonate B: any ammonium salt Common mistake: Writing chemical name instead of formula [1] [1] A1(b) Effervescence stops OR no more bubbles produced Common mistakes: Wr iting that effervescence forms; testing for carbon dioxide [1] A1(c) Mr of CO2 = 12 + 16 +16 = 44 Mr of NH3 = 14 + 1 +1 + 1 = 17 Ammonia gas diffuses faster (reaches mosquitoes faster) as it has a lower molecular mass. Common mistakes: Wrong terminology; wrong units [1] [1] [Total: 5] A2(a) The melting point increases from Na 2O to MgO and decreases from MgO to SO2. Common mistakes: Trend not clearly spelt out; pointing out individual values instead of outlining a trend; describing increase in mp to Gp III instead of II [1] A2(b) (i) In SiO 2, Si and O atoms are held together by strong covalent bonds in giant molecular structure. In P 4O6, the molecules are held together by weak intermolecular forces of attraction in a simple molecular structure. Hence, SiO 2 has a higher melting point than P4O6. Structure and bonding for each molecule must be mentioned to [1] [1]

2 No Answers Marks obtain 1 full mark each. If only structure is mentioned for both molecules, award 1 mark only. If only bonding is mentioned for both molecules, award 1 mark only. Common mistakes : Being unclear what constitutes “bonding” and what constitutes “structure”; describing silicon dioxide as ionic A2(b)(ii) Both NaCl and MgCl2 are held together by strong ionic bonds in a giant lattice structure. Hence, they have similar melting point. Common mistake: Omitting description of structure [1] A2(c) Any specifically stated value lower than (but not including) -73oC but greater than - 200 oC (can use M r of other compounds to estimate how low the mp will be) Common mistake: Not stating a specific value, but instead stating “low” or “lower than -73 oC”; no units [1] [Total: 5] A3(a)(i) Ca3(PO4)2 (s) + 6HNO3 (aq)  3Ca(NO3)2 (aq) + 2H3PO4 (aq ) No marks if either Balanced equation or State symbols are incorrect/ missing or wrong formula [1] A3(a)(ii) Phosphoric acid is a weak acid, which partially dissociates in water. Nitric acid is a strong acid, which fully dissociates in water. OR [1m] Strength of acid [1m] Dissociation of acid Accept: Nitric acid is a stronger acid than phosphoric acid [1] as it dissociates/ionises more completely/fully than phosphoric acid in water [1] if student wrote either “both acids are strong as they completely dissociate in water” or “both are weak as they partially dissociate in water”  1 m only Reject: - One of them is stronger as it dissociates completely in water [1] [1]

3 No Answers Marks - Nitric acid dissociates more in water than phosphoric acid - Nitric acid deionized in water A3(b)(i) No. of moles of Ca(NO3)2 = 246 000 000 / 164 = 1 500 000 mol Molar mass of NH4NO3 = (14 x 2) + 4 + (16 x 3) = 80 g mol-1 Mass of NH4NO3 produced = 1 500 000 x 2 x 80 = 240 000 000 g = 240 tonnes [1] [1] [1] A3(b)(ii) % yield = (181/240) x 100 = 75.4 % (to 3 s.f.) [allow ecf] [1] [Total: 7] A4(a) Step 1: To react/neutralise with excess/unreacted Mg/MgO To remove/ dissolve Mg/MgO (BOD) Reject: - if they stated the product as MgCl (wrong formula, but common mistake) - If they said that the purpose was to produce MgCl2 or water with no additional explanations (accept if they continue to elaborate that it was to produce a soluble salt from an insoluble base, MgO) - To remove B or B2O3 Step 2: To react with (excess/unreacted) hydrochloric acid in step 1 To remove excess acid (BOD) Reject: - to react with MgO (no such reaction) - B2O3 as it was the limiting reagent, so should not have any left - MgCl2 with NaOH [1] [1]

4 No Answers Marks Step 3: To remove any impurities such as unreacted NaOH, MgCl2, NaCl Accept: - To remove any solid/aqueous impurities - To remove/wash off any contaminants/unwanted residues/ excess solutions Reject: - To remove liquid impurities(no liquids produced except water) - To remove chemicals (B can be considered a chemical as well - vague) [1] A4b Melt the boron and check if it melts/boils at a fixed temperature. Accept: - Check if it melts over a range of temperatures - Check if the melting / boiling point is the same as that of pure boron / is below the melting point of pure boron/ boiling point above pure boron Reject: - Heat the sample. - Calculate the percentage purity of the sample - Calculate the theoretical yield based on number of moles of reactants and the actual yield from the mass of B obtained, then find percentage purity - Chromatography - React the sample with an acid/ alkali - Filteration/ Dry the residue with filter paper - Crystallisation Comments: Boiling boron should not be the correct method to determine the purity of boron, as it was given from the equation that boron is a solid at room temperature  melting point should be used. In general, most students confused percentage purity with percentage yield based on their description. Note that percentage yield is only based on the mass of the product obtained and the theoretical mass, but does not tell you anything about the purity of the sample. Chromatography was rejected as it is unlikely to find a solvent to dissolve [1]

5 No Answers Marks boron in. A4c No. of moles of Boron extracted = 22 000 / 11 = 2000 mol [1] No. of moles of Mg used in reaction = 2000 x 3/2 = 3000 mol [1] Mass of Mg used in reaction = 3000 x 24 = 72000 g or 72kg [1] Accept any answer more than 72 kg since Magnesium is in excess. [3] [Total: 14] A5(a) A: Lead (II) carbonate [1] B: carbon dioxide [1] C: Lead (II) oxide [1] D: Lead (II) nitrate [1] E: Lead (II) hydroxide [1] F: calcium carbonate [1] Reject answers in chemical formulae. [6] A5(b) The white precipitate dissolves to give a colourless solution [1] A5(c) calcium oxide and carbon dioxide Reject answers in chemical formulae. [1] [Total: 8] A6(a) Size and shape of particles will affect surface area of solids which will affects the rate of reaction and hence affects the temp. rise / vol. of CO2 evolved after 4 minutes interval. Keeping MgCO3 of the same size and shape will ensure that the surface area of solids remain constant for the various experiments for a fair comparison. [1] [1] A6(b) The higher the concentration of the hydrochloric acid, the faster the reaction rate/ more CO2 evolved / greater rise in temperature. [1] A6(c) More reactant particles per unit volume increases the frequency of collisions between the reactant particles. [1]

6 No Answers Marks Reject: • Acid molecules are referred to as reactant particles. • Per unit volume not mentioned. • Per unit area/ space This in turns increases the frequency of effective collisions which increase the rate of reaction. [1] A6(d)(i) Both HCl and HNO3 are monobasic acids. No. of moles of H+ ions produced by the two acids is the same/ same concentration of H+ ions Reject: • Only same concentration of acid is mentioned without basicity. • Both are strong acids hence concentration of H+ is the same. [1] [1] A6(d)(ii) 0.30°C [1] [Total:8] Section B B7 (a) (i) Isotopes are atoms of the same element with the same number of