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