YJC_H1_CHEM_Suggested_Answers

8872 / YJC / 2017 / Preliminary Examination / Suggested Answers 1 2017 H1 Chemistry 8872 Preliminary Examinations Suggested Answers Paper 2 Section A: Structured Questions 1 (a) (i) Al(OH)3 + 3H+  Al3+ + 3H2O (or Al(OH)3 reacting with HCl) (ii) Aluminium hydroxide or calcium carbonate is insoluble in water and therefore will not increase the pH of blood. (b) (i) 2 550 0.024554 0.0246 mol22400 COn    (ii) 32 0.0246 molCaCO COnn  3 0.024554 {40.1 12.0 3(16.0)} 0.024554 100.1 2. 4565 2.46 gCaCOm         (iii) mass of CaCO3 in one tablet = 2.4565 0.491 g5  The claim is valid, as the mass if CaCO 3 is approximately the same as what was claimed by the manufacturer [Total: 7 marks] 2 (a) (i) The melting point of the elements increases from Na to Si (with Si significantly higher than that for Al), The melting point decreases drastically from Si to P and is relatively low from P to Cl (or the melting point of P to Cl is much lower than that for the Na to Si). (ii) trend: atomic radius decreases from Na to Cl explanation: nuclear charge increases and shielding effect remains constant (b) sodium: metallic bonding and giant metallic structure ‘sea’ of delocalised electrons are available to conduct electricity and so it has high electrical conductivity silicon: covalent bonding and giant molecular structure or giant covalent structure In the giant molecular structure, there are some free electrons and ‘holes’ which can be used to conduct electricity, and so silicon is a semi-conductor chlorine: covalent bonding and simple molecular structure There are no mobile electrons to conduct electricity, and so chlorine is a non-conductor (c) (i) There are 17 protons and 18 neutrons concentrated (within a very small volume) at the nucleus / centre of the atom There are 17 electrons surrounding the nucleus and moving randomly (ii) 1s2 2s2 2p6 3s2 3p5

8872 / YJC / 2017 / Preliminary Examination / Suggested Answers 2 (d) (i) (ii) There are two bond pairs and two lone pairs of electrons (allow e.c.f.) Hence the electron pairs spread themselves out as far apart as possible to minimise repulsion giving rise to the bent shape. [Total: 16 marks] 3 (a) (i) C3nH6n + 9 2 n O2  (3n)CO2 + (3n)H2O (ii) carbon (soot) (iii) benzene and methyl benzene (iv) brown Br2(aq) is decolourised CH2CH2 + Br2 + H2O  BrCH2CH2OH + HBr or CH2CH2 + Br2  BrCH2CH2Br (accept equations involving propene as well) (v) hydrogen and methane collected can be used as fuels. or benzene collected (in tube B) can be used to manufacture styrene and phenol or reduce the emission of CO2(g) to the atmosphere (accept any other reasonable answer) (b) cis–hex–2–ene trans–hex–2–ene cis–hex–3–ene trans–hex–3–ene