JJC_H2_CHEM_P3_Ans

© Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 Suggested Answers to 2015 JC2 H2 Chemistry Preliminary Examination Paper 3 1 (a) (i) Hf(NH3(g)) = 1 2 (+994) + 3 2 (+436) – 3(+390) = –19.0 kJ mol1 (ii) Hr = 2(–119) – 2(–19.0) = –200 kJ mol1 (iii) Na is oxidised because O.S. of Na increases from 0 (in Na) to +1 (in NaNH2). NH3/H is reduced because O.S. of H decreases from +1 (in NH3) to 0 (in H2). (iv) (v) NH2  + CH3OH  NH3 + CH3O (b) (i) Electron-withdrawing –OH groups in ‘Tris’ makes the lone pair of electrons on N atom less available for protonation. OR Presence of intramolecular hydrogen bonding between –OH and –NH 2 groups in ‘Tris’ makes the lone pair on N atom less available for protonation. Thus, ‘Tris’ is a weaker base and hence a smaller Kb than tert-butylamine. (ii) (CH2OH)3CNH2 + HCl  (CH2OH)3CNH3 + Cl (iii) [OH] = Kb [base] [salt] 10(147.40) = (1.20  106) [base] [salt] OR pOH = pKb + lg [salt] [base] 14 – 7.40 = lg (1.20  106) + lg [salt] [base]  [base] [salt] = 0.209 Since HCl  salt, [salt] in buffer A = 100 1000  0.500 = 0.0500 mol dm3 [base] in buffer A = 0.209  0.0500 = 0.0105 mol dm3 Mass of ‘Tris’ needed = (0.0500 + 0.0105)  121 = 7.31 g (c) (i) Amide (ii) Step 1: substitution; Step 2: acid-base reaction (iii) (iv) ONH3C C

2 © Jurong Junior College 9647/03/ J2 PREIMINARY EXAMINATION/2015 2 (a) A transition element is a d-block element which forms one or more stable ions with incompletely filled d-orbitals. (b) (i) A catalyst provides an alternate reaction path of lowered activation energy, Ea’. Thus, the number of particles with energy ≥ Ea’ increases. Therefore, the frequency of effective collisions increases and hence rate increases. (ii) electrophilic substitution Br2 + FeBr3  Br+ + FeBr4 – (iii) p–p orbital overlap results in the delocalisation of lone pair of electrons on N atom into the benzene ring of phenylamine, making the benzene ring highly electron rich and thus more susceptible towards electrophilic substitution. Hence, there is no need of FeBr 3 to generate stronger Br+ electrophile for the halogenation of phenylamine. (c) (i) Nickel(II) complexes are coloured because Ni( II) has d8 configuration/ partially unfilled d-orbitals so electron transition between dorbitals is possible. In an octahedral nickel( II) complex ion, the presence of ligands causes the dorbitals to split into 2 (different) energy levels. Energy gap is relatively small such that radiation from the visible light spectrum is absorbed when an electron transits from a dorbital of lower energy to a dorbital of higher energy. Hence, the colour seen is the complement of the colours absorbed. (ii) Since G < 0 for both reactions, it implies that the ligand exchange is energetically feasible for both reactions.  H2O is the weakest lig