YJC_H2_Chem_ANS Prelim

1 H2 Chemistry 9746 Preliminary Examinations 2009 Suggested Solutions Paper 1: Multiple Choice Questions Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 A D B C A D C D C C Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 D D C D B C A D B D Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 B A D B D C D A D A Q31 Q32 Q33 Q34 Q35 Q36 Q37 Q38 Q39 Q40 B A B D A B C C B D Paper 2: Structured Questions 1 (a) (i) 1s2 2s2 2p6 3s2 3p6 3d10 4s1 (ii) Screening effect on the outermost electron of Cu is more or less the same as that in K, as the electrons in the d-subshell do not contribute significantly to screening effect As nuclear charge of Cu > K, the attraction of the outermost electron by the nucleus of Cu is much stronger than in K ( atomic radius of Cu is smaller than K) (iii) As atomic radius of Cu < K, the copper atoms can pack close together / there will be more copper atoms per unit volume / copper would occupy a smaller volume for the same number of particles As mass of one atom of Cu > mass of one atom of K, hence density (mass / volume) of copper is higher than K. (b) (i) HCl HBr HI Bond Energy (kJmol−1) 431 336 229 (awarded as long as candidate mentions at least two of the bond energies in their answer) ease of breaking the H−X bond: HCl < HBr < HI thermal stability: HCl > HBr > HI (ii) Hydrogen bonding exist between HF molecules (c) 1st I.E.: He = 2370 kJ mol1; Ne = 2080 kJ mol1 Potential difference required = 0 . 242370 2080 = 21.1 V [Total: 11] 9746 / YJC / 2009 / Preliminary Examination / Suggested Solutions

2 2 (a) (i) (ii) CC OO O O 2- 120o trigonal planar with respect to both carbon bond angle = 120 (b) (i) The experiment should be conducted in the fume cupboard to prevent inhalation of the toxic CO(g). (Or any feasible precaution, but must mention the toxic nature of CO. Vague reasons, such as CO is a pollutant, are not accepted.) (ii) amount of CaC2O4 = ) 0 . 16 ( 4 ) 0 . 12 ( 2 1 . 40 128 . 0   = 1.0  103 mol mass of B produced = 028 . 0 128 . 0 = 0.100 g Mr of B = 310 0 . 1 100 . 0  = 100 (iii) CaCO3 (iv) CaC2O4  CaCO3 + CO (v) Any temperature well above 400 C ionic radius of Ba 2+ > ionic radius of Ca2+ or charge density of Ba 2+ < charge density of Ca2+ Ba 2+ ion is less able to polarise / distort electron cloud of the anion. [Total: 12] 3 (a) (i) A is: HC C C HHH H H B is: HC C C H H H OH H H H (ii) Step II: H3PO4, 300 C, 70 atm Step III: KMnO4 / K2Cr2O4, H2SO4(aq), heat / reflux (b) (i) HCN + NaOH  Na+ + CN− + H2O 9746 / YJC / 2009 / Preliminary Examination / Suggested Solutions

3 (ii) (iii) H2SO4(aq) / HCl(aq), heat (c) (i) 335 10 43 . 1 100 . 0 10 05 . 2] [        moldmc K Ha 84 . 2 ) 10 43 . 1 log(3    pH (ii) HAHA NaOHNaOH HA Na