NJC Prelim P1_Solutions

1 NJC Preliminary Examination 9729/01/18 [Turn Over + – R S T NJC 2018 SH2 H2 Chemistry Paper 1 Solutions: 1 Use of the Data Booklet is relevant to this question. The following are flight paths of charged particles when accelerated in an electric field. Which correctly identifies S, T and U? S T U A 15O+ 14C+ 14N+ B 15O⎯ 15O+ 28Si+ C 14N⎯ 28Si2+ 14C2+ D 14N⎯ 14C+ 28Si2+ C: Negatively charged ions attracted to positive plate, positive ions to negative plate. particles 14N⎯ 14C2+ 14C+ 28Si2+ Charge /mass 1/14 2/14 = 1/7 1/14 2/28 = 1/14 Since angle of deflection is charge/mass ratio, S and T have roughly the same angle of deflection but different polarity, while U has almost double the angle of deflection as T. 2 In which sequence is the molecules listed in the order of increasing dipole moment? A SO 3, CO2, AlCl3 B H2O, H2S, HBr All non-polar. Dipole moments cancel out due to shape of molecule. H2O and H2S have bent shape. H2O is more polar than H2S as O is more electronegative than S. C CF4, CO, HF D NH3, HF, BeCl2 CF4 is non-polar, dipole moments cancel out due to tetrahedral shape of molecule. CO and HF are linear. HF is more polar than CO, as electronegativity difference is greater between H and F than between C and O. NH 3 and HF are polar but BeCl2 is non- polar, dipole moments cancel out due to linear shape of molecule. S T U

2 NJC Preliminary Examination 9729/01/18 [Turn Over 3 In which row are the molecules arranged in order of increasing bond angle? 1 CH4, AlCl3, XeF2 CH4 (tetrahedral, 109 degrees) AlCl3 (trigonal planar, 120 degrees) XeF2 (linear, 180 degrees) 2 H2S, PH3, NH3, H2S (tetrahedral, 109 degrees) PH3 and NH3 (trigonal pyramidal, around 107 degrees) NH3 has a larger bond angle than PH3 as 1) N is more electronegative than P, N pulls electron density of bond pairs more towards itself, leading to greater bond-pair bond-pair repulsion. 2) N has a smaller lone pair region than P (N is above P in group 15). Lone-pair bond- pair repulsion is smaller, leading to a larger bond angle. 3 NF3 , NCl3, SO3 NF3 , NCl3 (trigonal pyramidal, around 107 degrees) NF3 has a smaller bond angle than NC l3. F is more electronegative than N and N is more electronegative than Cl). F pulls electron density of bond pairs more towards itself/ away from central N atom, leading to smaller bond-pair bond-pair repulsion. SO3 (trigonal planar, 120 degrees) A 1, 2 and 3 B 1 and 2 C 2 and 3 D 1 only 4 Propadiene and propyne both have the same molecular formula, C 3H4. They exist in equilibrium as shown: H2C=C=CH2 CH 3C≡CH propadiene propyne Which bond is present in propadiene but not present in propyne? A a σ bond formed by s – sp overlap B a π bond formed by p – p overlap C a σ bond formed