2019 RVHS Prelim P3 ANS

River Valley High School 9729/02/PRELIM/19 [Turn over 2019 Preliminary Examination Suggested Answers for 9729 H2 CM 2019 JC 2 Prelim Paper 3 1 (a) (i) When dissolved in a polar solvent such as water, they dissociate to form strong acids, releasing H+ and X – ions forming ion-dipole interactions with water. [1] (ii) HF has an exceptionally high boiling point compared to the other hydrogen halides as hydrogen bonds exist between HF molecules but not between the rest of the HX molecules. More energy is needed to overcome the stronger hydrogen bonds. Boiling point of the hydrogen halides increases from HCl to HBr to HI. The strength of instantaneous dipole -induced dipole interactions between the hydrogen halide molecules increases as the number of electrons in the m olecules increases. More energy is needed to overcome the stronger id-id interactions. [2] (iii) When a red-hot steel needle is introduced, HBr produces red brown bromine vapour. HI gives violet fumes of iodine . HF and H Cl do not/show little tendency to decompose. Bond Bond Energies/ kJ mol-1 H–F +562 H–Cl +431 H–Br +366 H–I +299 Less energy is required to break the weaker H–X bond. The thermal stability of hydrogen halides decreases down the Group due to decreasing H–X bond energy. [3] (b) (i) Solubility of AgBr = 0.140  10–3 / (107.9 + 79.9) = 7.45  10–7 mol dm–3 [Br–] = 4.00 10–12 / 0.100 = 4.00 10–11 mol dm–3 [Ag+] [Br–] = Ksp [Ag+] (4.00 10–11) = (7.45  10–7)2 [Ag+] = 0.0139 mol dm–3 [2] (ii) In the saturated AgCl, [Ag+] [Cl–] = Ksp (0.0139)(0.0139 + 0.001) = Ksp Ksp = 0.000206 mol2 dm–6 Solubility = (0.00206)1/2 = 0.0144 mol dm–3 [2] (iii) Mass of AgCl dissolved in 1 dm3 = (0.0139)(107.9 + 35.5) = 1.99 g Mass of residue = 5.00 – 1.99 = 3.01 g [1]

2 River Valley High School 9729/02/PRELIM/19 2019 Preliminary Examination (c) (i) The higher the number of electron donating groups attached to carbocation, the more stable the carbocation. As the 2 possible intermediate contains a primary carbocation and secondary carbocation respectively, intermediate with the secondary carbocation is more stable. [2] (ii) [2] (iii) Compound B has a 4 membered ring form in place of the 5 membered ring presented in the reaction scheme. The 4 membered ring consist of 2 carbon atoms with sp2 hybridised orbitals 120° from each other and 2 carbon atoms with sp3 hybridised orbitals at 109.5° from each other. To fit into 4 membered ring, all 4 carbons are forced into 90° bond angle which exerts high angle strains making the 4 membered rin g formation unfavourable. [2] [Total: 17]

3 River Valley High School 9729/PRELIM/19 [Turn over 2019 Preliminary Examination 2 (a) (i) [1] (ii) H2O/Water [1] (b) (i) For p K1, the H + is removed from a neutral molecule. For p K2, the removal of a H+ from the anion that already carries a negative charge is