AJC Prelims H2 Chem Paper 2 Answers

AJC H2 Chemistry Prelim 2008 - Paper 2 Suggested Answers 1 (a) (i) M (g) → M+ (g) + e (ii) (b) (i) P4 + 6I2 Æ 4PI3 (ii) P I I I Shape of molecule trigonal pyramidal (iii) Iodine has a higher nuclear charge and exerts a higher screening effect Hence, the valence electrons in iodine experiences the similar effective (c) (i) (ii) Ka (1) H3PO3 ⇔ H2PO3 - + H+ Ka (2) H2PO3 - ⇔ HPO3 2- + H+ (iii) (iv) P-H bond is non-polar, hence H+ cannot be formed easily. nuclear charge as that for the valence electron in phosphorus. Energetically unfavorable to arrange five iodine atoms around small central P Atom, OR Steric hindrance prevents five iodine atoms to be bonded to P atom OP O H O H H More difficult to dissociate another H+ from a negatively charged ion. AJC/H2 Chemistry/Prelims2008/P2Answers 1

2 (a) (i) NaCl + H2SO4 → NaHSO4 + HCl (ii) HI is more reducing than HCl, hence it is oxidized to I2 by conc H2SO4 (b) (i) 0.0500 mol min-1 (ii) 0.0250 mol (iii) 0.0750 mol (iv) 25% (v) (c) (i) reagent(s) Any one of the following: - All the HCl that emerged from chamber was absorbed by NaOH - C2H4 does not undergo combustion inside the chamber condition(s) (ii) Cl2 Room temperature Type of reaction mechanism Electrophilic addition H C H C H H Cl Cl CH 2Cl CH 2 + Cl - CH 2ClCH 2Cl δ+ δ- slow fast AJC/H2 Chemistry/Prelims2008/P2Answers 2

(iii) (iv) By-product, CH2ClCH2OH, can be formed because H2O is a nucleophile and will react with the CH2ClCH2 carbocation. 3 (a) (i) (ii) M(NO3)2 → MO + 2NO2 + ½O2 (iii) AJC/H2 Chemistry/Prelims2008/P2Answers 3 Energy Reaction Pathway Ethene + Cl2 CH2ClCH2Cl CH2ClCH2 ⊕ Ea ’ Ea ’’ Ea ’ > Ea ’’ Mg reacts with slowly with hot water, effervescence observed and a white ppt is formed. Ba reacts rapidly with cold water effervescence observed and a colourless solution is formed. Mg(NO 3)2 decomposes at the lower temperature Mg2+ is smaller than Ba2+, hence possesses higher charge density / higher polarizing power Electron cloud of NO 3 - is distorted more easily N-O bond in NO3 - breaks more easily / is more weakened in Mg(NO3)2

(b) (i) Ksp = [Mg2+][F-]2 (ii) 2.10 × 10-3 mol dm-3 (iii) [F-] in fluoridated drinking water = 5.26 × 10-5 mol dm-3 Good source. (iv) 7.40 × 10-4 mol dm-3 4 (a) (i) Kp = 2 2 O 2 NO 2 N .P P P O (ii) 300K K is the largest at 300K, (iii) 0.1 (b) (c) (d) From the Kp values, the formation of NO is endothermic which is favoured by the high temperature in car engines. The formation of NO2 is exothermic which is favoured by the lower . temperature outside Time is needed for the NO emitted from the car engine to cool down to 300K Lowering the pressure will not reduce the formation of NO Pressure has no effect on the position of equilibrium between N2, O2 and NO as there is no change in the number of gas molecules. AJC/H2 Chemistry/Prelims2008/P2Answers 4

5 (a) structural formula no. of stereoisomers AJC/H2 Chemistry/Prelims2008/P2Answers 5 N CH2 CH3 NH O2N NO2 CH3 O H3CC H 3 C O CH2CH3 CH3 CH3 CH2Br H3C Br Br Carvone 2 Menthol 8 (b) terpenoid reagent and condition structural formula of the product Carvone 2,4-dinitrophenylhydrazine, room temperature Menthol CH3CH2COCl, room temperature (c) (i) H2C2O4 + [O] → 2CO2 + H2O (ii) O CH2 CH3 CH3 OH H3CC H 3 CH3 C CH3 O HO2CC H 2CH2 C CH3 O compound B compound C

AJC/H2 Chemistry/Prelims2008/P2Answers 6 (iii) test Na test / Na2CO3 test / PCl or SOCl2 test observations For C: H2 / CO2 / HCl fumes produced For B: No H2 / CO2 / HCl fumes produced