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