NJC_Prelim_P2 ANS

Solutions to 2009 prelim H2 Chemistry P 2 1 1 a i Pe(CH4) = 1.25 atm; Pe(H2S) = 2.5 atm; Pe(CS2) = 1.25 atm; Pe(H2) = 5.0 atm ii Kp = 2 S H CH 4 H CS 24 22 P P P P ) )( ( ) )( ( atm2 iii K p = 100 atm2 iv (I) Equilibrium shifts left. (II) Equilibrium shifts right. Q1b)(i) pH = 5.13 (2 dec pl.) (ii) amt of NH 4 + to be neutralized to form buffer of max buffer capacity = vol of NaOH required = 10 cm 3 pH = pK a = 9.26 Q2a) (i) It is a polypeptide chain joined by amino acid units through peptide bonds. (ii) IIe-Gly-Asp-Glu-Asn-Tyr (b) (i) -CH 2SH + -CH2SH + [O]  -CH2S-SCH2- + H2O -CH2SH + -CH2SH  -CH2S-SCH2- + H2 -CH2SH + -CH2SH  -CH2S-SCH2- + 2[H] (ii) tdid : Valine – phenylalanine: ionic interaction : lysine- glutamic, lysine- aspartic acid H-bonding: Serine – Aspartic acid: (Serine- glutamic acid), (Aspartic – glutamic acid) (c) (i) optimum working pH for this enzyme is 2.5. At pH lower or above 2.5, the existing ionic or H-bonding interactions are destroyed . changing the tertiary structure of protein rendering it inactive. (ii) addition of heavy metal ions which will destroy the disulfide bonds by forming ppt with sulfur. Protein denatured therefore lost its activity. Supply of heat/ high temp also acceptable. (i) CH3CONHC H COOH (CH2)4NHCOCH3

(ii) Both Lysine and compound A can react with alkali to form soluble salt. However, when compound A is formed, it loses its basic property as it forms the amide (not peptide) linkage. No reaction therefore insoluble in acid. However it is still soluble in alkaline medium as it still carries the COOH group that can react to form the soluble salt in alkaline solution. 3(a) (i) Al 2O3 + 6H+  2Al3+ + 3H2O Al2O3 + 2OH + 3H2O  2[Al(OH)4] 2  (b) (i) Ionic Radius Na Mg Al Si P S Cl pH Na2O MgO Al2O3 SiO2 P4O10 7 (ii) Ionic Radius: SO There are two isoelectr onic series in period 3. Within each series, an increasing number of protons results in increasingly stronger nuclear attraction experienced by the valence electrons. Valence electrons are pulled closer to nucleus, leading to a decreasing trend in ionic radius. Anionic series has a larger size than cationic series as the valence electrons are in a higher principal quantum shell, experience weaker nuclear attraction and are less tightly held. pH: Na 2O and MgO are ionic oxides that dissolve completely in water to form basic solution. Al2O3 and SiO2 are insoluble in water pH of solution is that of water. P4O10 and SO3 are covalent oxides that hydrolyse in water to give acidic solution. (c) (i) [Mg(H2O)6]2+ + H2O [Mg(H2O)5(OH)]+ + H3O+

3 S. (ii) H = +4336 kJ mol1 (iii) Formatio