TJC_H2_CHEM_P3_STUDENTS_ANS Prelim

11 Essay Answers: 1 (a) (i) PV = nRT m / V =  = P  Mr RT  density of NO 2 = 1.01  105  46.0 8.31  298  = 1880 g m3 (to 3 s.f)  = 1.88  103 g cm3 (ii) NO2(g) + N2O(g)  3NO(g)  H r = H  f (products) – H f (reactants) = 3(+90.4) – (+33.9 + 81.6) = +155.7 kJmol -1  At 800 K, Gr = H r – TS r = +155.7 – 800(+171.4  103) = +18.6 kJ mol1  Since Gr > 0 at 800 K, the reaction is not spontaneous. (iii)  OR [Correct number of dots and crosses for N and 2 O atoms.]  Linear molecule (b)  For AgCl to precipitate , [Ag +] [Cl] > 1.8  1010 (1.8  102)  [Cl] > 1.8  1010 [Cl] > 1.8  108 mol dm3  For PbCl 2 to precipitate, [Pb2+] [Cl]2 > 1.7  105 (2.0  102)  [Cl]2 > 1.7  105 [Cl] > 2.9  102 mol dm3  AgCl will precipitate out from the solution first as it requires a much smaller concentration of Cl. (c) (i)  X+(g)  X2+(g) + e  Going down the X + ions of Group II, both the nuclear charge and screening effect increase. As the increase in screening effect outweighs the increase in nuclear charge, the effective nuclear charge of the X + cations decreases down the group.  Hence moving down the group, the valence electron in the X + cation becomes further away from the nucleus and is less strongly attracted, thus needing less energy for its removal. N N O       N N O    

12 (ii)  Mg 2+ ion has a higher charge density than Ba 2+ ion and hence is more polarising. Mg2+ ions polarise the large NO3  ions to a greater extent and hence causes the N – O bonds to be weakened. Thus Mg(NO 3)2 is less thermally stable and decomposes at a lower temperature than Ba(NO3)2.  Equations: Mg(NO3)2  MgO + ½ O2 + 2NO2 Ba(NO 3)2  BaO + ½ O2 + 2NO2 (iii)  M is magnesium.  MgCO 3 undergoes decomposition when heated. Since it forms CO2(g) which escapes when it is formed, there is a loss in mass during heating. MgCO 3(s)  MgO(s) + CO2(g)  When X which is MgO is added to water, it reacts to form insoluble Mg(OH) 2. As the concentration of aqueous OH  is low, the resulting solution is only weakly alkaline. MgO + H 2O  Mg(OH)2  Adding H 2SO4(aq) to the above mixture produces MgSO 4 which is soluble in water. Hence no precipitate is observed.  1 mark for the two equations for decomposition of MgCO