TJC H2 Chem 2013 Prelim P3 Soln

9647/03/TJC Prelim 2013 1 9647 TJC H2 Chemistry Prelim P3 Answers: 1 (a)  M(NO 3)2(s) → MO(s) + 2NO2(g) + ½O2(g)  Charge density of the cation decreases down the group as charge of cation is constant and ionic radius increases.  Polarising power of the cation decreases resulting in less distortion of the electron cloud of the NO3 - ion, and the N-O covalent bond is weakened to a smaller extent. Therefore, ease of thermal decomposition decreases down the group. (b) (i)  Mg 2+(aq) + 2OH-(aq) → Mg(OH) 2(s) A  A l3+(aq) + 3OH-(aq) → A l(OH)3(s)  A l(OH)3 + OH-(aq) → A l(OH)4 -(aq) B (ii)  When carbon dioxide is passed into Q, the following reaction occurs. CO 2(g) + OH-(aq) → HCO 3 - (aq)  The concentration of OH - decreases, causing the position of the equilibrium below to shift to the left according to Le Chatelier’s principle. Thus Al(OH)3 (white solid C) is precipitated. A l(OH)3 + OH-(aq) A l(OH)4 -(aq) Or  Carbon dioxide is an acidic gas, and there will be an acid/base reaction with the Al(OH)4 - complex to form salt and water.  CO 2(g) + Al(OH)4 -(aq) → HCO 3 -(aq) + Al(OH)3(s) C or 2CO2(g) + 2Al(OH)4 -(aq) → 2HCO 3 -(aq) + Al2O3(s) + 3H2O(l) C (iii) Residue A is Mg(OH)2.  Mass of Mg = 24.3/58.3 x 0.18 = 0.0750 g % by mass Mg = 0.075/1.75 x 100% = 4.29 % After heating to constant mass, the residue is Al2O3(s).  Mass of A l = 54/102 x 3.16 = 1.67 g % by mass A l = 1.67/1.75 x 100% = 95.4% (c) (i)  (ii) 2O3 3O 2 1381

9647/03/TJC Prelim 2013 2 At t = 0 s, vol. of O3 = 38 cm3 and vol. of O2 = 762 cm3 At t = 1500 s, let vol. of O3 = (38 – y) cm3 and vol. of O2 = (3y/2 + 762) cm3 (38 – y) + (3y/2 + 762) = 818 y = 36.0  Volume of O3 = 38 – 36 = 2.00 cm3 (iii) Plot of Volume of O3 vs time. (correct labeling of axes, plotting of points)  Both half-lives are constant = 400 s (indicate at least 2 t 1/2 on graph)  Therefore, the reaction is first order with respect to ozone. (iv) k = ln2 / t1/2 k = 1.73 x 10-3 s-1 Rate = k[O3] 1382

9647/03/TJC Prelim 2013 3 (v)  A catalyst provides alternative pathway of lower activation energy, which is the minimum amount of energy the particles must collide with for reaction to take place.  The number of molecules having energy greater than or equal to activation energy increases, resulting in an increase in frequency of effective collisions and the rate of decomposition.  [Total: 20] 2 (a) (i)  The primary structure of a protein refers to the number and sequence of amino acids in the polypeptide chains.  The amino acids are bonded by peptide bonds in the polypeptide chain. (ii)  The -