DHS Prelim P3_Answer_scheme

1 © Dunman High School 2015 H2 Chemistry Prelim P3 2015 Year 6 H2 Chem Preliminary Examination Paper 3 (Suggested Solutions) Answer any four questions on writing paper. 1 (a) (i)  The difference in ionic radius/ size between the cation and anion is large. This leads to poor packing of the ions in the solid state. The cationic radius is large which results in a low magnitude of the lattice energy/ less exothermic lattice energy. OR the dispersion of charge of the cation due to resonance weakens the ionic bonds /interaction. (ii) A large amount of energy is required to overcome the strong ionic bonds/ interaction in [emim]BF4 hence they have high boiling point. (iii) It allows volatile organic compounds to be easily removed from the ionic liquids by simple distillation. OR They can be used as a medium for reactions involving ionic reagents since ionic reagents can dissolve readily in it. (b) Let the equilibrium partial pressure of NO be 2x. N2 (g) + O2 (g) 2NO (g) Initial partial pressure/atm 0.80 0.80 0 Change in partial pressure/atm –x –x +2x Equilibrium partial pressure/atm 0.8 – x 0.8 – x 2x 𝐾𝑝 = (𝑃𝑁𝑂)2 (𝑃𝑁2)(𝑃𝑂2) 0.40 = 4𝑥2 (0.8−𝑥)2 x = 0.1922 PO2 = PN2 = 0.80 – 0.1922 = 0.608 atm PNO = 2 x 0.1922 = 0.384 atm Note: Since the moles of reactants : products is 1 : 1, total pressure remains unchanged as volume of flask is fixed at 1 dm3. PO2 + PN2 + PNO = 0.80 x 2 = 1.60 atm.

2 © Dunman High School 2015 H2 Chemistry Prelim P3 (c) (d) The catalyst will increase the rate of both the forward and the reverse reactions hence equilibrium will be established more quickly. This means that the same equilibrium partial pressure of N 2 and NO will be established before 30 minutes. The rate of change of partial pressure for N 2 and NO will also be faster OR the graphs have steeper gradient s. The equilibrium partial pressure of all species will stay unchanged. (e) ΔH = B.E. (bonds broken) – B.E. (bonds formed) = 994 + 496 – 2(607) = +276 kJ mol–1 > 0 Since ΔH of the fwd reaction is endothermic, equilibrium will shift forward when temperature increases. 0.608 0.384 NO N2

3 © Dunman High School 2015 H2 Chemistry Prelim P3 (f) (g) L: 0.2 mol of L undergoes redox with Na to form 0.2 mol of H2 gas  There are 2 alcohol groups in L L turns moist red litmus paper blue  It contains a basic group/ it is basic/ it contains an amine group. L has a higher pKb than (CH3CH2)2NH but a lower pKb than C6H5NH2  L is less basic than (CH3CH2)2NH but more basic than C6H5NH2.  Lone pair of N atom in L less available than that of (CH 3CH2)2NH due to the electron withdrawing alcohol groups.  Lone pair of N atom in L more available than that of C 6H5NH2 as there is no delocalisation of the lone pair on N for L.  L has pKb of 4.0 (accept any value between 3.1 – 9.4) 2 (a)