HCI Prelim P3_ANSWERS

1 2015 HCI Prelim H2 Chemistry Paper 3 Answers 1 (a) (i) Heterogeneous catalysis [1] (ii) Optical isomerism [1] [1] (iii) The addition of H 2 to one side (bottom face) of the planar alkene molecule creates a chiral alkane molecule. [1] When the opposite side (top face) of the alkene is now adsorbed to the surface of the Ni catalyst, the addition of H 2 produces a chiral alkane molecule which is the non - superimposable mirror image of the product formed at the bottom face. [1] As there is 50% chance for each side (face) of the alkene to bind to the surface of the catalyst, the two enantiomers are produced in a 1:1 ratio resulting in a racemic mixture. Hence the product mixture does not rotate the plane of plane-polarised light. [1] ================================================================= If a diagram is used, it will substitute the 1st two marks: Show opposite faces adsorbed to the catalyst surface [1] Indicate which carbon forms the chiral alkane and the corresponding enantiomer. [1] (b) (i) H reaction = (B.E. of Bonds Broken) – (B.E. of Bonds Formed) = [B.E (C = C) + B.E. (H – H)] – [B.E (C – C) + B.E. (C – H) x 2] = (610 + 436) – (350 + 410 x 2) = – 124 kJ mol-1 Correct bond energies used [1] Correct equation and answer [1]

2 (ii) H reaction is exothermic. S reaction is negative as there is a decrease in the number of moles of gas in the reaction. [1] Given that G reaction = H reaction – TS reaction G reaction is expected to be negative only at low temperatures and so the reaction would be spontaneous only at low temperatures. [1] (iii) Although the reaction is energetically feasible, the activation energy is very high. Or as the low temperatures are necessary for the reaction to be feasible, the rate of reaction may be very slow. [1] Hence the catalyst provides an alternative pathway with a lower activation energy and the reaction speeds up. [1] (d) (i) A homogeneous catalyst acts in the same phase as the reactants. [1] (ii) Oxidation number of Rh in compound 2: +3 [1] x + (-1) + [(-1) x 2] = 0 x = +3 (working is not required) (iii) The coordination number indicates the number of dative bonds about the central atom or ion. [1] Wilkinson’s catalyst: coordination number = 4 Compound 3: coordination number = 6 [½] for each correct coordination number (iv) Rh is able to exhibit variable oxidation states. [1] The proposed mechanism requires the addition of H 2 to Rh in the form of two hydride ligands, which causes the oxidation state of Rh to increase from +1 to +3. [1] Rh is able to form complexes of variable coordination numbers. [1] The proposed mechanism requires the alkene and H 2 to bind to Rh as ligands before the addition reaction occurs. Or coordination number changes from 4 to 6 from Wilkinson