MI H2 Chem 2012 Prelim P3 Soln

- 2 - Answer ANY FOUR quest ions 1 (a) When studying Hydrocarbons, we recognised the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine. Because of these environmental concerns, alongside with high oil prices and oil being a limited natural resource, development of cleaner alternative fuels and advanced power systems for vehicles has become a high priority for many governments and vehicle manufacturers around the world. An alternative fuel vehicle is a vehicle that runs on a fuel othe r than "traditional" petroleum fuels (petrol or diesel); and also refers to any technology of powering an engine that does not involve solely petroleum (e.g. electric car, hybrid electric vehicles, solar powered). One such source of alternative fuel is the hydrogen / oxygen fuel cell, which is increasingly used in space crafts. Describe, providing details, the hydrogen / oxygen fuel cell. Include the following in your answer: · Draw a well-labeled diagram for the cell. · Outline the reactions taking place at each electrode, assuming an alkaline medium. · State one advantage of using this fuel cell. [6] ( 1 mark for correct electrolyte, 1 mark for labelled anode, 1 mark for labelled cathode) In alkaline electrolyte Reaction at anode: H 2(g) + 2OH–(aq) ® 2H2O(l) + 2e Reaction at cathode: ½O2(g) + H2O(l) + 2e ® 2OH–(aq) O verall cell reaction: H 2(g) + ½O2(g) ® H2O(l) ;;; ; ;

- 3 - Advantage: Clean products are produced. OR Water produced in hydrogen-oxygen fuel cell used in spacecraft can be used for drinking and washing. ; (b) Cyanogen is a highly toxic gas, composing of 46.2% carbon and 53.8% nitrogen by mass. At 25 ºC and 1 atm, 1.05 g of cyanogen occupies 0.500 dm3. (i) Determine the molecular formula of cyanogen. C N % mass 46.2 53.8 Mole ratio 3.85 3.84 Simple ratio 1 1 E mpirical formula of cyanogen is CN. Using PV = nRT 1.01 x 105 x 0.5 x 10-3 = (1.05 / Mr) x 8.31 x 298 Mr = 51.5 (1 d.p) Thus, molecular formula of Cyanogen = C2N2 ; ; ; (ii) Draw the dot-and-cross diagram for cyanogen. Hence, state its shape. N≡C-C≡N , linear ; ; (iii) ‘Cyanogen is soluble in ethanol.’ Explain this statement with the aid of a suitable diagram. [7] Cyanogen can form hydrogen bonds with ethanol as it has a lone pair of electrons on nitrogen, thus soluble in ethanol. (lone pairs on N and O, +, - must all be clearly shown) ; ; (c) Sketch and explain the trend observed for the atomic and ionic radii of the elements (from Na to Cl) in Period 3. [4] ;

- 4 - Atomic Radius: Across the period, nuclear charge ↑ as proton number ↑ change in the screening effect is negligible (Same no. of inner shells of e-s across the period). Thus the outer e-s are more strongly attracted by the nucleus \ atomic radii ↓ Cations (Na+ to Si4+) cations have one shell less than neutral atoms. Thus, the outer e-s are more strongly attracted by th