HCI_H2_CHEM_P3_ANSWERS Prelim

1    Hwa Chong Institution 2011 Prelim Paper 3 Answers 1 (a) (i) At the water surface, the surfactants are orientated such that the hydrophilic heads are exposed to water for favourable ion-dipole interactions, whereas the hydrophobic tails are exposed above the water surface, maintained by dispersion forces. (ii) Mechanical agitation during washing causes oil or grease to be surrounded by surfactant molecules and broken into small droplets so that relatively small micelles are formed. The charged surfaces of the micelles prevent the oil or grease droplets from coalescing again and the micelles are easily washed away. (b) (i) PCl 5 (s), room temperature (ii) acid-base reaction (iii) H2NCH2CH2OH is bifunctional and the acid chloride would be able to react with both the amine and the alcohol to give side products (C 11H23CO2CH2CH2NH2 or C11H23CONHCH2CH2OCOC11H23). (iv) CH3CH2Cl CH2=CH2 CH2BrCH 2OH H2NCH 2CH2OHethanolic NaOH heat Br2 (aq) conc NH 3 heat in a sealed tube (c) Electrophilic Addition NHBr Br  NHBr Br N Br slow fast - HBr N Br H Br Alternatively, Electrophilic Addition (followed by nucleophilic substitution)

2    NHBr Br  NHBr Br NH Br Br   N Br slow fast - H+ (d) (i) Kp  PSO3 2 PSO2 2  PO2 PSO2 = 0.15 atm PO2 = 0.075 atm PSO3 = 8 – 0.15 – 0.075 = 7.775 atm 075.015.0 775.7 2 2  pK = 3.58 x 104 atm–1 (ii) V2O5 acts as a heterogeneous catalyst. It is in a different phase from the reacting mixture. The reactant molecules readily adsorb onto the surface of the catalyst. The adsorption process brings the reactant molecules closer thus increasing their concentrations at the catalyst surface ; adsorption also weakens the strong S=O and O=O bonds ; and allows these molecules to be orientated in the right positions for reaction . The catalysed pathway thus involves lower activation energy. When the reaction is complete, the molecules will desorb and diffuse away from catalyst surface so that the active sites are exposed for further reaction.

3    2 (a) Liquid fuels must be vaporized before combustion can take place. As diesel oil has more C atoms than gasoline and hence a larger electron cloud , the dispersion forces between its molecules are stronger. Its boiling point is higher and hence vaporizes much less easily and therefore is not easily ignited. (b) (i) C2H5OH (l) + H2O (l) + O2 (g)  4H2 (g) + 2CO2 (g) OR 2C2H5OH (l) + 4H2O (l) + O2 (g)  10H2 (g) + 4CO2 (g) (ii) The sign of entropy change should be positive. There is a greater number of