AJC H2 Chem 2012 Prelim P2 Soln

1 (a) (i) AgNO3(aq) 2012 H2 Chemistry Prelim Examinations Paper 2 Solutions (ii) NH3(aq) 2 (b) Step Expected Location of each Observations anion 1. Add excess AgNO3(aq). Yellow ppt Cl– and I– exists as Filter AgX in the pptthe mixture. 2. Add excess NH3(aq) to Yellow residue I– exists as AgI in the residue. Colourless filtrate residue Filter the mixture. Cl– remains in filtrate 3. Add HNO3(aq) to the White ppt Cl– exists as AgCl in filtrate. the ppt Filter the mixture after adding excess AgNO3(aq) and after adding excess NH3 to the residue correct identification of ions Yellow ppt with AgNO3 in step 1. White ppt negates the mark. (Note: white ppt is obscured by the yellow ppt.) Yellow residue and colourless filtrate in step 2. White ppt with acid in step 3. FYI The mixture has to be filtered after adding AgNO3 to separate AgCl and AgI from the cations so that insoluble metal hydroxides would not be formed when NH3(aq) is added. (c) To remove other anions (e.g. CO3 2- and SO3 2-) that form insoluble compound with Ag+ (aq). (All nitrates are soluble). (d) Reagent: NH3(aq) and cation: Al3+ Al(OH)3 is insoluble while Zn(OH)2 is soluble in excess NH3(aq) due to complex formation. Al3+ + 3OH– Al(OH)3(s) Zn(OH)2 + 4NH3 [Zn(NH3)4]2+(aq) + 2OH– [allow 2 separate equations showing dissolving of Zn(OH)2] Zn(OH)2 Zn2+ + 2OH– [Zn(H2O)6]2+ + 4NH3 [Zn(NH3)4]2+ + 6H2O

2 (e) Step 1: To 2 cm depth of each of the unknown in a test tube, add a few drops of AgNO3(aq). (allow distilled water – gives white fumes) C6H5Br C6H5CH2Br CH3COBr No ppt No ppt Cream ppt Step 2: To fresh samples of 2 cm depth each of the other 2 compounds in a test tube, add NaOH(aq) and heat gently for 5 minutes. Step 3: To the cooled samples of each of the remaining 2 compounds in a test tube, add excess HNO3(aq), followed by AgNO3(aq). C6H5Br C6H5CH2Br No ppt Cream ppt Appropriate reagents: AgNO 3(aq), NaOH(aq), HNO3(aq) Appropriate conditions : heat (not reflux, warm), (cool), excess Quantities mentioned in all 3 steps: 2 cm depth / 1 – 5 cm3, (a few drops) Correct observations in steps 1 and 3 Alternatives: Add distilled water or dil HCl. Only ethanoyl bromide gives white fumes. Add aqueous silver nitrate and heat. Only ethanoyl bromide and (bromomethyl)benzene give cream ppt.

3 2 (a) (i) Graph B is CO2 at 473 K whilst graph C is SO2 at 298 K. SO2 deviates more than CO 2 at 298 K. SO2 molecules are held by stronger permanent dipole–permanent dipole attractions (or van der Waals forces of attraction) as compared to weaker instantaneous dipole– induced dipole attractions (or van der Waals forces of attraction) between CO2 molecules. CO2 at 473 K deviates less than CO 2 at 298 K. At higher temperature, CO 2 molecules possess higher average kinetic energy and are more able to overcome forces of attraction between the molecules. (ii) p = 2 2 ) ( V a n nbV nRT -- = 2 3 2 53 )105 . 0 ( )687. 0 ( ) 1 ( )10 68. 5105 . 0 ( )298)(31.