AJC H2 Chemistry 9647 N2012 P3 Suggested Solutions

©2013AndersonJC/CHEM 1 H2 Chemistry 9647 N2012 P3 Suggested Solutions 1 (a) Both Al2O3 and MgO behave as basic oxides in the presence of acids. MgO + 2H+ Mg2+ + H2O Al2O3 + 6H+ 2Al3+ + 3H2O Both Al2O3 and P4O10 behave as acidic oxides in the presence of alkalis. P4O10 + 12OH– 4PO43– + 6H2O Al2O3 + 2OH– + 3H2O 2[Al(OH)4]– Al2O3 is amphoteric because it is an ionic oxide with covalent character (acidic) due to the highly charged A l3+ ions distorting the electron cloud of the O 2– ions. Hence Al2O3 has both acidic and basic properties. [1] [1] [1] [1] [1] Comments: Students should emphasise on the interaction between A l3+ and O 2– instead of with H 2O molecules because the question specified Al2O3 instead of Al3+(aq). (b) (i) Al2Cl6 Al Cl Cl Cl Cl Al x x x x x x Cl Cl [1]: correct dot and cross diagram with two dative bonds (circled) between Cl and Al (each with 2 dots or 2 crosses) clearly indicated [1] (ii) Trigonal planar [1] (iii) Increasing the temperature favours the forward endothermic reaction. Position of equilibrium shifts right to absorb the increase of heat energy. This results in an increase in amount of AlCl3 and decrease in amount of Al2Cl6. [1] [1] (iv) mpV RTM= M = pV mRT ))(250x10(1.16x10 1)(500)(1.50)(8.3 65 −= = 215 g mol–1 Average Mr = 215 [1]

©2013AndersonJC/CHEM 2 (v) x = 1 – y 214.9 = 267(1–y) + 133.5y 214.9 = 267 – 267y + 133.5y y = 0.39 x = 0.61 [1] [1] (vi) p(Al2Cl6) = 0.61 x 1.16 x 105 = 70760 Pa p(AlCl3) = 0.39 x 1.16 x 105 = 45240 Pa [1] [1] (vii) 62 3 ClAl 2 AlCl p p pK = 70760 (45240)2 = = 28924 Pa [1]: correct Kp expression [1]: correct answer and units [2] (c) (i) A and B reacts with 2,4–DNPH  aldehyde / ketone functional group is present O O A B [1] each [2] Comments: The reaction is similar to the mechanism of electrophilic aromatic substitution. For B, intramolecular electrophilic substitution occurs as the carbocation is generated at the end of the alkyl side chain. The ketone ring should only join to positions 1 and 2 of the benzene ring. (ii) Warm separate samples of A and B with I2(aq) and NaOH(aq). A will give a yellow precipitate whereas B will not. The products are CHI3 (iodoform) and sodium benzoate. [1] [1] Comments: Hot, acidified KMnO 4 should not be used as both A and B will react with it. A gives benzoic acid and B gives benzene–1,2–dicarboxylic acid.

©2013AndersonJC/CHEM 3 2 (a) (i) [2]: correct components of each half–cell [1]: half–cells connected via a salt bridge and voltmeter [1]: direction of electron flow Note: “a fully labelled diagram”  one electrochemical cell [4] (ii) Measure the potent