ACJC_H2_CHEM_P2 ans Prelim

- 1 - H2 CHEMI STRY PRELIMS SUGGESTED SOLUTIONS PAPER II 1 (a) (i) Nucleophilic addition. [1] (ii) The gas is HF. [1] (iii) COF2(g) + H2O(l) → 2HF (g) + CO2 (g) [1] (b) (i) Amt of NaOH that has reacted = 100 . 01000 00 . 25 = 2.50 x 10-3 mol [1] (ii) 1CO 2 ≡ 2HF 1CO2 ≡ 2OH- 2HF ≡ 2OH- No of moles of CO2 produced = ½ x ½ x (25.0/ 1000) x 0.100 = 6.25 x 10-4 moles [3] (iii) PV = nRT V = nRT / P = (6.25 x 10 -4)(8.314)(273+30) / 1.01 X 105 = 1.559 x 10-5 m3 = 1.56 x 10-5 m3 [1] (iv) Mass of COF 2 = 6.25 x 10-4 x Mr = 6.25 x 10-4 x 66.0 = 0.0413 g [1] (c) (i) 2 COF 2  CO2 + CF4 Initial / mol 0.500 0 0 Change / mol -2x +x +x Eqm / mol 0.500 – 2x x x 2 2 2 2 ) 2 500 . 0 ( V x V x  = 2.00 Solving, x = 0.1847 mol [1] No of moles of COF 2 that remains undissociated at eqm = 0.500 – 2(0.1847) = 0.131 mol [4] © ACJC 2009 9746/02/Prelim/09 [Turn over

- 2 - (ii) There is no change in the yield of CO2. There are equal number of moles of gaseous products and reactants. [2] 2 (a) X = Mg Y = Ca or Sr or Ba Reason: Reducing power increases down the group. [2] (b) High melting point of MgO [1] (c) (i) SiO2 does not dissolve in water. pH  7 Na 2O dissolves readily in water to give a colourless solution. Na2O + H2O  2NaOH pH  13-14 CaO dissolves sparingly in water. CaO + H2O Ca(OH)2 pH  9-12 [5] (ii) The basic metal oxide will react / undergo neutralisation with the acidic SO 2. OR Acid-base reaction [1] (d) (i)  Br 2 has a smaller electron cloud size than I2.  electron cloud of Br 2 is less polarisable  weaker Van der Waals’ forces between the molecules  More energy is required to overcome the interaction between I 2 molecules, leading to a higher boiling point. [2] (ii)  Cl - has one more electron; hence its p/e ratio is smaller  attraction of the nucleus for the outer electrons is weaker . Therefore it has a bigger radius.  OR electron-electron repulsion [2] (e) Disproportionation 3Cl 2(g) + 6OH-(aq)  5Cl-(aq) + ClO3 -(aq) + 3H2O(l) Smell of Cl2 disappears OR greenish-yellow/pale green/green colour of Cl2 disappears (hard to observe) [2] © ACJC 2009 9746/02/Prelim/09 [Turn over

- 3 - 3 (a) (i) Slow decolourisation of reddish brown solution of bromine. OR Intensity of reddish brown color of bromine fades with