JPJC Prelim P3 Ans

¤ Jurong Pioneer Junior College [Turn Over 1 (a) Deductions B is soluble in water Not MgO, Al2O3, SiO2 Aq. solution of B reacts with Na2CO3 in 2:3 ratio to form CO2(g) Aq. solution of B is acidic (9) Since the reacting ratio is 2:3, the acid formed must be tribasic ∴ B is P4O10/P4O6 [1] C is soluble in water Not MgO and Al2O3, SiO2 Aq. solution of C reacts with NH4+ to give NH3(g) Aq. solution of C is basic (9) ∴ C is Na2O [1] A reacts with both B and C A is amphoteric (9) ∴ A is Al2O3 [1] 2-39: 1m Equations for reaction with water: B: P4O10(s) + 6H2O(l) o 4H3PO4(aq) [1] C: Na2O(s) + H2O(l) o 2NaOH(aq) [1] For ([DPLQHU¶V Use (b) (i) 3Mg(NH2)2(s) ĺ Mg3N2(s) + 4NH3(g) (ii) Mg(NH2)2 is less thermally stable than Ba(NH2)2. (9) Mg2+ has the same charge but a smaller ionic radius hence a higher charge density than Ba2+. Thus Mg2+ polarises the large NH2³ anion more. (9) This weakens the N-H bond in the Mg(NH2)2 more (9) and thus a lower temperature is needed to decompose magnesium amide. 49: 2m; 2-39: 1m (iii) Rxn 1: Mg3N2(s) + 6H2O(l) ĺ 3Mg(OH)2(s) + 2NH3(aq) Rxn 2: NH3(aq) + HCl(aq) ĺ NH4+(aq) + Cl²(aq) 3 3 32 3 32 Amount of acid From rxn 2: 1 H 1 NH Amount of NH produced from reaction with air 0.00600 mol From rxn 1: 1 Mg N 2 NH Amount of Mg N formed Mass o

{ { 12.0×0.50 0.00600 mol1000 0.006000.00300 mol2 [1] 32 32 f Mg N in 1.00 g sample percentage of Mg N in 1.00 g sample ? 0.00300×100.9 0.303 g 0.303×1001.00 30.3 %[1]

2 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAM/2021 1 (c) (i) Precipitate formed is AgCl. initial sp Ag at point of mixing Cl 30 Cl at point of mixing 30 5 For precipitation to take place: ionic product (AgCl) (AgCl) ionic product K

ªº ¬¼ ªº u¬¼ªº ¬¼ t -3 -3 - 0.0100×5 30+5 1.428×10 mol dm 1.428×10 [1] initial C l §·ªº¬¼¨¸ t¨¸ ©¹ ªº?t¬¼ - 3 -10 initial -7 -3 Cl ×30 1.8×1030+5 1.47×10 mol dm [1] (ii) Cream ppt is AgBr, halide present is Br³. (iii)

‡‡ ‡ ‡ ‡ ' ' ' ' ?' [1] [1] -10 4 -1 -1 4 ppt -1 -1 8.31 298 ln 1.8×10 5.56×10 J mol -55.6 kJ mol -5.56×10 =¨+ -178000 J mol -178 kJ mol ppt ppt ppt ppt Using / G GH T S H (iv)

‡ ‡' ' ' ' ? ¦ [1] [1] -1 -1 +178 kJ mol +178=-LE+ -473 + -378 -1030 kJ mol 1 ppt soln soln hyd Since 178 kJ mol , hence Since ions HH HL EH LE (v) This difference indicates that AgCl is not purely ionic / there exists covalent character in the ionic bond in AgCl. [1] The electronegative difference between Ag and Cl is so small that complete transfer of an electron from the silver to the chlorine is not possible. [1] OR Cl³ has a large anion radius allowing it to be readily polarised by Ag+ ions. [1] [Total: 20]

3 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAM/2021 [Turn Over 2 (a) (i) Indicator for first end point: cresol red Indicator for second end point: