NJC_H2_CHEM_P3_Solutions.pdf - 1 NJC H2 Chemistry Prelim...

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1 NJC H2 Chemistry Prelim Paper 3 Suggested Answers 1 (a) (i) BaCO 3 (s) BaO(s) + CO 2 (g) (ii) Down the group, cations of Group 2 metals have the same charge but their size increases. The 𝑐ℎ𝑎?𝑔𝑒 ?𝑖𝑧𝑒 ratio of the cation decreases. This leads to a decrease in polarising power. The electron clouds in the carbonate anion is being distorted to a smaller extent down the group and the weakening of C O bond is less significant down the group. More energy is required to break the C O bonds in the carbonate anion down the group. Therefore, the ease of thermal decomposition of the carbonates decreases down the group. (b) (i) Shape: trigonal planar Bond angle: 120 o (ii) Reaction 1: (NH 2 ) 2 CO + H 2 O 2NH 3 + CO 2 Reaction 2: Ba(OH) 2 + CO 2 BaCO 3 + H 2 O (c) (i) K a = [HCO 3 - ][H 3 O + ] [H 2 CO 3 ] K a = ( x) 2 0.100−x = 4.3 × 10 7 mol dm 3 [H + ] = √4.3 × 10 −7 × 0.1 = 2.074 × 10 4 mol dm 3 pH = 3.68 (ii) H 2 CO 3 + 2NaOH Na 2 CO 3 + 2H 2 O Amount of H 2 CO 3 = 25 1000 ×0.1 = 0.0025 mol Amount of NaOH required = 0.005 mol Volume of NaOH required = 0.005 0.125 = 0.04dm 3 = 40.0 cm 3 (iii) At maximum buffering capacity, pH = pK a1 = 6.37
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