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1 NJC H2 Chemistry Prelim Paper 3 Suggested Answers1 (a) (i) BaCO3(s) BaO(s) + CO2(g) (ii) Down the group, cations of Group 2 metals have the same charge but their size increases. The 𝑐ℎ𝑎?𝑔𝑒?𝑖𝑧𝑒ratio 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: (NH2)2CO + H2O 2NH3+ CO2 Reaction 2: Ba(OH)2 + CO2BaCO3+ H2O (c) (i) Ka= [HCO3-][H3O+][H2CO3]Ka= (x)20.100−x= 4.3 × 10–7mol dm–3 [H+] = √4.3 × 10−7× 0.1= 2.074 × 10–4mol dm–3pH = 3.68 (ii) H2CO3+ 2NaOH Na2CO3+ 2H2O Amount of H2CO3= 251000×0.1 = 0.0025 mol Amount of NaOH required = 0.005 mol Volume of NaOH required = 0.0050.125= 0.04dm3= 40.0 cm3 (iii) At maximum buffering capacity, pH = pKa1 = 6.37
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