2018 HCIS IB2 HL Chem Prelim P2 (ans)

2018 IB2 HL Chem Prelim P2 – Answers 1 2018 PRELIMINARY EXAMINATION International Baccalaureate 2 Chemistry Higher level Paper 2 1. (a) (i) Weighted average mass of its isotopes when compared to 1 12 the mass of a 12C atom. [1] (ii) Ar = 69(65)+71(35) 100 = 69.7 [1] (b) (i) 1s2 2s2 2p6 3s2 3p6 3d10 [1] (ii) Negative Decrease in number of gaseous particles, less disordered [1] [1] (iii) ΔG = ΔH – TΔS Since ΔH < 0 and ΔS < 0, For a reaction to be spontaneous, ΔG < 0. Hence, |ΔH| > |TΔS| Reaction is only spontaneous at low temperatures. [1] [1] (iv) nGa = 10.0 69.72 = 0.1434 mol nN2 = 1 2 (0.1434) = 0.07172 mol vol N2 = 0.07172(22.7) = 1.63 dm3 Absolute uncertainty = 0.2 69.72 ( 1 2 )(22.7) = 0.04 dm3 (1sf) [1] [1] [1] (c) (i) nNaOH = 14.00 1000 (0.100) = 0.0014 mol [1] (ii) nHCl reacted with NaOH = 0.0014 mol total nHCl = 200 1000 (0.300) = 0.06 mol nHCl in excess = 0.06 – 0.0014 = 0.0586 mol [1] [1] (iii) nGa2O3 = 0.0586 ( 1 6) = 0.009767 mol mGa2O3 = 0.009767 (69.72 × 2 + 16.00 × 3) = 1.83 g [1] [1]

2018 IB2 HL Chem Prelim P2 – Answers 2 2. (a) Na2O + H2O → 2NaOH NaOH releases OH− ions. pH = 13 P4O6 + 6H2O → 4H3PO3 H3PO3 releases H+ ions. pH ≈ 2 / 3 / 4 [Award 1 mark only if both pH are stated correctly.] [1] [1] [1] (b) (i) Both have giant ionic structure / ionic lattice structure, with strong ionic bonds. Charge of Mg2+ > Na+ and radius of Mg2+ < Na+ MgO has stronger ionic bonds, more energy needed to overcome. [1] [1] [1] (ii) SiO2 has giant covalent structure, and strong Si−O covalent bonds. P4O6 has simple covalent/molecular structure, and weak van der Waals’ forces / intermolecular forces. Covalent bonds are stronger than van der Waals’ forces, more energy required to overcome. [Award full 3 marks only if comparison of the strength of the bonds to be overcome is stated.] [1] [1] [1] (c) (i) Lewis structure I Lewis structure II [Award 1 mark each for each correct Lewis structure.] [Award 1 mark each for correct formal charges indicated on both S AND O atoms on each structure.] [4] (ii) Lewis structure I. All atoms have a formal charge of 0. Recap: More stable: (i) structure with greater no. of 0 FC; (ii) negative FC on electronegative atom e.g. F, O, N [1] (iii) sp3 (SiO2 is like diamond) [1] (d) (i) Energy change when 1 mol of MgO (s) is formed from its elements Mg (s) and O2 (g) under standard conditions. [Accept general definition.] [1] (ii) A : enthalpy change of atomization of Mg D : Sum of 1st and 2nd electron affinity of O (g) [1] [1] (iii) ΔH = −(−602) + (702) + (2186) + (248) + (150) = +3888 kJ mol−1 [Award 1 mark for final answer only if sign is shown.] [1] [1] (iv) 2nd IE = 2186 – 738 = +1448 kJ mol−1 After an electron is removed, the remaining electrons experience greater [1] [1]

2018 IB2 HL Chem Prelim P