2019 HCIS IB2 HL Chem Prelim P2 (ans + comments)

2019 IB2 HL Chem P2 (Answers) 1 2019 IB2 HL Chemistry Prelim Exam Paper 2 – Answers 1. 9.74 tonnes of magnesium is need to react with 1.60 x 102 tonnes of impure titanium (IV) oxide, TiO2, ore. All the titanium is completely extracted from the sample of impure ore. The following are reactions involved in the extraction process: Step 1: TiO2 + C + 2Cl2  TiCl4 + CO2 Step 2: TiCl4 + 2Mg  Ti + 2MgCl2 (1 tonne = 106 g) (a) Write an overall equation for extraction of titanium from TiO2. [1] TiO2 + C + 2Cl2 + 2Mg  Ti + CO2 + 2MgCl2 (b) Calculate the amount, in mols, of TiO2 present in the sample of impure ore. [1] nMg nTiO2 = 2 1 9.74 𝑥 106 24.31 nTiO2 = 2 1  nTiO2 = 1 2 x 9.74 𝑥 106 24.31 mol = 200329.08 mol = 2.00 x 105 mol [Note: e.c.f. (a) equation] Some students did not convert tonnes to g, hence, no marks awarded. Some students uses 1.60 x 102 tonnes for calculation. Note that not all 1.60 x 102 tonnes are TiO2. ( ½ for nMg) ( ½ nTiO2) (c) Calculate the mass, in tonnes, of TiO2 in the sample of impure ore. [2] Mass of 200329.08 mol TiO2 = 200329.08 x (47.87 + 2x16.00) (½) = 200329.08 x 79.87 g = 15997961 g = 1.60 x107 g (½) = 1.60 x101 tonnes (accept 16 tonnes) (1) [Note: e.c.f. (a) equation] If student did not convert to g, and get 16 tonnes, only award 1.5. Student must ensure units consistency in their calculation. Some students uses the wrong Ar for Ti. (d) Calculate the percentage by mass of TiO2 in the sample. [1] % by mass of TiO2 in the sample = 1.60 x 101 1.60 x 102 x 100 = 10.0% [Note: e.c.f. (a) equation] Ecf is allowed as long as student uses the answer (in tonnes) calculated in part (c) for calculation of % by mass here.

2019 IB2 HL Chem P2 (Answers) 2 (e) (i) Titanium (IV) oxide, TiO2, is a solid, however titanium (IV) chloride, TiCl4, is a liquid at room temperature. Suggest a reason for this difference between the two compounds, in terms of structure and bonding. [2] TiCl4 is a simple molecular structure, weak intermolecular forces of attraction between the molecules./ Owtte TiO2 is an giant ionic compound, with strong ionic bonds / strong electrostatic forces of attraction between oppositely charged ions/ owtte 5 points : 2 structure, 2 bonding, strength comparison. 2 points stated 0.5, 3 – 1, 4 – 1.5, 5 – full marks (ii) Titanium (IV) ch