RVHS H2 Chem 2012 Prelim P2 Soln

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 2 1 Planning (P) Calcium salts are used in many industrial processes. Calcium oxide is used in the production of cement while calcium nitrate is sometimes used to accelerate the setting of concrete. Calcium oxide is a white hygroscopic solid. Calcium oxide can react with water and as such can cause severe irritation when in contact with skin, eyes or if inhaled. Calcium nitrate is a lso a white hygroscopic solid. Calcium nitrate decomposes on heating to form the oxide, nitrogen dioxide and oxygen. (a) Write a balanced equation, including state symbols, for the thermal decomposition of calcium nitrate. Ca(NO3)2(s) ® CaO(s) + 2NO2(g) + ½ O2(g) [1] (b) Calculate the change in mass of the solid you would expect if 1.00 mol of calcium nitrate underwent thermal decomposition. Loss in mass = Mass of NO2 and O2 = 2(14.0 + 2(16.0)) + ½(2(16.0)) = 108 g [1] (c) A container of calcium nitrate was contaminated with calcium oxide. Design an experiment to determine the percentage purity of calcium nitrate by a gravimetric method. You are provided with about 2 g of the impure calcium nitrate. (In gravimetric methods, the o bjective is to form a thermally stable solid so that the mass can be determined accurately for analysis.) Your plan should include the following: · A full description of the procedure. · Details of how you would calculate the percentage purity from the measurements made. [Mr of Ca(NO3)2 = 164] 1. Weigh accurately a clean and dry crucible (Accept: hard glass tube). Add the sample and weigh the crucible with its contents. 2. Heat the crucible strongly for about five minutes to decompose the calcium nitrate. 3. Place crucible in a dessicator to cool. Then weigh the crucible and its contents. 4. Repeat the heating, cooling and weighing process until the mass of the crucible and its contents is consistent within 0.05 g in difference. Loss in mass = initial mass of crucible and contents – final mass of crucible and contents after final heating [8]

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 3 Amount of Ca(NO3)2 = loss in mass / 108 Mass of Ca(NO3)2 = amount of Ca(NO3)2 ´164 % purity = mass of Ca(NO3)2 / initial mass of solid ´100% (d) State and explain clearly one precaution you would take to improve the accuracy of your results. Sample should be stored in dessicator while cooling to prevent absorption of water from the air. [1] (e) Identify one potential safety hazard in this experiment and state how you would minimise the risk. NO2 is toxic. Perform experiment in fume cupboard. OR CaO causes skin / eye irritation / irritation if inhaled. Wear gloves, mask etc. [1] [Total: 12] 2 Titanium(IV) oxide TiO2 is the naturally occurring oxide of titanium. It is the main ingredient in sunscreen lotion due to its high refractive index and strong UV-light absorbing capability. (a) Using the following information as well as relevant data from the Data Booklet, draw a Born- Haber cycle for the formation of TiO 2(s) and calculate the lattice energy of TiO2(s). Standard enthalpy change of formation of TiO2(s) −945 kJ mol−1 Standard enthalpy change of atomisation of Ti +471 kJ mol−1 1st electron affinity of oxygen −142 kJ mol−1 2nd electron affinity of oxygen +844 kJ mol−1 Ti(s) + O2(g) TiO2(s) Ti(g) + 2O(g) Ti 4+(g) + 2O2−(g) [3] +471 −945 +661 + (+1310) + (+2720) + (+4170) +496 2[(−142) + (+844)] Lattice energy of TiO2

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 4 By Hess’ Law, −945 = +471 + (+661) + (+1310) + (+2720) + (+4170) + (+496) + 2[(−142) + (+844)] + LE(TiO2) LE(TiO2) = −12177 kJ mol−1 = −1.22 ´ 104 kJ mol−1 (b) Crude titanium( IV) oxide is purified via the Chloride process. In this two-step process, the crude ore (containing at least 70% titanium( IV) oxide) is first reacted with carbon and chlorine, represented by the equation shown below. TiO2(s) + C(s) + 2Cl2(g) ® TiCl4(l) + CO2(g) Standard enthalpy change of formation of TiCl4(l) −804 kJ mol−1 Standard enthalpy change of formation of CO2(g) −394 kJ mol−1 Standard molar entropy of TiO2(s) +49.9 J mol−1 K−1 Standard molar entropy of C(s) +5.7 J mol−1 K−1 Standard molar entropy of Cl2(g) +223 J mol−1 K−1 Standard molar entropy of TiCl4(l) +252 J mol−1 K−1 Standard molar entropy of CO2(g) +214 J mol−1 K−1 Using the above information, as well as relevant data from (a) and the Data Booklet, calculate, for the first step of the Chloride process, (i) the standard enthalpy change of reaction, Standard enthalpy change of reaction = [(−804) + (−394)] − [(−945) + 0 + 2(0)] = −253 kJ mol−1 (i i) the standard entropy change of reaction. Note: DS, = SS,(products) – SS,(reactants) Standard entropy change of reaction = [(+252) + (+214)] − [(+49.9) + (+5.7) + 2(+223)] = −35.6 J mol−1 K−1 [2]

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 5 (c) Using your answers in (b) , determine if the first step of the Chloride process is spontaneous at 2000 K. DG = ΔH − TDS DG = (−253) − (2000)(−35.6/1000) DG = −182 kJ mol−1 < 0 Hence, the reaction is spontaneous at 2000 K. [1] (d) The second step of the Chloride process involves the titanium (IV) chloride being distilled off and oxidised in a pure oxygen flame to give pure titanium(IV) oxide and chlorine gas. Write an equation for the reaction in the second step. TiCl4 + O2 ® TiO2 + 2Cl2 [1] [Total: 7] 3 Neidong Forest Recreation National Park, located in the Wulai District in Taipei, has the highest concentration of anions in Taiwan due to the numerous waterfalls and lush forestation in the park. Anions are said to help boost the immune system, among other health benefits. (a) Bromide ions are thought to be one of the anions present in abundance. The concentration of bromide ions in the river waters can be used as an estimate of the concentration of the ions in the park. A 25.0 cm 3 sample of river water was collected and 50.0 cm 3 of 1.00 mol dm –3 Pb(NO3)2 was added. The precipitate formed was filtered off and 3.00 g of precipitate was collected. The filtrate can be regarded as a saturated solution of PbBr2. (i) Calculate the concentration of Pb2+ ions in the filtrate. Amount of PbBr2 collected = 3.00 207+2(79.9) = 8.18 ´ 10–3 mol Amount of Pb2+ in filtrate = ( ) -æö ´-´ç÷èø 350 1.00 8.18 101000 = 0.0418 mol

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 6 [Pb2+] in filtrate = 0.0418 0.075 = 0.557 mol dm–3 (i i) Write an expression for the solubility product, Ksp, of PbBr2. Ksp = [Pb2+][Br –]2 (iii) The numerical value for the Ksp of PbBr2 is 3.9 ´ 10–5. Determine the concentration of bromide ions in the sample of river water. [Br –] in filtrate = -´ 53.9 10 0.557 = 8.37 ´ 10–3 mol dm–3 Total amount of Br– in sample of river water = Amount of Br– in filtrate + Amount of Br– in ppt = (8.37 ´ 10–3 ´ 75 1000 ) + (8.18 ´ 10–3 ´ 2) = 0.0170 mol [Br –] in sample of river water = ¸ 250.0170 1000 = 0.680 mol dm–3 [5] (b) Another anion, W, found in Neidong National Park can be formed when the oxide of a Period 3 element reacts with an alkali. When dilute nitric acid is added to a solution containing W, a colour