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School of Chemistry and Physics UNIVERSITY OF KWAZULU-NATAL, WESTVILLE CAMPUS MAIN EXAMINATION CHEM 161: CHEMICAL ENGINEERING CHEMISTRY 1 DURATION: 3 Hours TOTAL MARKS: 100 Internal Examiner: S. Singh Internal Moderator: H. B. Friedrich IMPORTANT: COMPLETE THIS SECTION IMMEDIATELY SURNAME AND INITIALS (Optional): ________________________ SIGNATURE: _____________________ Student Number Seat Number INSTRUCTIONS 1. Non programmable calculators may be used but all working must be shown. 2. You must write legibly in black or blue ink. Pencils and Tipp-Ex are not allowed. 3. Your answers must be written on the question paper in the spaces provided. The left-hand pages may be used for extra space or for rough work. 4. This question paper consists of 15 pages (including the cover page) and 6 questions. Please check that you have them all. A periodic table and a data sheet are provided separately. 5. You will also require a copy of Section A - Multiple Choice Questions. Question No. Internal Mark External Mark 1 2 3 4 5 6 Total School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination SECTION B TOTAL FOR SECTION B: 70 MARKS Answer All Questions Question 1 (a) The atomic masses of the two stable isotopes of bromine, 79Br (50.69%) and 81Br (40.31%), are 78.9183 u and 80.9163 u respectively. (i) State the number of neutrons in the heavier isotope of bromine. (1) (ii) Calculate the average relative atomic mass of bromine. (2) 2 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (b) An organic solvent containing carbon, hydrogen and sulfur yields CO2, H2O and SO2 on combustion. When subjected to combustion analysis, a 1.086 g sample produces 2.272 g of CO2, 0.4650 g of H2O and SO2. Determine the empirical formula of the solvent. 3 (4) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (c) A 50.00 g sample of Mg(OH)2 solution is mixed with 70.00 g of H3PO4 solution. A neutralization reaction takes place which is represented by the equation 3Mg(OH)2 + 2H3PO4 Mg3(PO4)2 + 6H2O (i) Determine the limiting reagent. (Show all calculations). (3½) (ii) Calculate the mass of excess reagent that remains after completion of the reaction. (2½) 4 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (d) A 0.4308 g sample of iron ore is dissolved in acid and the iron converted into Fe2+. This solution is reacted with a solution of KMnO4. The reaction requires 27.35 mL of a 0.02469 mol dm-3 solution of KMnO4. Calculate the percentage of Fe in the ore. The equation for the reaction is: 5Fe2+ + MnO4- + 8H+ 5 Fe3+ + Mn2+ + 4H2O (3) [16] 5 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination Question 2 (a) Calculate the frequency (in s-1) of the electromagnetic radiation emitted by a hydrogen atom when its electron undergoes a transition from the n = 4 level to the n = 2 level. (b) (3) In the following assignments of the four quantum numbers, write acceptable values for the missing ones. What are the orbitals corresponding to these quantum numbers? (i) n = ________ l=1 ml = 0 ms = ________ l=0 ml = ________ ms = +½ orbital = ________ (ii) n = ________ orbital = ________ 6 (3) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (c) Draw the Lewis structure and assign formal charges for nitric acid (HNO 3) in which the three O atoms are bonded to the central N atom and the ionizable H atom is bonded to one of the O atoms. (d) (4) Showing all the relevant steps, use VSEPR theory to predict the molecular geometry of: (i) CH4 (2) (ii) NH3 (2) [14] 7 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination Question 3 (a) A chemist has synthesized a greenish-yellow gaseous compound of chlorine and oxygen and finds that its density is 7.71 g L-1 at 36 °C and 2.88 atm. Calculate the molar mass of the compound and determine its molecular formula. (b) (4) A mixture of 15 g methane (CH4) and 22 g ethane (C2H6) are forced into a container until the total pressure is 2.2 atm. What are the partial pressures of methane and ethane? 8 (3) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (c) Calculate the standard enthalpy of formation of acetylene (C2H2) from its elements. The equations for each step and the corresponding enthalpy changes are (a) C(graphite) + O2(g) CO2(g) Horxn = -393.5 kJ mol-1 (b) H2(g) + ½O2(g) H2O(l) Horxn = -285.8 kJ mol-1 (c) 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l) Horxn = -2598.8 kJ mol-1 (5) [12] 9 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination Question 4 (a) The following data were determined for the reaction: A(g) + B(g) AB(g) Experiment [A] (M) [B] (M) Initial Rate (M/s) 1 5.0 × 10-3 2.0 × 10-3 1.3 × 10-5 2 10.0 × 10-3 2.0 × 10-3 5.0 × 10-5 3 10.0 × 10-3 4.0 × 10-3 10.0 × 10-5 Determine the rate expression and the value of the rate constant k. 10 (5) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (b) Iodine atoms combine to form molecular iodine in the gas phase I(g) + I(g) I2(g) This reaction follows second-order kinetics and has a high rate constant of 7.0 × 109 M-1 s-1 at 23°C. The initial concentration of I was 0.086 M. (i) Calculate the concentration after 2.0 min. (3) (ii) Calculate the half-life of the reaction. (2) [10] 11 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination Question 5 (a) The following equilibrium process has been studied at 230 oC: 2NO(g) + O2(g) ⇌ 2NO2(g) The concentration of the reacting species at equilibrium was found to be [NO] = 0.0542 M, [O2] = 0.127 M and [NO2] = 15.5 M. Calculate the equilibrium constant, Kc. (b) (2) The equilibrium constant (Kc) for the reaction N2O4(g) ⇌ 2NO2(g) is 4.63 × 10-3 at 25 °C. What is the value of Kp at this temperature? 12 (2) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (c) In an experiment 1.50 mol of H2(g) and 1.50 mol of Br2(g) were initially placed in a 1.0 L flask at 50 oC. Calculate the equilibrium concentration of the reactants and products if the equilibrium constant is 54.3. (5) [9] 13 School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination Question 6 (a) Identify the acid-base conjugate pairs in the following reaction: HClO + CH3NH2 ⇌ CH3NH3+ + ClO- (b) Determine the [OH-] in aqueous ammonia at a pH of 11.28. 14 (2) (2) School of Chemistry and Physics University of KwaZulu-Natal, Westville Campus CHEM161: Chemical Engineering Chemistry May 2012 Main Examination (c) What is the pH of a 0.0250 M methylamine (CH3NH2) solution in water Kb = 4.2 × 10-4. (5) [9] 15