FACULTY OF SCIENCE MID-TERM EXAMINATION 2 MARCH 18, :30 TO 8:30 PM CHEMISTRY 120 GENERAL CHEMISTRY

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1 FACULTY OF SCIENCE MID-TERM EXAMINATION 2 MARCH 18, :30 TO 8:30 PM CHEMISTRY 120 GENERAL CHEMISTRY Examiners: Prof. B. Siwick Prof. A. Mittermaier Dr. A. Fenster Name: Associate Examiner: A. Fenster INSTRUCTIONS 1. Enter your student number and name on the computer scorecard provided, by filling in the appropriate circles. Check that your scorecard has the correct version number filled in (version 1). If not, fill that in. One mark will be awarded for correctly recording your student number on the scantron card. Double check that this has been done properly! 2. This examination comprises 30 questions but will be marked out of 31 (see point 1 above). All questions are of equal value. 3. Transfer all answers to the scantron computer scorecard provided. 4. ONLY the SHARP EL510RB calculator may be used in the Midterm. Translation dictionaries are also permitted. NO notes or texts are allowed. 5. The Examination Security Monitor Program detects pairs of students with unusually similar answer patterns on multiple-choice exams. Data generated by this program can be used as admissible evidence, either to initiate or corroborate an investigation or a charge of cheating under Section 16 of the Code of Student Conduct and Disciplinary Procedures. Page 1 of 16

2 1. The heat of fusion for water is 6.01 kj/mol and for ethyl alcohol is 5.01 kj/mol. The amount of heat that would melt 25.0 grams of water would melt how many grams of ethyl alcohol (C2H5OH)? a) 8.2 g b) 53.3 g c) 76.7 g d) 32.0 g e) 11.7 g 2. Which compound is most likely to be soluble in both water and benzene? a) propyl alcohol (C3H7OH) b) hexane (C6H14) c) calcium chloride (CaCl2) d) naphthalene (C10H8) e) toluene (C7H8) 3. The equilibrium constants for the chemical reaction: N2(g) + O2(g) 2NO (g) are Kp = and at 2,200 K and 2,500 K respectively. Which one of the following statements is true? a) Higher total pressure shifts the equilibrium to the left. b) The partial pressure of NO(g) is less at 2,200 K than at 2,500 K. c) The total pressure at 2,200 K is the same as at 2,500 K. d) The reaction is exothermic; i.e. ΔH < 0. e) Kp is less than Kc by (RT). 4. For the reaction: H2(g) + I2(g) 2 HI(g), Kc = When equilibrium concentrations of HI and I2 are [HI] = M and [I2] = M, the equilibrium concentration of [H2] is: a) M b) M c) M d) 9.56 M e) M Page 2 of 16

3 5. A total of 0.12 g of a compound is dissolved in 10.0 g benzene. The resulting solution freezes at 4.62 C. Pure benzene freezes at 5.48 C and has a freezing point depression constant of Kf = 4.90 C/m. What is the molecular mass of the compound, assuming it is not an electrolyte? a) 68 g mol -1 b) 106 g mol -1 c) 570 g mol -1 d) 11 g mol -1 e) 57 g mol For the formation of NO2, Kc = [NO2] 2 /([NO] 2 [O2]). At equilibrium in a 2.50 L container, there are 3.00 mol NO, 4.00 mol O2 and 22.0 mol NO2. The value of Kc is: a) 3.75 b) 13.4 c) 33.6 d) 5.38 e) A reaction is always spontaneous if: i) ΔG is a negative value ii) both enthalpy and entropy increase iii) ΔH is negative and ΔS is positive iv) both enthalpy and entropy decrease v) ΔH is positive and ΔS is negative a) i and iii b) cannot be determined without temperature c) i and ii d) iii and iv e) ii and v 8. Henry s Law constants for aqueous solutions at 25 C are molal/mmhg for N2 and molal/mmhg for O2. Determine the solubility of nitrogen in water under an atmospheric pressure of 760 mmhg, assuming that air is 80% N2 and 20% O2, by mole. a) m b) m c) m d) m e) m Page 3 of 16

4 9. The element oxygen was prepared by Joseph Priestley in 1774 by heating mercury(ii) oxide. HgO(s) Hg(l) + ½O2(g) ΔH = kj Use the data given below to estimate the temperature at which this reaction will become spontaneous under standard state conditions. S (Hg) = J/K mol S (O2) = J/K mol S (HgO) = J/K mol a) 840 K b) 108 K c) 620 K d) 430 K e) 775 K 10. Commercial perchloric acid is 70.0% by mass, HClO4(aq), and has a density of 1.67 g/ml. Calculate the molarity of perchloric acid in the solution, assuming the molecular weight of HClO4 is g/mol. a) 23.7 M b) 1.17 M c) 16.5 M d) 0.12 M e) 11.6 M 11. The following reaction is exothermic: 2N2O(g) 2N2(g) + O2(g) This means the reaction a) will be spontaneous only at low temperatures. b) is not spontaneous at any temperature. c) will be spontaneous only at high temperatures. d) will be spontaneous at all temperatures. e) none of the above 12. If ΔG is positive for a certain reaction at a given T and P, then: a) the reaction is endothermic b) the reaction is spontaneous at high temperatures. c) one would need to know the Kelvin temperature to determine spontaneity. d) the reverse of the reaction is spontaneous. e) the system is in equilibrium. Page 4 of 16

5 13. For Cl2O(g) + 3/2 O2(g) 2 ClO2 ΔH = 126 kj/mol and ΔS = J/mol-K at 377 C. What is Keq? a) 2.66 b) c) d) e) Consider the following reaction: C(s) + H2O(g) CO(g) + H2(g) At equilibrium at a certain temperature, [H2O(g)] = 0.12 M, and [CO(g)] = [H2(g)] = 1.2 M. If suddenly these concentrations are increased by 0.50 M, which of the following is true? a) more H2O(g) will be formed b) Kc = 4.66 c) Since Kc does not change, nothing happens. d) more products are formed e) All the additional H2O(g) is consumed 15. Given the following: I) N2O(g) + 1/2 O2(g) 2NO(g) Kc = II) N2(g) + O2(g) 2NO(g) Kc = Find the value of the equilibrium constant for the following equilibrium reaction: N2(g) + 1/2 O2(g) N2O(g) a) b) c) d) e) Calculate the temperature at which Keq for a reaction is if ΔH = kj/mol and ΔS = -246 J/mol K. a) 307 K b) 274 K c) K d) cannot be determined without ΔG e) K Page 5 of 16

6 17. The vapor pressures of pure hexane and pure heptane at 25 C are mmhg and mmhg respectively. An ideal solution contains mol fraction hexane and mol fraction heptane. What is the composition of the vapor in equilibrium with this solution at 25 C? a) 70.0% hexane, 30.0% heptane b) 88.6% hexane, 11.4 % heptane c) 23.2% hexane, 76.8% heptane d) 76.8% hexane, 23.2% heptane e) 11.4% hexane, 88.6% heptane 18. Consider the following reaction at equilibrium: 2SO2(g) + O2(g) 2SO3(g) ΔH = kj/mol The equilibrium is displaced to the left if: a) the pressure is raised b) some oxygen is added c) some sulfur dioxide is added d) some sulfur trioxide is removed e) the temperature is raised 19. For the reaction: 2 Cl2(g) + 2 H2O(g) 4 HCl(g) + O2(g), Kp = at 500 K. If a fixed volume is filled with initial concentrations of these gases at 227 C such that [Cl2] = 0.5 M, [H2O] = 0.40 M, [HCl] = 0.5 M, and [O2] = M, in which direction will the reaction proceed? a) The reaction volume must be specified to answer this question. b) The reaction proceeds to the left. c) The value of Kp at 25 C must be specified to answer this question. d) The reaction is already at equilibrium. e) The reaction proceeds to the right. 20. A liquid in a closed container is at equilibrium with its vapor. If the container is briefly opened and some of the vapor is allowed to escape, what is the immediate result? a) vaporization rate increases b) condensation rate increases c) vaporization rate decreases d) condensation rate decreases e) the vapor pressure increases Page 6 of 16

7 21. The Van t Hoff factor for electrolytes, i, a) depends only on the molecular weight of the solute. b) decreases with increasing solute concentration. c) increases with increasing solute concentration. d) has no effect on freezing point depression. e) depends only on the molecular weight of the solvent. 22. The equilibrium constant for the reaction: HNO2(aq) + H2O(l) NO2 - (aq) + H3O + (aq) is 7.2 x 10-4 at 298 K and 1 atm. When [HNO2(aq)] = 1.0 M and [NO2 - (aq)] = [H3O + (aq)] = 1.0 x 10-5 M, calculate ΔG. a) kj/mol b) kj/mol c) kj/mol d) kj/mol e) kj/mol 23. A liquid will exhibit a concave meniscus and undergo capillary action whenever a) it has a high vapor pressure b) adhesive interactions are strong compared to cohesive interactions. c) the surface tension is greater than the viscosity d) it has a low vapor pressure e) adhesive interactions are weak compared to cohesive interactions. 24. An aqueous glucose solution is placed in a container that is permeable to water, but not permeable to glucose. The container is submerged in pure water and the system is allowed to equilibrate at 25 C. At equilibrium, the pressure inside the container is found to be 2.05 atm greater than the pressure outside the container, due to osmosis. What is the final concentration of glucose inside the container? a) M b) M c) M d) 1.0 M e) M Page 7 of 16

8 25. For the reaction: PCl5(g) PCl3(g) + Cl2(g) at 298 K, Keq = , ΔS = J/mol K. What is Keq at 200 K if ΔH = 92.6 kj/mol? a) b) c) 16.8 d) e) At 400 C, Kc = 64 for the equilibrium H2(g) + I2(g) 2HI(g) If 3.00 mol H2 and 3.00 mol I2 are introduced into an empty 4.0 L vessel, find the equilibrium concentration of HI at 400 C. a) 5.8 M b) 2.4 M c) 4.8 M d) 0.15 M e) 1.2 M 27. The normal boiling point of acetone is 56.2 C and the molar heat of vaporization is 32.0 kj/mol. At what temperature will acetone boil under a pressure of 50.0 mmhg? a) 6.0 C b) -6.0 C c) 73.6 C d) 156 C e) 40.7 C Page 8 of 16

9 28. According to the phase diagram below, which of the following statements is false? a) If liquid Z at the pressure and temperature of point 2 is heated, it will undergo evaporation. b) At the temperature of point 4, the substance Z cannot be liquefied at any pressure. c) At the temperature of point 5, liquid Z is denser than solid Z. d) If solid Z at the pressure and temperature of point 3 is heated, it will undergo fusion (melting). e) At the temperature and pressure of point 1, Z can exist as a three-phase system at equilibrium. 29. The heat of deposition, ΔHdeposition, is equal to a) ΔHfusion + ΔHsolidification b) -ΔHvaporization c) ΔHfusion + ΔHvaporization d) ΔHfusion e) ΔHcondensation + ΔHsolidification Page 9 of 16

10 30. Based on the boiling-point diagram for a mixture of two substances A & B, shown below, which of the following statements is true? a) The vapor above a boiling solution of 25% A contains 75% A. b) The boiling point of pure substance A is 90 C c) The vapor above a boiling solution of 75% A contains 50% A. d) A mixture of 25% A and 75% B is an azeotrope. e) The boiling point of pure substance B is 70 C. Page 10 of 16

11 UNITS Conventional S.I. Volume ml or cm 3 = cm 3 or 10-3 dm 3 Liter (L) = dm 3 Pressure atm = 760 torr = Pa torr = Pa Temperature C 0 C = K PV L-atm = dm 3 Pa =101.3 dm 3 kpa =101.3 J Energy 1 cal = J (J= kg m 2 /s 2 ) 1 erg = 1 g cm 2 /s 2 = 10-7 J Current ampere (A) = 1 C/s CONSTANTS Avogadro s number N x mol -1 Boltzmann s constant k x J K -1 Faraday s constant F C mol -1 Values of R for various unit combinations cal mol -1 K L atm mol -1 K cm 3 atm mol -1 K J mol -1 K erg mol -1 K dm 3 kpa mol -1 K cm 3 kpa mol -1 K -1 Useful equations: P ln P1 H = R 1 T1 T 2 vap 1 2 PV = nrt m = MPV RT PV 1 n T = P2 V n T MP d = P = hdg RT π = imrt C=kP gas P A =χ A P A Page 11 of 16

12 G (non-standard conditions): ΔG = ΔG + RT ln Q Van t Hoff Equation: o K2 H 1 1 ln = K1 R T2 T1 Standard states for various elements under STP conditions: Hydrogen: H 2 (g) Carbon: C(s, graphite) Nitrogen: N 2 (g) Oxygen: O 2 (g) Copper: Cu(s) Sulphur: S(s) The Periodic Table of the Elements Page 12 of 16

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MCGILL UNIVERSITY FACULTY OF SCIENCE MIDTERM EXAMINATION CHEM 120 MONDAY MARCH 16, :30PM 8:30PM VERSION NUMBER: 1

MCGILL UNIVERSITY FACULTY OF SCIENCE MIDTERM EXAMINATION CHEM 120 MONDAY MARCH 16, 2009 6:30PM 8:30PM VERSION NUMBER: 1 Instructions: BEFORE YOU BEGIN: Enter your student number and name on the computer