Chem 102H Exam 1 - Spring 2005: Physical Constants and Thermodynamics, Exams of Chemistry

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Chem 102H Exam 1 - Spring 2005 Name I.D. # PHYSICAL CONSTANTS/CONVERSION FACTORS Speed of light = 3.00 108 m/s Planck s const. = 6.63 10-34 J•s Avagadro s Number = 6.02 1023 Electron charge = 1.602 10-19 C Faraday s const. = 96487 C mol-1 (e–) Gas const.(R) = 0.0821 L atm mol-1 K-1 = 62.4 L torr mol-1 K-1 = 8.314 J mol-1 K-1 = 1.987 cal mol-1 K-1 Mass of e = 0.00055 amu = 9.1094 10-31 kg H Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra Sc Y Ti Zr Hf V Nb Ta Cr Mo W Mn Tc Re Fe Ru Os Ce Co Rh Ir Pr Ni Pd Pt Nd Cu Ag Au Pm Zn Cd Hg Sm B Al Ga In Tl Eu C Si Ge Sn Pb Gd N P As Sb Bi Tb O S Se Te Po Dy F Cl Br I At Ho Ne Ar Kr Xe Rn Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr He UnqUnpUnhUnsAc Lanthanide series Actinide series 140.1 581.1 140.9 59 144.2 60 (145) 61 150.4 62 152.0 63 157.2 64 158.9 65 162.5 66 164.9 67 167.3 68 168.9 69 173.0 70 175.0 711.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.0 1.2 232.0 901.2 231.0 911.3 238.0 921.5 (237) 931.3 (244) 94 (243) 95 (247) 96 (247) 97 (251) 98 (252) 99 (257) 100 (258) 101 (259) 102 (260) 1031.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.5 1.008 12.1 6.941 31.0 22.99 111.0 39.10 190.9 85.47 370.9 132.9 550.8 (223) 870.8 9.012 41.5 24.30 121.2 40.08 201.0 87.62 381.0 137.3 561.0 (226) 881.0 44.96 211.3 88.91 391.2 138.9 571.1 (227) 891.1 47.88 221.4 91.22 401.3 178.5 721.3 104 50.94 231.5 92.91 411.5 180.9 731.4 1052.1 52.00 241.6 95.94 421.6 183.8 741.5 106 54.94 251.6 (98) 431.7 186.2 751.7 107 55.85 261.7 101.1 441.8 190.2 761.9 58.93 271.7 102.9 451.8 192.2 771.9 58.69 281.8 106.4 461.8 195.1 781.8 63.55 291.8 107.9 471.6 197.0 791.9 65.39 301.6 112.4 481.6 200.6 801.7 10.81 52.0 26.98 131.5 69.72 311.7 114.8 491.6 204.4 811.6 12.01 62.5 28.09 141.8 72.61 321.9 118.7 501.8 207.2 821.7 14.01 73.0 30.97 152.1 74.92 332.1 121.8 511.9 209.0 831.8 16.00 83.5 32.07 162.5 78.96 342.4 127.6 522.1 (209) 841.9 19.00 94.0 35.45 173.0 79.90 352.8 126.9 532.5 (210) 852.1 20.18 10 39.95 18 83.80 36 131.3 54 (222) 86 4.003 2 (261) (262) (263) (262) La 1.008 12.1 H Pauling Electronegativity Atomic molar mass (Atomic weight) 0 ˚C = 273 K 1.00 atm = 760 torr 1 inch = 2.54 cm 1.00 lb = 454 g 1 Å = 1.0 10-8 cm 1 eV = 1.602 10-19 J 1.0 cal = 4.184 J (exact) Mass of p = 1.0073 amu = 1.6726 10-27 kg Mass of p = 1.0087 amu = 1.6749 10-27 kg Scores 1 /8 2 /10 3 /6 4 /7 5 /6 6 /8 7 /10 8 /15 9 /15 10 /15 bonus /10 Tot. /100 Read questions carefully - some questions may provide information not needed to answer the question (1) (8 pts) Choose the process below for which H and U are the most different. Assuming that one mole of the first “reactant” is involved, calculate H - U. (Give the correct letter, and then compute the value of H - U. If you don’t know how to get H - U, give a short explanation for your choice to receive partial credit. No credit for guessing and providing no explanation.) (a) C(s, graphite) + O2(g) CO2(g) (b) CaO(s) + CO2(g) CaCO3(s) (c) NaCl(s) + excess H2O(l) Na+(aq) + Cl–(aq) (salt dissolves in water) (d) H2O(l) H2O(s) (water freezes to ice) (2) (10 points) For each of the following thermodynamic properties, decide whether the value should be positive, negative, or zero. (Please answer by writing ‘+’, ‘-’, or ‘0’ in the blank to the left of each one.) (2 pts. each) ______ (a) H 0 f for Cl2(g) ______ (b) S 0 for F2(g) ______ (c) S 0 for the evaporation of liquid methanol ( CH3OH(l) CH3OH(g) ) ______ (d) G for the combustion of propane: C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) ______ (e) H for the combination of 2 oxygen atoms: O(g) + O(g) O2 (g) (3) (6 points) For a process carried out at constant pressure, the heat measured in a calorimetry experiment, qP, is related to X, where “X” is a state function. What is “X”? (4) (7 points) Heat capacities measured at constant volume and constant pressure are respectively referred to as CV and CP. Which of these is generally larger than the other? (Explain why in one or two sentences.) For full credit, give an expression for the difference between the two for one mole of a gas that behaves ideally. (9) (15 pts) Consider the following facts when answering the questions given below: (I) NH3(g) + HCl(g) NH4Cl(s) H˚ = -176 kJ/mol | S˚| = 285 J/mol-K (II) Both NaCl and NH4Cl are solid, white powders under standard conditions. (III) The melting point of NaCl is approx. 800 ˚C. At atmospheric pressure, NH4Cl does not form a liquid at any temperature. (IV) Both NaCl and NH4Cl are very soluble in water and separation of the two salts can not be achieved by exploiting differences in solubility. (a) (5 pts) What is the sign of S˚? (briefly explain your answer). (b) (10 pts) Suppose you have a sample in which NaCl and NH4Cl are intimately mixed. Use the thermodynamic and other chemical information given to devise a simple method for removing the NH4Cl from NaCl to obtain pure NaCl. Your answer should be concise (no more than a few sentences), but you should be clear in explaining why your method will work. (Answers which are unclear and difficult to understand will be given no credit.) (10) (15 points) Nitrogen, N2, is one of the top chemicals produced in industry. The nitrogen phase diagram is shown below. Hfus(N2) and Hvap(N2) are the respective enthalpy changes for melting and boiling one mole of N2 at 1.0 atm pressure. Important information: Hfus(N2) = 0.721 kJ/mol Hvap(N2) = 5.565 kJ/mol; S 0 (N2) = 191.61 J/mol•K (a) (3 pts) What phases of matter (gas, liquid, solid) are stable in regions A, B, and C? A: B: C: (b) (5 pts) From the information given above and/or on the diagram, calculate Sfus(N2) and Svap(N2). (c) (7 pts) Estimate the heat absorbed ( H) in warming one mole of N2 gas from the boiling point to 298 K. Assume that the heat capacity (Cp) of N2 gas is constant over this range and that vibrational contributions to N2 can be ignored. BONUS! (10 pts) (d) (5 pts) Use the same heat capacity estimate you used in part (c) to estimate the entropy change for the process (An integral formula must be used.): N2(g,bp) N2(g,298.15 K) (The experimental value is S = 40.12 J/mol•K; your task is to show that a simple approximation is pretty good.) (e) (5 pts) The heat capacity (Cp) for liquid nitrogen is approximately 56.5 J mol –1 K –1 over the entire liquid range (m.p. < T < b.p.). Using this information and information given in the problem or used in earlier parts of the problem, compute the entropy of solid N2, just below the melting point.