exam_answers_Exam_1 - K£V@ CHEMISTRY 152B HOUR EXAM I...

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Unformatted text preview: K£V@ CHEMISTRY 152B HOUR EXAM I (Version A) Winter 2007 Monday, January 29, 2007 Name: TA Section: Student Number: TA Name: Score 1 ’ 2 GOOD LUCK! Watch sig figs and units. 3 4 _._____ Total __ (x/100) I. CONCEPTS AND SHORT ANSWER: Answer all questions (subtotal = 48 pts) 1. (a) (T® For a real, polyatomic gases, Cp > Cv + R (b) For real, polyatomic gases, why is Cv always > (3/2)R? ; I” 0’47/{613 s'n/4rh9/mao/{faag 4/3 Gin.» VJ) 69)%+/ rank/'93 . 2. Consider an ideal gas at 25 °C in a 1.0 liter bulb connected to an evacuated volume of . 5.0 liters by a closed valve. Now open the valve, letting the gas expand adiabatically into C the larger volume. For this expansion, give the sign (+ or — or O) of each of the following: / q w AE AH AS V AG 0 o 0 O +- *— 3. Indicate which of the following statements is true (T) or false (F): _ (a) _7; All real processes are irreversible 3 (b) L In an irreversible cyclical process, the system returns to its original state, but the universe does not. (c) J: All spontaneous processes increase the disorder of the system 4. For each of the following forms of matter, give the standard thermodynamic state(°): (a) solutions /'M (b) pressure of a gas I «9% ' (c) temperature 298/? H (d) pure solids, or liquids flmv sol/day A} w’a/ £2 Chemistry 152C Page 2 5. Consider the following sequence of reactions: H12A—>B ’ H2 B—>C+2D H3 E—>D In terms of the enthalpies of the individual reactions (H1 etc), write an expression for the enthalpy Hrxn for the following net reaction: 2A —> C + 2E Al, 2/4 -——>v3/ 7’ Ala .2/ -—‘7 C 4‘” ~‘243 ‘2@/—92A: ’fl______________, ,Aflam:A0+Aé—Zfis 24L4>C+Z¢ 6. Consider the exothermic gas-phase reaction: A2 (9) + 32 (g) —> 2AB(g)- Q (a) Give the sign of AH0 and A30 for this reaction? A} H<O [Pro/1:" m'c) A€>O (b) Under what conditions of temperature might the reverse reaction occurr «4/04 23‘ ’m'WV ymzézmap icy/obj 7. (a) At what temperature T (in K) does bromine boil (P=1 atm) ? Br2 (I) —> Bl'z (g) Vaporization enthalpy AH°= 31.0 kJ/mole, entropy change AS° = 93.0 J/(K mole) “ A60: Afl'O-'7'A§.: 6/ f0 4/9: 7 : ill/V” 3/.ox/o7T/7w/c A5“: W ‘ L\ (b) At this boiling point, what is the value of: AG ASuniver, AS u" 0 O 8. For the chemical reaction A 4) B, give the appropriate temperature conditions for spontaneity for each of the following (i.e. spontaneous at all T, at no T, T high, or T low) (a) AS<0 and AH >0 mew-f“ spew/$75“) >477». q (b) AS<0 and AH <0 (0) and AH >0 6/ jfoaw’s s/Qm/é/éea 7' (d) AS>0 and AH <0 q/Ma's' jflm/fi q/L q// 7‘ Version A Chemistry 152C Page 3 9. Which one of the following species has the highest heat capacity Cv and why? , » He H2 O=C=0 bx (W”‘//""¢¢') F0/y9’4wfc ma/Pca/e 445‘ MG”? V/Z"°/7lm/ 9MJ arr/Z,//g7’ry' ( 0" 110 Vne/ 10. (a) In Boltzman’s formula for entropy S: kan , what does 0 represent? 2 7é'Ié/470Mévv ‘2ij 2'VOS’4i/45 [flat/74’mg/ on Mam/37)) my: 0 /2 9,37 * (b) For a perfect crystal at absolute zero temperature (0 K), what is the value of Q ? 12 == 1 15": )2 («a z 0 11. For each of the following, give the sign of AS: evaporating alcohol freezing Water dissolving salt in water compressing an ideal gas at constant temperature Ll 99.69: Inl+lil+ 12. (a) Name the two most important greenhouse gases in our atmosphere. #2 0 / Caz (b) The natural greenhouse effect warms the Earth about (3, 100) K (circle answer) (c) Increasing atm heric 002 is predicted to cause additional warming in this century _ ’1 of about (0.3, 30) K (circle answer) (d) The most abundant fossil fuel ispetroleum, natural gas) (circle answer) "Chemistry 152C Page 4 ll. QUANTITATIVE: Do all four (4) of the following (subtotal = 52 pts) I Calculate the energy required to warm 1.00 kg of ethane (CZHS) from an initial temperature of 25.0 °C to a final temperature of 75.0 °C in a rigid vessel (constant volume ' :onditions). The heat capacity of ethane is Cv = 44.60 J/(K mole). Repeat the calculation of the energy required to achieve this same temperature change for conditions of constant pressure. Calculate work (w) for each of these cases. lmol 1.00 x103gc H6>< , 2 30.07 g 56.0 7,” 9% 3 qv=AE = 11ch =.33.3 mol (44.60 Jmol“ K“) (434’ K) =7>12 x 104 J=m k1 ~3 . 3 ( :4. At constant volume, . H of energy are reqmred and AB = M U. At constant pressure (assuming ethane acts as an ideal gas): = 33.3 mol W7 CP _= Cv + R= 44.60 + 8.31 = 52.91 J mol'1 K'l ' sf- W m Energy required = qp = AH = nCPAT = 33.3 mol (52.91 J mol'1 K") ( . K) = 8753, x 10‘ J = 953 k] A w: 6 Arfllk-‘g 0U: ~PAV= —/3.8/é.T 74v mas/B3 2. Consider the reaction: 2 POCI3(g) —> 2 PCI3(g) + 02(9) (a) Calculate Aern° for this reaction, using the following values: AGf° (POCl3) = - 502 kJ/mole AGfo (PCI3) = -270 kJ/mole +c AG° = 2(-270. 1d) - 2(-502 k1) = 464 H (b) At what temperature does this reaction become spontaneous for standard (°) conditions? \ For this reaction, Aern° = 179 J/K (Assume that AH° and AS° do not depend on temperature). AG° = AH°- TAS°, AH° = AG" + TAS° = 464 k] + 298 K(0.179 kJ/K) = 517 k] We need to solve for the temperature when AG° = 0: AG°=0=AH°-TAS°, T= AH:— =fl_ =2890K AS° 0.179 kJ/K ' This reaction will be spontaneous at standard conditions (AG° < 0) when T > 2890 K. At these temperatures the favorable entropy term will dominate. ‘ Version A Chemistry 152C Page 5 3. Consider the following reaction for which AH°rxn = -118 kJ/mole: 2HC| (aq) + Ba(OH)2 (aq) __) BaCl2 (aq) + 2‘H20 (I) (a) Calculate the heat released when 100.0 mL of 0.500 M HCI is mixed with 300 mL of 0.100 M Ba(OH)2 ' - 0.100 L x WEI-{91 = 5.00 x 10'2molHCl 0.100 mol Ba 0 0.300 L X = 3.00 X 10'2 mol Ba(OI-I)2 To react with all the HCl present, 5.00 X 10'2/2 = 2.50 X 10'2 mol Ba(OH)2 is required. 0.0300 mol Ba(OH)2 is present, then HCl is the limiting reactant. \ (b) Calculate the final temperature of the mixture. Assume that both solutions were initially at a temperature of 25 °C, and that the final mixture has a mass of 400.0 g and a specific heat capacity of 4.18 Jl(gram °C), ' 5.00 x 10'2 mol HCl x Jig—Ii. = 2.95 k] of heat is evolved by reaction. 2 mol HCl . - Heat gained by solution = 2.95 X 103 J = 4'10? X 400.0.g X AT g AT =1.76°C = Tf— Ti =Tf- 25.0°C, Tf= 263°C 4. Calculate q, w, AE and AH for the process in which 88.0 grams of nitrous oxide (N20) is cooled from 165 °C to 55 °C at a constant pressure of 5.00 atm (Cp = 38.70 J/K mole). 1 mol N20 44.02 g N20 At. constant pressure, qp = AH - 88.0 g N20 X = 2.00 mol N20 -, AH = nCpAT = (2.00 mol) (38.70 J mol" 8C") (55°C - 165 °C) AH= -8510 J = -8.51kJ= qp w = - PAV = -nRAT = -(2.00 mol) (8.3145 J mol"1 K") (—110. K) = 1830 J = 1.83 k] AE=q+w=-8.51kJ+1.83kJ=-6.68kJ Version A ...
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exam_answers_Exam_1 - K£V@ CHEMISTRY 152B HOUR EXAM I...

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