Physical Chemistry - Midterm 2005 Solved

Physical Chemistry - Midterm 2005 Solved - CHM2132B Midterm...

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1 of 8 CHM2132B Midterm Friday October 28 th , 2004 Name: _________________________________ Student #: _________________________________ This is a closed book exam with no notes allowed. Calculators are permitted. Write all the formulas that you use to solve the questions and show all your work. Remember to include units in all your calculations . Marks will be deducted if units are not shown. Constants and Data: R = 8.314 J K -1 mol -1 = 0.08206 L atm mol -1 K -1 = 0.08314 L bar mol -1 K -1 1 L =10 -3 m 3 1 atm = 101325 Pa 1 L atm = 101.325 J 1 bar = 10 5 Pa T sub (CO 2 (s)) = 194 K ± sub H m o CO 2 () = 26.1 kJ mol -1 C p , m o CO 2 (s) =18.1 J K -1 mol -1 C p , m o CO 2 (g) =37.1 J K -1 mol -1 C p , m o He(g) =20.786 J K -1 mol -1 ± f H m o Ag 2 O(s) =-31.05 kJ mol -1 Formula sheet is on last page - this can be removed and does not have to be handed in. Q1: __________/9 Q2: __________/9 Q3: __________/7 Q4: __________/10 Q5: __________/15 Bonus: __________/3 Total = __________/50
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Name: _____________________________ 2 of 8 Question 1: Write balanced chemical equations for the following reactions. Be sure to indicate the phases. (9 marks) a) Combustion of solid oxalic acid (COOH) 2 C 2 O 4 H 2 (s) + ± O 2 (g) ± 2CO 2 (g) + H 2 O(l) 5 marks - 1 for each correctly balanced compound +1 for correct phases b) Formation of liquid 1,2-dichloro-4-hydroxybenzene: 6C(s) + Cl 2 (g) + ± O 2 (g) + 2H 2 (g) ± C 6 Cl 2 OH 4 (l) HO Cl Cl H H H 4 marks - 1 for each correctly balanced element Question 2: In order to generate a spooky Halloween mist, 0.50 mol of dry ice is taken from a freezer at -79 o C and allowed to completely sublime under 1 bar of constant pressure at a room temperature 25 o C. a) Calculate the total change in entropy for the CO 2 using the available data (see first page). (7 marks) Given: n= 0.50 mol; T 1 = 273 K - 79 K = 194 K; p = 10 5 Pa (constant) T 2 = 298 K We need to account for the entropy change associated with the solid to gas phase transition at the sublimation phase transition temperature, followed by the entropy change associated with heating up the CO 2 gas to 25 o C. ² S = ² sub S + ² heat S 1 mark ± S = n ± sub H m o T sub + C p , m dT T T 1 T 2 ² ³ ´ µ µ · ¸ ¸ = n ± sub H m o T sub + C p , m ln T 2 T 1 ¹ º » ¼ ½ ¾ ³ ´ µ µ · ¸ ¸ = 0.05 mol () 26100 J mol -1 194 K + 37.1 J K -1 mol -1 ln 298 K 194 K ¹ º » ¼ ½ ¾ ³ ´ µ · ¸ = 75 J K -1 1 mark for sublimation entropy equation; 1 mark for entropy of heating equation; 1 mark for using C p,m for CO 2 (g), 1 mark for multiplying by number of moles, 1 mark for using correct temperature in entropy of phase transition calculation, 1 mark for correct answer
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Name: _____________________________ 3 of 8 b) Suppose the actual heat of the process in part a) is 15 kJ. Based on this information and your answer above, will this process occur spontaneously? You must show the calculation supporting your answer. (2 marks) ± S univ = ± S sys + ± S surr = ± S sys + ² q irr , sys T surr =
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Physical Chemistry - Midterm 2005 Solved - CHM2132B Midterm...

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