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Lecture

Problem Set: Spontaneity

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Department
Chemistry
Course
CHEM 123
Professor
Angie O' Neill
Semester
Summer

Description
CHEMISTRY 123 Problem Set #4 Reversibility, Entropy and Free Energy Concept questions 1. In an imaginary experiment, you take a steel ball from the freezer and drop it in a Styrofoam cup of water (an isolated system). You hypothesize that the water temperature increases and the ball gets colder. Describe ∆S for this transition of the ball and water. What can you conclude about your hypothesis? 2. Ideal solutions that obey Trouton’s Rule have ∆S ◦vap= 88 J mol −1 K −1? Water and ◦ methanol fail to obey this rule. Verify this by calculating ∆S vapfor water and methanol, given the data in the table below. Explain why these liquids are special. • -1 -1 Example S (J mol K ) H 2 (l) 70.0 H 2 (g) 188.7 CH OH 126.3 3 (l) CH O3 (g) 239.9 Problems 3-6. When 44.0 g of solid carbon dioxide at 1.00 atm pressure are reversibly converted to vapour at 1 atm pressure and −78 °C, 25.33 kJ of heat are absorbed by the system. The density of solid CO 2 is 1.56 g cm . 3. ∆H for this process is (in kJ): (1) −23.71 (2) +23.71 (3) −25.33 (4) +25.33 (5) +26.95 −1 4. ∆S for the process is (in J K ): (1) 130.0 (2) −130.0 (3) 121.5 (4) −121.5 (5) 138.1 5. ∆G for the process is (in kJ): (1) 50.66 (2) −50.66 (3) −46.71 (4) 46.71 (5) 0 6. If S° of CO (g) is 197.7 J K−1 mol −1at −78 °C, the standard entropy of solid CO is (in J K ): 2 2 (1) 67.70 (2) −67.70 (3) 327.7 (4) 0 (5) 130.0 7. At 279 K and 1 atm pressure, pure benzene crystallizes reversibly with an entropy change of −35.66 J mol −1 K . What is the molar heat of fusion of benzene (in kJ mol ) at its melting point? 8−10. Consider the following processes at constant T and P: Process ∆H ∆S (1) positive positive (2) positive negative (3) negative positive (4) negative negative Answer the following questions, using the process numbers as the answer choices: 8. Which processes are ALWAYS spontaneous? 9. Which processes can be both spontaneous or non-spontaneous (i.e. the spontaneity depends on the temperature)? 10. Which processes are NEVER spontaneous? 11−15. Consider the following data at 298 K: Ag(s) H 2(g) O 2g) Ag 2(s) H 2(g) −1 ∆H °fkJ mol ) −20.5 −242 S°(J K −1 mol )1 42.6 206 205 189 For the reaction 4Ag(s) + 2H S2g) + O (g2 → 2Ag S(s) 2 2H O(g) 2 −1 at a temperature of 25 °C, ∆H° = −507 kJ and ∆S° = −119 J K . −1 11. ∆H f of Ag 2 (s) is (in kJ mol ): (1) +475 (2) −32 (3) −64 (4) −285.5 (5) −728.5 −1 −1 12. S° of Ag2S (s) is (in J K mol ): (1) 290.4 (2) 264.2 (3) 145.6 (4) 145.2 (5) 383.6 13. ∆G° for the reaction is (in kJ): (1) + 396.3 (2) −471.5 (3) −43302 (4) −75300 (5) −35.6 14. K for the reaction is: (1) 2.2 × 10 −83 (2) 1.9 × 102 (3) 3.4 × 10−70 (4) 5.25 (5) 4.5 × 1082 15. If the reaction were to occur at a higher temperature, more Ag 2S(s) would be formed. (1) True (2) False 16. For the reaction Cl2(g) → 2Cl(g), ∆S° is likely to be: (1) positive (2) zero (3) negative 17. For the reaction, ½N (2) + ½O (g)2- NO(g), −2 −2 K = 1.11 × 10 at 1800 K, and 2.02 × 10 at 2000 K. The enthalpy of reaction at 2000 K is nearest to (in kJ): (1) −89.6 (2) 90.3 (3) −90.3 (4) 89.6 (5) 0.0896 18. Consider the following data (values are tabulated at 25 °C and 1 atm pressure): NO(g) O 3g) NO 2g) SO (2) O 2g) SO 3g) −1 +90.39 +142.1 +33.9 −296.8 0.0 −394.6 ∆H f(kJ mol ) ∆G f(kJ mol )−1 +86.5 +163.0 +51.8 −1 −1 S° (J mol K ) +248.3 +204.8 +255.8 (a) Calculate ∆G°, ∆H°, ∆S°, and K at 25 °C for the reaction, NO (g) + O 3g) - NO (g2 + O (g)2 (b) For the reaction, SO 2g) + ½O (2) - SO (g3 calculate ∆H°, ∆S°, ∆G°, and K at 25 °C. Assuming that ∆H° and ∆S° do not change with temperature, also calculate K at 327 °C. 19. Suppose you have to investigate the thermodynamic feasibility of converting coal and hydrogen at 25 °C either to a synthetic natural gas such as propane, C3H 8 or to the industrially useful acetylene,
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