WA 09_CHE-122-jan18 Sterile

Written Assignment 9: Spontaneity Reactions Answer all assigned questions and problems, and show all work. 1. How does the entropy of a system change for each of the following processes? (14 points) a. A solid melts. - increases b. A liquid freezes. - decreases c. A liquid boils. - increases d. A vapor is converted to a solid. - decreases e. A vapor condenses to a liquid. - decreases f. A solid sublimes. - increases g. Urea dissolves in water. - increases (Reference: Chang 17.5) 2. For each pair of substances listed here, choose the one having the larger standard entropy value at 25ºC. The same molar amount is used in the comparison. Explain the basis for your choice. (12 points) a. Li( s ) or Li( l ) - Li( l ) b. C 2 H 5 OH( l ) or CH 3 OCH 3 ( l ) - C 2 H 5 OH( l ) c. Ar( g ) or Xe( g ) - Xe( g ) d. CO( g ) or CO 2 ( g ) - CO 2 ( g ) e. O 2 ( g ) or O 3 ( g ) - O 3 ( g ) f. NO 2 ( g ) or N 2 O 4 ( g ) - N 2 O 4 ( g ) (Reference: Chang 17.9) 3. Using the data in Appendix 3 of the textbook, calculate the standard entropy changes for the following reactions at 25ºC. (8 points) a. S( s ) + O 2 ( g ) → SO 2 ( g ) 248.5 - 31.88 - 205.0 = 11.62 J/K b. MgCO 3 ( s ) → MgO( s ) + CO 2 ( g ) 26.78 – 213.6 – 65.69 = 174.69 J/K (Reference: Chang 17.11) 4. Without consulting Appendix 3 in the textbook, predict whether the entropy change is positive or negative for each of the following reactions. Give reasons for your predictions. (12 points) a. 2KClO 4 ( s ) → 2KClO 3 ( s ) + O 2 ( g ) i. Positive – change of phase from solid to gas is a significant increase in entropy b. H 2 O( g ) → H 2 O( l ) i. Negative – change of phase from a gas to a liquid is a significant decrease in entropy 1 Copyright © 2017 by Thomas Edison State University. All rights reserved.