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Unformatted text preview: 19 Chemical Thermodynamics Visualizing Concepts 19.1 (a) (b) S is positive, because the disorder of the system increases. Each gas has greater motional freedom as it expands into the second bulb, and there are many more possible arrangements for the mixed gases. By definition, ideal gases experience no attractive or repulsive intermolecular interactions, so H for the mixing of ideal gases is zero, assuming heat exchange only between the two bulbs. (c) The process is irreversible. It is inconceivable that the gases would reseparate. (d) The entropy change of the surroundings is related to H for the system. Since we are mixing ideal gases and H = 0, H surr is also zero, assuming heat exchange only between the two bulbs. 19.4 (a) At 300 K, H = T S. Since G = H T S, G = 0 at this point. When G = 0, the system is at equilibrium. (b) The reaction is spontaneous when G is negative. This condition is met when T S > H. From the diagram, T S > H when T > 300 K. The reaction is spontaneous at temperatures above 300 K. Spontaneous Processes 19.7 Analyze/Plan . Follow the logic in Sample Exercise 19.1. Solve . (a) Nonspontaneous; 5 C is below the melting point of ice, so melting does not happen without continuous intervention. (b) Spontaneous; sugar is soluble in water, and even more soluble in hot coffee. (c) Spontaneous; N 2 molecules are stable relative to isolated N atoms. (d) Spontaneous; the filings organize in a magnetic field without intervention. (e) Nonspontaneous; CO 2 and H 2 O are in contact continuously at atmospheric conditions in nature and do not form CH 4 and O 2 . 239 19 Chemical Thermodynamics Solutions to Red Exercises 19.9 (a) NH 4 NO 3 (s) dissolves in water, as in a chemical cold pack. Naphthalene (mothballs) sublimes at room temperature. (b) Melting of a solid is spontaneous above its melting point but nonspontaneous below its melting point. 19.11 Analyze/Plan . Define the system and surroundings. Use the appropriate definition to answer the specific questions. Solve . (a) Water is the system. Heat must be added to the system to evaporate the water. The process is endothermic. (b) At 1 atm, the reaction is spontaneous at temperatures above 100 C. (c) At 1 atm, the reaction is nonspontaneous at temperatures below 100 C....
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This homework help was uploaded on 03/19/2008 for the course CHEM 102 taught by Professor Todd during the Spring '08 term at UNC.
- Spring '08