CH. 15 CHEMICAL THERMODYNAMICS (Part 2)

CH. 15 CHEMICAL THERMODYNAMICS (Part 2) - CHAPTER 15:...

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CHAPTER 15: CHEMICAL THERMODYNAMICS. (Part 2) SPONTANEITY OF PHYSICAL AND CHEMICAL CHANGES Water always flows downhill from a position of higher gravitational potential energy to one of lower gravitational potential energy. The flow of water is spontaneous in the direction of lower gravitational potential energy. In the case of an exothermic reaction, formation of the product is accompanied by release of heat energy. Hence, the products have lower energy than the reactants. Consider the following exothermic reaction: Hg (l) + ½O 2(g) HgO (s) H = - 21.7 kcal. The heat content of the product, mercury(II) oxide is less than that of the reactants, mercury and oxygen. We expect HgO to form spontaneously, proceeding from a higher energy level to a lower energy level. In actual practice, when the temperature is moderate, the reaction does proceed spontaneously. However, at high temperatures (above 400 ° C), the reaction does not occur at all. Instead, the reverse reaction occurs. HgO (s) Hg (l) + ½O 2(g) H = 21.7 kcal. The decomposition of HgO is endothermic, yet it occurs spontaneously if the temperature is high enough. Contrary to our expectations, the reaction proceeds from a lower heat content (lower energy) to a state of higher heat content when the temperature is high. (This is like the water reversing its direction and flowing uphill on a warm day.) Studying the energy changes in the formation and decomposition of HgO clearly indicates that for chemical changes, change in energy is not the only factor that determines the direction of chemical change. There is another factor that influences the direction of a chemical change. This factor is called entropy.
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2 Entropy, S Thermodynamic property entropy, S, is a state function like enthalpy and internal energy. It is typically expressed in J/K or cal/K. Entropy has been defined in a number of different ways. It is easy to consider entropy of a system as: (i) a measure of the dispersal of energy and matter in the system, or (ii) as a measure of the randomness in the system, or (iii) as a measure of the ‘disorder’ or lack of organization in the system. A process that occurs with an increase in entropy causes the energy and matter in the system to be more dispersed (scattered) and less organized. Entropy change in several processes can be predicted. Six examples of processes in which entropy changes can be predicted are discussed on pages 618-620. Examples:
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This note was uploaded on 09/13/2008 for the course CHEM 1412 taught by Professor Davis during the Spring '08 term at Alamo Colleges.

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CH. 15 CHEMICAL THERMODYNAMICS (Part 2) - CHAPTER 15:...

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