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Chapter 20 thermodynamics

# Chapter 20 thermodynamics - Chapter 20 Thermodynamics...

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20 Chapter Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions 20.1 Spontaneous processes proceed without outside intervention. The fact that a process is spontaneous does not mean that it will occur instantaneously or even at an observable rate. The rusting of iron is an example of a process that is spontaneous but very slow. The lighting and resulting explosion of dynamite is an example of a process that is not spontaneous but very fast. 20.2 A spontaneous process is capable of proceeding as written or described without need of an outside source of energy. Under a specific set of conditions, if a process is spontaneous in one direction, it is not spontaneous in the other. By changing the set of conditions, a nonspontaneous process can sometimes be made spontaneous. 20.3 a) The energy of the universe is constant. b) Energy cannot be created or destroyed. c) E system = -∆ E surroundings The first law is concerned with balancing energy for a process but says nothing about whether the process can in fact occur. 20.4 Entropy is related to the number of possible equivalent arrangements of the components of a system. A system with many equivalent arrangements has high entropy and is highly probable. The highest entropy state corresponds to the state of greatest probability. a) All heads up has a probability of, 1/2 n where n = the number of coins. For six coins, all up has a probability of 1/2 6 or 1/64. b) and c) Probability is so remote as to be virtually impossible. Both would require the simultaneous, coordinated movement of a large number of independent particles, so are very unlikely. 20.5 The larger value for S vap results largely from the increased volume in which the molecules may be found. An increase in volume means an increase in randomness. Gases have much higher entropy than liquids or solids, due to the random motion of their molecules. 20.6 S surr 0 for an exothermic reaction. S surr < 0 for an endothermic reaction. Melting of ice cream at room temperature. 20.7 a) zero b) increase c) The third law states that the entropy of a pure, perfectly crystalline element or compound may be taken as zero at zero Kelvin. Since the standard state temperature is 25°C and entropy increases with temperature, S ° must be greater than zero for an element in its standard state. d) Since entropy values have a reference point (0 entropy at 0 K), actual entropy values can be determined, not just entropy changes. 20.8 (a), (b) and (c) are spontaneous. 20.9 (a) and (c) are spontaneous. 20.10 (a) and (b) are spontaneous. 20.11 (a) and (c) are spontaneous. 20.12 a) S sys 0. b) S sys < 0. c) S sys < 0. 130

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20.13 a) S sys 0. b) S sys 0. c) S sys < 0. 20.14 a) S sys 0. b) S sys 0. c) S sys 0. 20.15 a) S sys < 0. b) S sys < 0. c) S sys < 0. 20.16 a) S sys < 0. b) S sys < 0. c) S sys 0.
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