Chapter05rw-final - 5 5.1 5.2(a(b(c 5.3(a(b Thermochemistry...

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106 5 Thermochemistry Visualizing Concepts 5.1 The book’s potential energy is due to the opposition of gravity by an object of mass m at a distance d above the surface of the earth. Kinetic energy is due to the motion of the book. As the book falls, d decreases and potential energy changes into kinetic energy. The first law states that the total energy of a system is conserved. At the instant before impact, all potential energy has been converted to kinetic energy, so the book’s total kinetic energy is 85 J, assuming no transfer of energy as heat. 5.2 (a) The internal energy, E, of the products is greater than that of the reactants, so the diagram represents an increase in the internal energy of the system. (b) Δ E for this process is positive, +. (c) If no work is associated with the process, it is endothermic. 5.3 (a) For an endothermic process, the sign of q is positive; the system gains heat. This is true only for system (iii). (b) In order for Δ E to be less than 0, there is a net transfer of heat or work from the system to the surroundings. The magnitude of the quantity leaving the system is greater than the magnitude of the quantity entering the system. In system (i), the magnitude of the heat leaving the system is less than the magnitude of the work done on the system. In system (iii), the magnitude of the work done by the system is less than the magnitude of the heat entering the system. None of the systems has Δ E < 0. (c) In order for Δ E to be greater than 0, there is a net transfer of work or heat to the system from the surroundings. In system (i), the magnitude of the work done on the system is greater than the magnitude of the heat leaving the system. In system (ii), work is done on the system with no change in heat. In system (iii) the magnitude of the heat gained by the system is greater than the magnitude of the work done on the surroundings. Δ E > 0 for all three systems. 5.4 (a). No. This distance traveled to the top of a mountain depends on the path taken by the hiker. Distance is a path function, not a state function. (b) Yes. Change in elevation depends only on the location of the base camp and the height of the mountain, not on the path to the top. Change in elevation is a statfunction, not a path function.
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5 Thermochemistry Solutions to Exercises 107 5.5 (a) w = –P Δ V. Since Δ V for the process is (–), the sign of w is (+). (b) Δ E = q + w. At constant pressure, Δ H = q. If the reaction is endothermic, the signs of Δ H and q are (+). From (a), the sign of w is (+), so the sign of Δ E is (+). The internal energy of the system increases during the change. (This situation is described by the diagram (ii) in Exercise 5.3.) 5.6 (a) The temperature of the system and surroundings will equalize, so the temperature of the hotter system will decrease and the temperature of the colder surroundings will increase. The system loses heat by decreasing its temperature, so the sign of q is (–). The process is exothermic.
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