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19 Chemical Thermodynamics Solutions to Exercises 19.21 Analyze/Plan. Define the system and surroundings. Use the appropriate definition to answer the specific questions. Solve. (a) An ice cube can melt reversibly at the conditions of temperature and pressure where the solid and liquid are in equilibrium. At 1 atm external pressure, the normal melting point of water is 0°C. (b) We know that melting is a process that increases the energy of the system, even though there is no change in temperature. is not zero for the process. 19.22 (a) The detonation of an explosive is definitely spontaneous, once it is initiated. (b) The quantity q is related to Since the detonation is highly exothermic, q is large and negative. If only PV-work is done and P is constant, AH = q. Although these conditions probably do not apply to a detonation, we can still predict the sign of q, based on if not its exact magnitude. (c) The sign (and magnitude) of W depend on the path of the process, the exact details of how the detonation is carried out. It seems clear, however, that work will be done by the system on the surroundings in almost all circumstances (buildings collapse, earth and air are moved), so the sign of W is probably negative. (d) = q + W. If q and W are both negative, then the sign of is negative, regardless of the magnitudes of q and W. Entropy and the Second Law of Thermodynamics (section 19.2) 19.23 (a) For a process that occurs at constant temperature, an isothermal process, AS = qrev/T. Here qrev is the heat that would be transferred if the process were reversible. Since AS is a state function, it is independent of path, so S for the reversible path must equal S for any path. (b) No. S is a state function, so it is independent of path. 19.24 Both vaporizations are isothermal; they occur at constant temperature. For an isothermal process, : = (a) Assuming that qrev is closely related to enthalpy of vaporization and is about the same at the two temperatures, is larger at 25°C than at 100°C. (b) No. Because is a state function, it is independent of path. We can calculate for a reversible pathway, even if the change does not occur that way. 19.25 (a) entropy increases, more mol gas in products, greater motional freedom. (b) 581