Thermo_ISM_ch08 - Thermo Chapter 8 Questions on Concepts...

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131 Thermo Chapter 8 Questions on Concepts Q8.1) At a given temperature, a liquid can coexist with its gas at a single value of the pressure. However, you can sense the presence of H 2 O( g ) above the surface of a lake by the humidity, and it is still there if the barometric pressure rises or falls at constant temperature. How is this possible? The statement that at a given temperature, a liquid can coexist with its gas at a single value of the pressure holds for a system with only one substance. For the case described, the system consists of water and air. The change in barometric pressure is equivalent to an external pressure exerted on a liquid. As discussed in Section 8.5, this will change the vapor pressure only slightly. Q8.2) Why is it reasonable to show the µ versus T segments for the three phases as straight lines as is done in <LINK>Figure 8.1? More realistic curves would have some curvature. Is the curvature upward or downward on a µ versus T plot? P S T µ = − . Because S increases with T , the realistic curves will curve downward. Q8.3) Figure 8.5 is not drawn to scale. What would be the relative lengths on the q P axis of the liquid + solid, liquid, and liquid + gas segments for water if the drawing were to scale and the system consisted of H 2 O? For the liquid + solid segment, the length of the segment is H fusion , for the liquid segment, the length is , liquid P m C T , and for the liquid + gas segment, the length is H vaporization . Numerically, for water, the relative lengths are 6008 : 7550 : 40656. Q8.4) Why is sublimation fusion vaporization H H H = ∆ + ∆ ? Because H is a state function, H for the process solid liquid gas must be the same as for the process solid gas if the initial and final states are the same. Q8.5) A triple point refers to a point in a P-T phase diagram for which three phases are in equilibrium. Do all triple points correspond to gas–liquid–solid equilibrium? No. If there are several solid phases, there can be a triple point corresponding to equilibrium between 3 solid phases. Q8.6) Why are the triple point temperature and the normal freezing point very close in temperature for most substances?
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