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 versus T plot? P S T ⎛⎞ =− ⎜⎟ ⎝⎠ . 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 Pm CT , 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 HH 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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132 This is the case because the freezing point changes only slightly with temperature. Q8.7) As the pressure is increased at –45ºC, ice I is converted to ice II. Which of these phases has the lower density? The higher density phase, in this case ice II, is more stable as the pressure is increased. Therefore ice I is the less dense phase. Q8.8) What is the physical origin of the pressure difference across a curved liquid–gas interface? The origin is the cohesive forces in the liquid. Across an interface, the resultant force vector is not zero in magnitude. If the interface is curved, the resultant force tends to minimize the surface area, leading to a pressure difference across the interface.
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This note was uploaded on 06/10/2010 for the course CHEM 127 taught by Professor Nefzi during the Spring '03 term at UCSD.

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Thermo_ISM_ch08 - Thermo Chapter 8 Questions on Concepts...

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