CHEM 105 Problem Set 25 PDF

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Problem Set 25 – Phase Diagrams Chem 105

1. List and briefly define (1 sentence) all possible phase changes. Freezing: the substance changes from liquid to a solid Melting: the substance changes back from the solid to the liquid Condensation: the substance changes form a gas to a liquid Vaporization: the substance changes from a liquid to a gas Sublimation: the substances changes direction from a solid to a gas without going through the liquid phase Deposition: the substance changes directly from a gas to a solid without going through the liquid phase 2. In the following phase diagram:

(a) Which phases corresponds to regions I, II, and III? Phase 1: solid Phase 2: liquid Phase 3: gas (b) What is the significance of points a-d? a = melting point b = boiling point c = critical point d = triple point (c) What do each of the lines represent? The solid lines identify the temperature and pressures at which an equilibrium exists between phases.

3. (a) What phase exists at the crossing of the dashed lines on the diagram to the right? The substance is a solid at 25 degrees Celsius and 1 atm What happens if you change the P or T along the dashed lines in direction?

phase

each

Increasing the temperature and decreasing the pressure would cause conversion of solid directly to gas (sublimation)

4. The image to the right is the phase diagram of an imaginary molecular compound X. a) Does the green line represent a series of (a) freezing points, (b) sublimation points, (c) boiling points, or (d) critical points? Transition from solid to gas. It represents a series of sublimation points b) Does the solid form of compound X (Figure P6.3)

float on the liquid as the liquid begins to freeze at P = atm? Solid is favored at higher pressure, therefore it is more dense. It won’t float on water

300

5. More questions about compound X. a) If a sample of X is left outside in a sealed container on a hot summer day, will the X in the container be a solid, liquid, or a gas? Solid b) Suppose you bring the sample of X inside and place it in an uncovered pot of boiling water on your kitchen stove. What phase changes, if any, will occur? On boiling the temperature reached 100 and the pressure is 1 atm. It changes from solid to gas c) Another sample of X is stored in a pressurized container (P = 50 atm) in a freezer at 0°C. If the sample is then transferred to an oven and slowly warmed to 250°C, what phase changes, if any, will X undergo? The point of 50 atm and 0 Celsius is the solid phase. The point for 50 atm and 250 is in the gas phase. The compound undergoes both melting (crossing the solid liquid equilibrium) and boiling (crossing the liquid gas equilibrium). This compound undergoes solid to liquid and liquid to gas changes. 6. To the right is the phase diagram of water. a) Label each region with the appropriate phase. b) As you go deeper in the ocean, the water gets colder, eventually reaching 0°C. Why do you not observe ice deep in the ocean? Use the information in the phase diagram (and the fact that pressures at the bottom of the ocean are about 1000 times greater than at sea level) in your explanation. It’s a liquid at 1000 atm and 0 degrees Celsi 7. Use the phase diagram of water in the previous problem for the following. a) Label points A-E. a = triple point b = critical point c = melting point d = boiling point e = sublimation b) Freeze-drying is used to preserve food at a low temperature with minimal loss of flavor. It is accomplished by freezing the food and then lowering the pressure with a vacuum pump to sublime the ice. Must the pressure be lower than the pressure at the triple point of H2O? The pressure has to be lower than the triple point; the line separating the solid and gas phases occurs at pressures below the triple point pressure, so if we want the solid H2o to vaporize the pressure must cross below that line 8. The melting point of solid hydrogen is 15.0 K at 1.00 atm. The temperature of its triple point is 13.8 K at 0.0695 atm. Does liquid H2 expand or contract when it freezes? Both the temp and the pressure of the triple point are lower than those of the normal melting point, thus the slope of the solid-liquid line must be positive and the solid phase has a higher density than the liquid phase. Therefore hydrogen contracts as it freezes

9. Sketch a phase diagram for material Z, which has a triple point at (152 K, 0.371 atm), a boiling point of 166 K at a pressure of 1.00 bar, and a normal melting point of 161 K at 1 bar....