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CHEMISTRY 123 Problem Set #2.pdf

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University of British Columbia
CHEM 123
Anka Lekhi

CHEMISTRY 123 Problem Set #2 Phase Diagrams Concept questions 1-3: A phase diagram for an unknown substance is shown below. This substance has a vapour and liquid phase, and three solid phases (S I S IIand S IIIAt point A on the diagram, S , III IId the liquid phase are in equilibrium. SIIs denser than S .III A B P [atm] T [K] 1. Label the states present in each of the five regions of the phase diagram. 2. Label all triple points with TP and the critical point with CP. 3. Which phase present at point B has the higher density? 4. You add heat to a liquid sample and some of it evaporates. How is this a Le Chatelier response to the addition of heat? 5. The line of liquid-solid equilibrium on a P, T phase diagram has a positive slope. How does this affirm that the solid has a higher density than the liquid? 6-20: For parts 6-20 below, consider the phase diagram for phenol, which has two solid phases, S and I S IIboth more dense than the liquid. The transition S → S iI exothIImic. For questions 6-11 and 16−20 use: (1) vapour or gas (2) liquid (3) solid S I (4) solid S II 6. Which phase is stable in region A? 7. Which phase is stable in region D? 8. Which phases are stable at 61 atm and 64 °C? 9. Which phases are in equilibrium at 2000 atm and 64 °C? 10. Which phases are in equilibrium at 3 × 10 atm and 40 °C? 11. Which phase is the most dense? 12. Which phase is more dense: B or C? (1) B (2) C 13. The normal boiling point is: (1) 40 °C (2) 64 °C (3) 420 °C (4) none of these 14. The critical temperature is: (1) 40 °C (2) 64 °C (3) 420 °C (4) none of these 15. The transition S →Iliquid is exothermic. (1) True (2) False 16. A tube is half filled with solid phenol, then the air is removed and the tube sealed. It is kept at a constant temperature of 35 °C, and the phenol comes to equilibrium. What phases are present in the tube at equilibrium? 17−20. Isobaric heating curve for phenol at 1 atm Identify the phase(s) present in each of the numbered regions on the following graph 21. Consider the P, V isotherm of a real gas at moderate temperature diagrammed to the right. Moving from large volume to small, the curve breaks once at large volume (point A), goes flat, then breaks again at small volume (point B) and bends sharply upward. What physical changes occur in the system at points A and B? 22. For the particular isotherm at temperature, T 1, consider the system as it approaches point B from the left. What can you say about the relative amounts of liquid and gas present? 23. Benzene freezes at +5.5 °C. Although you can easily skate on a frozen lake of water at −5 °C, you would not be able to skate very well on frozen benzene at 0 °C (i.e., also about 5 °C below its melting point). Explain why not by discussing the fact that the phase diagram for benzene is “normal.” (Ignore toxicity.) 24. If you’re skiing at the top of Whistler and you’re cooking spaghetti in boiling water, why does it take a bit longer than usual (i.e., compared to at home, at sea level) to cook? (Consider only the time after which the spaghetti is added to the boiling water, not the time needed to heat the water to a boil.) 25. A substance has the following properties: boiling point = 75 °C and melting point = –15 °C. Sketch a heating curve for the substance from –50 °C to 100 °C in the diagram shown below. Clearly label the phases present at each individual step (five steps). T [K] 373 273 223 Time [min] 26. Which of the following compounds is expected to have the highest vapor pr
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