11.E: Liquids (Exercises)

A general chemistry Libretexts Textmap organized around the textbook
Chemistry: Principles, Patterns, and Applications
by Bruce A. Averill

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These are homework exercises to accompany the Textmap created for "Chemistry: Principles, Patterns, and Applications" by Bruce A. Averill and Patricia Eldredge. Complementary General Chemistry question banks can be found for other Textmaps and can be accessed here. In addition to these publicly available questions, access to private problems bank for use in exams and homework is available to faculty only on an individual basis; please contact Delmar Larsen for an account with access permission.

Application Problems

Please be sure you are familiar with the topics discussed in Essential Skills 6 (Section 11.9 "Essential Skills 6") before proceeding to the Application Problems. Problems marked with a ♦ involve multiple concepts.

1. During cold periods, workers in the citrus industry often spray water on orange trees to prevent them from being damaged, even though ice forms on the fruit.

   1. Explain the scientific basis for this practice.
   2. To illustrate why the production of ice prevents damage to the fruit during cold weather, calculate the heat released by formation of ice from 1000 L of water at 10°C.

2. ♦ Relative humidity is the ratio of the actual partial pressure of water in the air to the vapor pressure of water at that temperature (i.e., if the air was completely saturated with water vapor), multiplied by 100 to give a percentage. On a summer day in the Chesapeake, when the temperature was recorded as 35°C, the partial pressure of water was reported to be 33.9 mmHg.

   1. The following table gives the vapor pressure of water at various temperatures. Calculate the relative humidity.

      T (°C)	0	10	30	50	60	80	100
      P (mmHg)	4.6	9.2	31.8	92.6	150	355	760

   2. Why does it seem “drier” in the winter, even though the relative humidity may be the same as in the summer?

3. ♦ Liquids are frequently classified according to their physical properties, such as surface tension, vapor pressure, and boiling point. Such classifications are useful when substitutes are needed for a liquid that might not be available.

   1. Draw the structure of methanol, benzene, pentane, toluene, cyclohexane, 1-butanol, trichloroethylene, acetic acid, acetone, and chloroform.
   2. Identify the most important kind of intermolecular interaction in each.
   3. Sort the compounds into three groups with similar characteristics.
   4. If you needed a substitute for trimethylpentane, from which group would you make your selection?

4. ♦ In the process of freeze drying, which is used as a preservation method and to aid in the shipping or storage of fruit and biological samples, a sample is cooled and then placed in a compartment in which a very low pressure is maintained, ≈0.01 atm.

   1. Explain how this process removes water and “dries” the sample.
   2. Identify the phase change that occurs during this process.
   3. Using the Clausius–Clapeyron equation, show why it is possible to remove water and still maintain a low temperature at this pressure.

5. ♦ Many industrial processes for preparing compounds use “continuous-flow reactors,” which are chemical reaction vessels in which the reactants are mixed and allowed to react as they flow along a tube. The products are removed at a certain distance from the starting point, when the reaction is nearly complete. The key operating parameters in a continuous-flow reactor are temperature, reactor volume, and reactant flow rate. As an industrial chemist, you think you have successfully modified a particular process to produce a higher product yield by substituting one reactant for another. The viscosity of the new reactant is, however, greater than that of the initial reactant.

   1. Which of the operating parameters will be most greatly affected by this change?
   2. What other parameter could be changed to compensate for the substitution?
   3. Predict the possible effects on your reactor and your process if you do not compensate for the substitution.

11.6: Critical Temperature and Pressure

Conceptual Problems

1. Describe the changes that take place when a liquid is heated above its critical temperature. How does this affect the physical properties?

2. What is meant by the term critical pressure? What is the effect of increasing the pressure on a gas to above its critical pressure? Would it make any difference if the temperature of the gas was greater than its critical temperature?

3. Do you expect the physical properties of a supercritical fluid to be more like those of the gas or the liquid phase? Explain. Can an ideal gas form a supercritical fluid? Why or why not?

4. What are the limitations in using supercritical fluids to extract organic materials? What are the advantages?

5. Describe the differences between a molten salt and an ionic liquid. Under what circumstances would an ionic liquid be preferred over a molten salt?

11.8: Liquid Crystals

Conceptual Problems

1. Describe the common structural features of molecules that form liquid crystals. What kind of intermolecular interactions are most likely to result in a long-chain molecule that exhibits liquid crystalline behavior? Does an electrical field affect these interactions?

2. What is the difference between an isotropic liquid and an anisotropic liquid? Which is more anisotropic—a cholesteric liquid crystal or a nematic liquid crystal?