Prince George's Community College
|Unit I: Atoms Unit II: Molecules Unit III: States of Matter Unit IV: Reactions Unit V: Kinetics & Equilibrium
Unit VI: Thermo & Electrochemistry Unit VII: Nuclear Chemistry
Problems marked with a ♦ involve multiple concepts.
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.
- Explain the scientific basis for this practice.
- 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.
♦ 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 Torr.
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 (Torr) 4.6 9.2 31.8 92.6 150 355 760
- Why does it seem “drier” in the winter, even though the relative humidity may be the same as in the summer?
♦ 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.
- Draw the structure of methanol, benzene, pentane, toluene, cyclohexane, 1-butanol, trichloroethylene, acetic acid, acetone, and chloroform.
- Identify the most important kind of intermolecular interaction in each.
- Sort the compounds into three groups with similar characteristics.
- If you needed a substitute for trimethylpentane, from which group would you make your selection?
♦ 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.
- Explain how this process removes water and “dries” the sample.
- Identify the phase change that occurs during this process.
- Using the Clausius–Clapeyron equation, show why it is possible to remove water and still maintain a low temperature at this pressure.
♦ 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.
- Which of the operating parameters will be most greatly affected by this change?
- What other parameter could be changed to compensate for the substitution?
- Predict the possible effects on your reactor and your process if you do not compensate for the substitution.
Modified by Joshua Halpern, Scott Sinex and Scott Johnson