CHEM 341
Example Problems for Exam 1

The actual exam will consist of three parts. Short sentence answer (conceptual), Short numerical answer (like the homework problems), and Application of physical chemical principles to real world problems. Here I give many examples of each type of problem (many more than will actually be on the exam). Please note that not all the material you are responsible for is covered by this preexam. Some aspects of what we have learned that are not covered here may be on the actual exam. On the actual exam, you will be given all constants and conversion factors that you will need (I will put a set of conversion factors and a few common equations on the front cover of the exam).

Short Answer (there will be five problems like this on the exam):

1) Name and give the units of four of the state variables that we have worked with (use English words, not symbols).
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2) Define the term work as it is used in thermodynamics.
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3) Define the term heat as it is used in thermodynamics.
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4) Consider a system that is adiabatic and at constant pressure (the volume can change allowing the system to remain at the external pressure of 1 atm at all times). Will an exothermic reaction inside the system most likely cause the internal energy of the system to increase, decrease or stay the same? Why?
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5) a) An ideal gas at greater than one atm is allowed to expand against the surrounding atmospheric pressure (at 1 atm) adiabatically. Is the work positive, negative or zero?
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b) Is the internal energy change in the last problem positive, negative or zero?
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c) In the last problem, would the temperature increase, decrease or stay the same upon expansion? Explain.
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d) Still considering expansion of a gas against a constant pressure, what would be the effect of using a gas that was not ideal and had a Joule Thompson coefficient that was negative?
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Short Numerical Answer (there will be three problems like this on the exam)

7) What would the final pressure be if 2.0 L of an ideal gas at 1 atm was compressed to 1.0 L at the same temperature?
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8) If the density of an ideal gas is 1 g/L at 300 K and 1 atm, what is its molecular weight (molar mass)?
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9) What is the mean free path of Nitrogen molecules at 100,000 Pa and 25 C ?

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10) 1 mole of an ideal gas is expanded isothermally and reversibly from 2 liters to 4 liters at 25 C. Calculate w, q, and the change in internal energy for this process.
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11) How much heat is required to raise the temperature of 2 moles of an ideal gas from 20 C to 40 C at a constant pressure of 1.0 atm? What is the change in enthalpy for this process? (Assume that the molar CV of an ideal gas is about 25 J/(K mol) )
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12) How much heat is required to raise the temperature of 1.5 moles of an ideal gas from 15 C to 50 C when it is confined in a rigid container of 1 liter? What is the change in internal energy for this process? (Assume that the molar CV of an ideal gas is about 25 J/(K mol) )
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13) Use standard enthalpies of formation to determine the standard reaction enthalpy for the following reaction. Is the reaction exothermic or endothermic?

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Applying the concepts and calculations (there will be one problem like this on the exam)

14) Let’s suppose that one absent-mindedly placed a sealed 1 liter glass jar on a hot burner (the jar originally contained 1 atm air at 25 C). If the jar breaks when it reaches 2 atm, what is the temperature inside and what is the internal energy of the system when it explodes? Assume that air is an ideal gas (molar CV approximately 25 J/(K mol)).
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15) A bicycle tire has circumference of about 27.5 inches and a tube diameter of about 1.125 inches. What volume of air must be isothermally compressed in order to bring the pressure in the tube to 90 psi at 25 C? If this compression was done reversibly, how much work would be involved? Assume air behaves as an ideal gas. Note that an inch is 2.54 cm and a pound per square inch (psi) is 6895 pascals or about 0.06805 atm.
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16) 300 ml of liquid nitrogen is left over after an experiment in a 10 liter dewar (an insulated container for very cold liquids) and then left on the bench. Unfortunately, the student had absent-mindedly sealed the dewar completely, leaving no vent for excess pressure to escape as the nitrogen vaporized. It sits for a couple of days until it reaches 25 C and all of the nitrogen has been converted to gas (the dewar is insulated, but heat slowly leaks in through the walls). What is the pressure inside the dewar when the temperature reaches 25 C (assume that nitrogen gas behaves ideally). The density of liquid nitrogen is about 0.8 g/ml. The molecular weight of Nitrogen gas is 28.
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