Kaplan MCAT General Chemistry Review

D. Ne and Kr

PASSAGE III (QUESTIONS 22–29)

Dry ice forms when carbon dioxide gas is cooled to -78° C at atmospheric pressure. After becoming solid, it reforms gas when heat is added as shown in the reversible reaction below:

CO₂ (solid, -78° C) + heat (120kJ/mol) CO₂ (gas, 25° C)

An experiment was done to test the change, over three days, in a block of dry ice placed in a rigid container at room temperature at 1 atmosphere of pressure. The container was closed to the outside environment for the duration of the experiment. The only components in the container were the dry ice and air (g). No liquid in the container was detected over the three-day period. The apparatus and results of the experiment are shown in Figure 1 below.

Figure 1

As a solid, carbon dioxide has many uses, not the least of which is cooling its surroundings. This transfer of energy is a main method by which coolants operate in many mechanical devices. The phase diagram for carbon dioxide is a major reason for its unique behaviors. The phase diagram for carbon dioxide is shown in Figure 2 below.

Figure 2

22. Which of the following BEST describes the equation in the passage?

A. Evaporation

B. Condensation

C. Fusion

D. Sublimation

23. Referring to figure 2, if liquid carbon dioxide were subject to increasing pressure at a constant temperature, it would

A. become solid.

B. become gaseous.

C. gain kinetic energy.

D. lose kinetic energy.

24. It can be inferred from the results of the experiment that the air in the container

A. lost kinetic energy.

B. gained kinetic energy.

C. gained volume.

D. lost volume.

25. When the dry ice molecules shown undergo phase changes, which of the following is a likely cause?

A. The attractive forces between the carbon dioxide molecules overcome the kinetic energy that keeps them apart.

B. The kinetic energy of the carbon dioxide molecules overcomes the attractive forces that keep them together.

C. The hydrogen bonds between the carbon dioxide molecules form at a more rapid rate in the solid phase.

D. The hydrogen bonds between the carbon dioxide molecules form at a more rapid rate in the liquid phase.

26. The process shown in the experiment from the passage was

A. endothermic, and the dry ice gained potential energy.

B. endothermic, and the dry ice lost potential energy.

C. exothermic, and the dry ice gained potential energy.

D. exothermic, and the dry ice lost potential energy.

27. If the experiment from the passage were allowed to continue until all the carbon dioxide changed phase, one could logically predict that the air in the container would have

A. increased in pressure.

B. decreased in volume.

C. become a solid.

D. increased in temperature.

28. In the heating/cooling curve for carbon dioxide shown below, what represents the location of the phase change described in the passage?

A. A

B. B

C. C

D. None of the above

29. In the phase diagram for carbon dioxide shown below, what represents the phase change shown in the experiment in the passage?

A. A

B. B

C. C

D. None of the above

QUESTIONS 30–36 ARE NOT BASED ON A DESCRIPTIVE PASSAGE.

30. Which of the following will result in a negative free energy change for a reaction?

A. The enthalpy change is negative.

B. The entropy change is positive.

C. The enthalpy change is negative and the entropy change is negative.

D. The enthalpy change is negative and the entropy change is positive.

31. Compared to the atomic radius of calcium, the atomic radius of gallium is

A. larger, because increased electron charge requires that the same force be distributed over a greater number of electrons.

B. smaller, because gallium gives up more electrons, decreasing its size.

C. smaller, because increased nuclear charge causes the electrons to be held more tightly.

D. larger, because its additional electrons increases the volume of the atom.

32. Under which conditions would water vapor demonstrate behavior closest to an ideal gas?

A. High pressure, low temperature

B. Low pressure, low temperature

C. High pressure, high temperature

D. Low pressure, high temperature

33. If the pressure of an ideal gas in a closed container is halved while the volume is held constant, the temperature of the gas

A. decreases by a factor of 2.

B. decreases by a factor of 4.

C. remains the same.

D. increases by a factor of 4.

34. “Greenhouse gases” are gases that will absorb IR radiation and trap energy between the Earth and the atmosphere. CO₂ and H₂O both strongly absorb radiation and are thus considered greenhouse gases, while N₂ and O₂ do not. One quality of greenhouse gases is that

A. they are composed of polar molecules.

B. they have a permanent dipole moment.

C. they experience hydrogen bonding.

D. they have polar covalent bonds.

35. What element contains unpaired electrons in its most common ionized state?

A. Fluorine

B. Aluminum

C. Zinc

D. Iron

36. Which of the following is the correct electron configuration for chromium in the ground state?