Kaplan MCAT General Chemistry Review

The solid block becomes smaller in the experiment and so must move from the solid phase to the gaseous phase, sublimation. The other answer choices require a liquid phase, which is not valid because the experimental results demonstrate that no liquid was detected when the solid shrunk in size.

23. A

The solid/liquid equilibrium line has a positive slope and so the phase change from liquid to solid at a constant temperature is the only correct possible phase change. (B) is wrong because the gas phase is not possible as pressure increases; the liquid would change to gas, however, if the temperature were increased while maintaining constant pressure. (C) and (D) are illogical because kinetic energy is equivalent to temperature. Because temperature is constant according to the question stem, there is no kinetic energy change.

24. A

You must deduce from both the experimental results and the chemical equation that the volume does not change. The volume does not change because it is described as a rigid container, and no change in volume is described in the experimental results. As a result, the dry ice must absorb energy from its surroundings (the air in the container) in order to change phase. Because the total energy must remain constant, the air initially present in the container must lose an equivalent amount of kinetic energy.

25. B

The chemical equation shows that the dry ice absorbs heat from its environment to change into a gas, requiring it to overcome the intermolecular forces that organize it in the solid form. (C) and (D) refer to hydrogen bonds, which are not present in carbon dioxide.

26. A

The chemical reaction shows that the conversion of solid carbon dioxide to the gaseous state requires an absorption of heat from the ambient air. This use of heat defines an endothermic reaction, which must therefore characterize the reaction occurring in the container. (C) and (D) refer to an exothermic reaction, which would release heat. (B) is incorrect since an endothermic reaction absorbs/uses heat, which increases the energy of some of the molecules in the reaction; thus, the potential energy should not decrease.

27. A

The air gains dry ice molecules and these exert a pressure on the container, according to the equation PV = nRT, where n represents the number of moles of gas. (B) is the opposite; its volume would increase, according to the same equation, if possible. However this experiment takes place in a rigid container, preventing any change in volume. (C) is incorrect because the question clearly states that the solid changes phase. In this endothermic reaction, air temperature decreases as the solid carbon dioxide absorbs energy in order to undergo sublimation.

28. D

None of the choices shows a substance moving from the solid directly to the gas phase in the process of sublimation. (A) and (B) show fusion and evaporation, respectively, but not sublimation. (C) indicates the heating of a gas; in the experiment, the carbon dioxide solid increases its kinetic energy; the gas is not being heated.

29. C

(C) indicates change from a solid to a gas (sublimation) as described in the passage for dry ice.

QUESTIONS 30-36

30. D

Because G = H - can be used to relate free energy to enthalpy and entropy, and T is always positive, a negative H (enthalpy) and a positive S (entropy) will always give a negative G value; that is, the reaction will occur spontaneously. (D) is correct.

31. C

Moving from left to right across the periodic table, atomic radii will decrease. Because calcium is on the left side of the table and gallium is on the right, in the same period, gallium should have a smaller atomic radius. As stated in (C), this is due to the greater number of protons in the nucleus holding the electrons more tightly.

32. D

The major forces that cause gases to deviate from ideal behavior are intermolecular attractions and the volume of the gas molecules. These factors are minimized when gas molecules are far apart and moving quickly, which occurs at low pressures and high temperatures.

33. A

The question says that this gas is ideal, so use PV = nRT. The ideal gas law shows that P is directly proportional to T. So with volume held constant, if pressure is reduced by a factor of 2, temperature will also be reduced by a factor of 2. (A) is correct.

34. D

Though this question begins by telling you about greenhouse gases, it ultimately asks you to identify similarities between CO₂ and H₂O which N₂ and O₂ lack. CO₂ is not a polar molecule; its linear geometry allows the opposing dipole moments to cancel out, so (A) is incorrect. Because its dipole moments cancel, (B) is incorrect. Furthermore, it lacks hydrogen atoms and is therefore incapable of hydrogen bonding, so (C) is incorrect. However, both CO₂ and H₂O have polar bonds, while N₂ and O₂ both have diatomic, nonpolar covalent bonds between two atoms of the same element.

35. D