Kaplan MCAT General Chemistry Review

The human body is a dynamic system that has to deal with significant environmental threats on a daily basis. One type of threat is from the effects of reactive oxygen species (ROS). ROS are ions or small molecules containing oxygen that have unpaired valence shell electrons. The superoxide anion, O₂^-, is a toxic threat, becoming lethal at intracellular levels of just 1 nM. It spontaneously forms O₂ and H₂O₂, but is also able to react with NO to form peroxynitrite. Peroxynitrite can cause extreme cellular damage. The enzyme NADPH oxidase produces the superoxide anion in the body to combat invading microorganisms. Because of the threat that it poses in such small quantities, the body has developed ways to dispose of this chemical.

The superoxide anion puts the concept of compartmentalization on display. It would be a waste of energy to both produce and destroy superoxide in the same cell, so it is only produced in phagocytes (immune cells that ingest infectious agents), and is broken down in any other cell of the body. In two steps, the enzyme superoxide dismutase (SOD) uses iron or other metals to create oxygen and hydrogen peroxide from superoxide and hydrogen ions:

Step 1: Fe³⁺- SOD + O₂^- Fe²⁺ - SOD + O₂

Step 2: Fe²⁺- SOD + O₂^- + 2H⁺ Fe³⁺ -

SOD + H₂O₂

A graduate student at a local university was given the task of determining the kinetics of this reaction. Her results are shown in table 1 below:

Table 1

The potential energy diagram for the reaction is shown in figure 1 below:

Figure 1

FOR QUESTIONS 9–14, ASSUME BOTH STEPS OF THE REACTION ARE IRREVERSIBLE.

9. What is the order of H⁺ in step 2 of the above reaction?

A. 0

B. 1

C. 2

D. 3

10. What is the rate of step 2 of the reaction if the following concentrations of reactants exist? Assume the rate constant, k = 0.50.

[H⁺] = 2 M, [O₂^-] = 2 M

A. 2.5 M/sec

B. 4 M/sec

C. 8 M/sec

D. 10 M/sec

11. What function might Fe²⁺ - SOD play in the overall reaction?

A. A substance used to create a product in the reaction

B. A substance that is created in the reaction

C. A substance that increases the rate of the reaction

D. A short-lived, unstable molecule in the reaction

12. Which of the following is NOT true when describing the kinetics of the previous overall reaction?

A. The rate of the reaction is proportional to the number of collisions between reacting molecules.

B. In some effective collisions, all of the colliding particles do not have enough kinetic energy to exceed activation energy.

C. A transition state is formed when old bonds are breaking and new bonds are forming.

D. The activated complex has greater energy than either products or reactants.

13. What section of the diagram in figure 1 represents the forward activation energy?

A. A

B. B

C. C

D. D

14. What section of the diagram in figure 1 represents the enthalpy change during the reaction?

A. A

B. B

C. C

D. D

FOR QUESTIONS 15–16, ASSUME BOTH STEPS OF THE REACTION ARE REVERSIBLE.

15. What is the equilibrium constant for the overall reaction in the passage and will an increase in pressure raise or lower the equilibrium constant? Assume [O₂^-] = 3 M, [H⁺] = 1 M, [H₂O₂] = 1 M, [O₂] = 2 M.

A. 0.22; raise

B. 0.27; lower

C. 2.2; raise

D. 2.7; lower

16. What would BEST explain the equilibrium shift to the right in reaction 2?

A. Increase in volume

B. Addition of product

C. Addition of reactant

D. Decrease in temperature

QUESTIONS 17–21 ARE NOT BASED ON A DESCRIPTIVE PASSAGE.

17. Latent heat flux is the loss of heat by the surface of a body of water caused by evaporation. To determine the latent heat flux over the Atlantic Ocean, one would need to know

A. H _fusion of water.

B. H _vaporization of water.

C. H _sublimation of water.

D. H _ionization of water.

18. If the pK_a of a weak acid is 5, the pH will be 6

A. when the concentration of dissociated acid is one-tenth the concentration of undissociated acid.

B. when half the acid is dissociated.

C. when the concentration of dissociated acid is ten times the concentration of undissociated acid.

D. only after a base has been added.

19. What compound would NOT be considered an electrolyte?

A. AgCl

B. CaO

C. LiI

D. HBr

20. Aluminum has a lower electronegativity than iron, but reacts extremely slowly with oxygen in moist air because of a hard, protective aluminum oxide coat that protects all exposed surfaces. Under which of the following conditions would aluminum be more readily eroded?

A. Immersed in a solution of HCl

B. Immersed in a bath of hot sodium metal

C. Immersed in a solution of NH₃

D. Immersed in a solution of NaOH

21. Two gases, X and Y, are combined in a closed container. At STP, the average velocity of a gas A molecule is twice that of a gas B molecule. Gases A and B are most likely which of the following?

A. He and Ar

B. He and Kr

C. Ne and Ar