Kaplan MCAT General Chemistry Review

Reversible chemical reactions will eventually “settle” into an energy minimum state (for which there is maximum entropy) called equilibrium. This is a dynamic equilibrium in which the concentrations of reactants and products are constant because the rates of the forward and reverse reactions are equal.

The law of mass action gives the equilibrium constant (K_eq) expression. It states that at equilibrium, the ratio of products to reactants will be constant. At equilibrium, the ratio of the forward rate to the reverse rate will be equal to one; however, the ratio of the equilibrium concentrations of products to reactants will usually not be equal to one.

The reaction quotient, Q_c, is a calculated value involving reactant and product concentrations at any time within a reaction. Comparison of the calculated Q_c value to the known K_eq value will tell you “where” the reaction is with respect to its equilibrium state.

Pure solids and pure liquids are not included in the law of mass action, and K_eq is temperature dependent.

Le Châtelier’s principle states that a chemical system that experiences a stress (changes to concentration, pressure, or temperature) will react in whichever direction results in the re-establishment of the equilibrium state.

EQUATIONS TO REMEMBER

Practice Questions

1. In a third-order reaction involving two reactants and two products, doubling the concentration of the first reactant causes the rate to increase by a factor of 2. What will happen to the rate of this reaction if the concentration of the second reactant is cut in half?

A. It will increase by a factor of 2.

B. It will increase by a factor of 4.

C. It will decrease by a factor of 2.

D. It will decrease by a factor of 4.

2. In a certain equilibrium process, the activation energy of the forward reaction is greater than the activation energy of the reverse reaction. This reaction is

A. endothermic.

B. exothermic.

C. spontaneous.

D. nonspontaneous.

3. Carbonated beverages are produced by dissolving carbon dioxide in water to produce carbonic acid

CO₂ (g) + H₂O(l) H₂CO₃ (aq)

When a bottle containing carbonated water is opened, the taste of the beverage gradually changes until all of the carbonation is lost. Which of the following statements best explains this phenomenon?

A. The change in pressure and volume causes the reaction to shift to the left, thereby decreasing the amount of aqueous carbonic acid.

B. The change in pressure and volume causes the reaction to shift to the right, thereby decreasing the amount of gaseous carbon dioxide.

C. Carbonic acid reacts with environmental oxygen and nitrogen.

D. Carbon dioxide reacts with environmental oxygen and nitrogen.

4. A certain chemical reaction is endothermic. It occurs spontaneously. Which of the following must be true for this reaction?

I. H > 0

II. G < 0

III. S > 0

A. I only

B. I and II only

C. II and III only

D. I, II, and III

5. A certain ionic salt, A₃B, has a molar solubility of 10 M at a certain temperature. What is the K_sp of this salt at the same temperature?

A. 10⁴

B. 3 × 10⁴

C. 2.7 × 10⁵

D. 8.1 × 10⁵

6. Acetic acid dissociates in solution according to the following equation:

CH₃COOH CH ₃COO^- + H⁺

If sodium acetate is added to a solution of acetic acid in excess water, which of the following effects would be observed in the solution?

A. Decreased pH

B. Increased pH

C. Decreased pK_a

D. Increased pK_a

7. A certain chemical reaction follows the following rate law:

Rate = k [NO₂] [Br₂]

Which of the following statements describe(s) the kinetics of this reaction?

I. The reaction is a second-order reaction.

II. The amount of NO₂ consumed is equal to the amount of Br₂ consumed.

III. The rate will not be affected by the addition of a compound other than NO₂ and Br_2.

A. I only

B. I and II only

C. I and III only

D. III only

8. The following data shown in the table were collected for the combustion of the theoretical compound XH₄:

XH₄ + 2 O₂ XO₂ + 2 H₂O

What is the rate law for the reaction described here?

A. Rate = k [XH₄] [O₂]

B. Rate = k [XH₄] [O₂]²

C. Rate = k [XH₄]² [O₂]

D. Rate = k [XH₄]² [O₂]²

9. Which of the following actions does NOT affect the equilibrium of a reaction?

A. Adding/subtracting heat

B. Adding/removing a catalyst

C. Increasing/decreasing concentration of reactants

D. Increasing/decreasing volume of reactants

10. In a sealed 1 L container, 1 mol of nitrogen gas reacts with 3 mol of hydrogen gas to form 0.05 mol of NH₃. Which of the following is closest to the K_eq of the reaction?

A. 0.0001

B. 0.001

C. 0.01

D. 0.1

FOR QUESTIONS 11-13, CONSIDER THE FOLLOWING ENERGY DIAGRAM SHOWN IN FIGURE 1:

11. The overall reaction depicted by this energy diagram is

A. endothermic, because point B is higher than point A.

B. endothermic, because point C is higher than point A.

C. exothermic, because point D is higher than point E.

D. exothermic, because point A is higher than point E.