Kaplan MCAT General Chemistry Review

Using the equation for determining order of a reactant detailed in the explanation for the answer to question 9, we find that O₂ is second order. Substituting the information into the equation for the rate of a reaction leads to:

Rate = (0.5)[2.0 M]¹[2.0 M]²

Simplifying this equation allows us to determine that the rate is 4 M/sec.

11. C

(C) is correct because it describes a catalyst, which is exactly the role Fe²⁺ - SOD plays. (A) is incorrect because it describes a reactant, and Fe²⁺ - SOD is not a reactant. (B) is incorrect because it describes a product, and Fe²⁺ - SOD is not a product. (D) describes a transition state, and is incorrect because transition states are temporary states of highest energy of the conversion of reactant to product.

12. B

(B) is not true (making it the correct answer) because all the colliding particles must have enough kinetic energy to exceed activation energy if a collision is to be effective. (A) is true because particles must collide to react, and thus the rate of reaction is both dependent on and proportional to the number of particles colliding. (C) is the accurate definition of a transition state. (D) is true because “activated complex” is another name for “transition state,” which by definition has greater energy than both reactants and products.

13. C

Section C shows the energy that must be put into the reaction to drive it in a forward direction. Section A in the diagram represents the change in enthalpy; it shows the difference between the starting and the final energy values. Section B does not represent any specific energy value. Section D represents the reverse activation energy. It is much greater than the forward activation energy because the reactant is starting at a much lower level of energy, yet must still reach the same amount of total energy to proceed with the reaction.

14. A

Section A is correct because it shows the difference between the starting and the final energy values.

15. A

The equation to determine the equilibrium constant for a reaction aA + bB = cC + dD is K_c = ([C]^c[D]^d)/([A]^a[B]^b). For this reaction, K_c = ([2]¹[1]¹/([3]²[1]² = 2/9 = 0.22. When the corresponding values are plugged into the equation, K_c = 2/9, or 0.22.

16. C

According to Le Châtelier’s principle, the addition of a reactant (C) will cause an equilibrium to shift to the right. An increase in volume (A) would have no effect in this particular reaction. An addition of product (B) would cause a shift to the left. A temperature decrease (D) would most likely cause a shift to the left due to fewer numbers of collisions between particles, assuming kinetic energy is proportional to temperature.

QUESTIONS 17–21

17. B

The question says that latent heat flux is caused by evaporation. Therefore, simply identify which value of H is related to this phase change. Vaporization is another way of saying evaporation, so (B) is correct. Fusion (A) refers to the change from a solid to a liquid, and sublimation (C) refers to the change from a solid to a gas. Ionization (D) is unrelated to a phase change.

18. C

The correct answer is (C) due to the Henderson-Hasselbach equation, which states that for a weak acid solution, the pH equals the pK_a plus the log of the ratio of the concentration of conjugate base to the concentration of acid.

pH = pK_a + log([conjugate base]/[acid])

For an acid HX, the conjugate base is the X^- ion that is formed when the acid dissociates. Now, if the pK_a of an acid is 5, then for its pH to be 6, the log of that concentration ratio must be 1, or in other words, the ratio must be 10. This means that the concentration of conjugate base, and therefore of dissociated acid, must be 10 times the concentration of acid.

19. A

The question requires you to know the solubility rules. Because an electrolyte must dissociate into its component ions in water, look for the substance that will not dissociate. The answer is (A), silver chloride (AgCl), which is insoluble in water. All of the other compounds will readily dissociate into their constituent ions.

20. A

(A) is correct. Because aluminum oxide is alkaline, immersion in an acidic solution would readily strip away the protective coating and allow the acid to oxidize the metal below.

21. D

How can you relate the average velocities of gases at the same temperature? Two gases at the same temperature will have the same average molecular kinetic energy. Because two gases at the same temperature will have the same average molecular kinetic energy, (1/2)m _A v _A² = (1/2)m _B v _B², which gives m_B /m _A = (v_A/v_B)². Since v_A /v_B = 2, we have m_B /m _A = 4. Only (D) lists two gases with a mass ratio around 4:1 (krypton 83.3g/mol and neon 20.2 g/mol).

PASSAGE III

22. D