6. A
In cisplatin, a molecule with square planar geometry, two chloride atoms and two ammonia molecules each are bonded directly to the central platinum, without any remaining lone pairs on the central platinum. The NH3 groups bond to the central platinum by donating a lone pair of electrons into an unfilled orbital of the platinum atom. As such, NH3 is acting as a Lewis base, and platinum is acting as a Lewis acid, and they form a coordinate covalent bond. A polar covalent bond (B) is not formed from this type of donation of a lone pair; an example of a polar covalent bond would be the N–H bonds in the NH3 group, with partial negative charge on the nitrogen and partial positive charge on the hydrogen. In (C) and (D), the Pt–N bond is formed by a Lewis acid/ base relationship, which is not the case for a nonpolar covalent bond or ionic bond.
7. B
Although the geometry of this carbon atom is likely to be changed somewhat by the ring strain on the adjacent ring structures, it is still most likely to approximate a tetrahedral geometry. This carbon is bonded to four groups: the two phenyl groups, a nitrogen, and the additional carbon atom in the adjacent carboxyl group. The tetrahedral geometry (B) maximizes the space among these four groups. Octahedral geometry (D) typically refers to a central atom surrounded by six groups, not four.
8. C
Resonance structures serve to spread out formal charge. The most important resonance structures minimize or eliminate the formal charge on individual atoms. As a result, polarity in any single bond might be minimized. (A) is essentially the opposite of this argument. (B) is incorrect; it is possible for molecules with or without resonance to be polar, and this is too broad of a generalization to be true. (D) is incorrect because polar bonds will intrinsically place a partial negative charge on the more electronegative atom.
Important resonance structures would likely further accentuate this inclination, placing extra electrons on more electronegative atoms. Counteracting this effect and essentially “removing” electrons from highly electronegative atoms to counterbalance the natural polarity of the bond would form an extremely high-energy, unfavorable, and thus unimportant, resonance structure.
PASSAGE II
9. B
The order of this reaction can be determined by the equation r = k[A]x [B]y. You must divide r3 by r1 to get:
10. B
The equation for the rate of a reaction is:
Rate = k [reactant1]order1[reactant2]order2
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 Kc = ([C]c[D]d)/([A]a[B]b). For this reaction, Kc = ([2]1[1]1/([3]2[1]2 = 2/9 = 0.22. When the corresponding values are plugged into the equation, Kc = 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
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 pKa plus the log of the ratio of the concentration of conjugate base to the concentration of acid.
pH = pKa + 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 pKa 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.