Kaplan MCAT General Chemistry Review

D. BF₃ is nonpolar, while NH₃ is polar.

16. Which of the following is a proper Lewis structure for BeCl₂?

17. Which of the following best describes an important property of bond energy?

A. Bond energy increases with increasing bond length.

B. The more shared electron pairs comprising a bond, the higher the energy of that bond.

C. Single bonds are more difficult to break than double bonds.

D. Bond energy and bond length are unrelated.

18. Which of the following is true about the polarity of molecules?

A. Polarity is dependent on the vector sum of dipole moments.

B. Polarity does not depend upon molecular geometry.

C. If a molecule is comprised of one or more polar bonds, the molecule is polar.

D. If a molecule is comprised of one or more nonpolar bonds, the molecule is nonpolar.

Small Group Questions

1. Why does bond length decrease with increasing bond strength?

2. Predict two elements whose hydrides (H^-) would contain incomplete octets.

Explanations to Practice Questions

1. B

Carbon monoxide, CO, has a double bond between carbon and oxygen, with the carbon retaining one lone pair and oxygen retaining two lone pairs. In polar covalent bonds, the difference in electronegativity between the bonded atoms is great enough to cause electrons to move disproportionately toward the more electronegative atom but not great enough to transfer electrons completely (i.e., to form an ionic bond). This is the case for CO. Oxygen is more electro-negative than carbon, so electrons will be disproportionately carried on the oxygen, leaving the carbon atom with a slight positive charge. Nonpolar covalent bonds occur between atoms that have the same electronegativity, for example, the carbon–carbon bond in CH₃–CH₃. Coordinate covalent bonding occurs through donation of a lone pair from a Lewis base to a Lewis acid.

2. B

To answer this question, you must understand the contribution of resonance structures to average formal charge. In CO₃^2-, there are three possible resonance structures. Each of the three oxygen atoms carries a formal charge of -1 in two out of the three structures. This averages to approximately -²/₃ charge on each oxygen atom, which is more than the other answer choices. To prove this, estimate the formal charges on each oxygen atom in the other answer choices. There are no formal charges in H₂O, which has no resonance structures. In ozone, O₃, there are two possible resonance structures. The central oxygen carries a positive charge in both structures, and each outer oxygen carries a negative charge in one of the two possible resonance structures. This leaves an average charge of -½ on the two outer oxygens, which is less than the -²/₃ in CO₃^2-. Finally, CH₂O has no resonance structures and, therefore, no formal charge on oxygen.

3. C

The two greatest contributors are structures I and II. Resonance structures are representations of how charges are shared across a molecule. In reality, the charge distribution is a weighted average of contributing resonance structures. The most stable resonance structures are those that minimize charge on the atoms in the molecule; the more stable the structure, the more that it will contribute to the overall charge distribution in the molecule. Structure I has a -1 charge on one oxygen and a single positive charge on the central nitrogen atom. Structure II has the exact same distribution, except the negative charge is on the other oxygen. It is equally as stable as structure I, so they both contribute equally to the hybrid. Structure III involves two negative charges (one on each oxygen) and a +2 charge on the central nitrogen. With so many charges, it’s less stable than structures I and II and, thus, is not an important resonance structure for NO₂.

4. D