Kaplan MCAT General Chemistry Review

The intermolecular interactions are electrostatic interactions that weakly hold molecules together. They generally are significant only over short distances and in the solid or liquid state. The weakest intermolecular interaction is the dispersion force (London force), which results from the moment-by-moment changing unequal distribution of the electron cloud density across molecules. A more moderate-strength intermolecular interaction is the dipole–dipole interaction, which exists between the opposite ends of permanent molecular dipoles. The strongest of these intermolecular interactions is the hydrogen bond, which is a special case of the dipole–dipole interaction between the hydrogen atom attached to either oxygen, nitrogen, or fluorine and another oxygen, nitrogen, or fluorine on another molecule—or sometimes, if the molecule is large enough, even the same molecule. One of the most important occurrences of hydrogen bonding is that between water molecules.

EQUATIONS TO REMEMBER

µ = qr

Formal charge = V - N_nonbonding -½N_bonding

Practice Questions

1. What is the character of the bond in carbon monoxide?

A. Ionic

B. Polar covalent

C. Nonpolar covalent

D. Coordinate covalent

2. Which of the following molecules has the oxygen atom with the most negative formal charge?

A. H₂O

B. CO₃^2-

C. O₃

D. CH₂O

3. Which of the following structure(s) contribute most to NO₂’s resonance hybrid?

A. I only

B. III only

C. I and II

D. I, II, and III

4. Order the following compounds shown from lowest to highest boiling point:

I.

II. KCl

III. Kr

IV. Isopropyl alcohol

A. I < II < IV < III

B. III < IV < I < II

C. II < IV < I < III

D. III < I < IV < II

5. Both CO₃^2– and ClF₃ have three atoms bonded to a central atom. What is the best explanation for why CO₃ has trigonal planar geometry, while ClF₃ is trigonal bipyramidal?

A. CO₃ has multiple resonance structures, while ClF₃ does not.

B. CO₃ has a charge of -2, while ClF₃ has no charge.

C. ClF₃ has lone pairs on its central atom, while CO₃ has none.

D. CO₃ has lone pairs on its central atom, while ClF₃ has none.

6. Which of the following has the largest dipole moment?

A. HCN

B. H₂O

C. CCl₄

D. SO₂

7. Despite the fact that both C₂H₂ and NCH contain triple bonds, the lengths of these triple bonds are not equal. Which of the following is the best explanation for this finding?

A. In C₂H₂, since the triple bond is between similar atoms, it is shorter in length.

B. The two molecules have different resonance structures.

C. Carbon is more electronegative than hydrogen.

D. Nitrogen is more electronegative than carbon.

8. Which of the following is the best explanation of the phenomenon of hydrogen bonding?

A. Hydrogen has a strong affinity for holding onto valence electrons.

B. Hydrogen can only hold two valence electrons.

C. Electronegative atoms disproportionately carry shared pairs when bonded to hydrogen.

D. Hydrogen bonds have ionic character.

9. Which of the following best describes the character of the bonds in a molecule of ammonium?

A. Three polar covalent bonds

B. Four polar covalent bonds

C. Two polar covalent bonds, two coordinate covalent bonds

D. Three polar covalent bonds, one coordinate covalent bond

10. Although the octet rule dictates much of molecular structure, some atoms can exceed the octet rule and be surrounded by more than eight electrons. Which of the following is the best explanation for why some atoms can exceed the octet rule?

A. Atoms that exceed the octet rule already have eight electrons in their outermost electron shell.

B. Atoms that exceed the octet rule only do so when bonding with transition metals.

C. Atoms that exceed the octet rule can do so because they have d-orbitals in which extra electrons can reside.

D. Some atoms can exceed the octet rule because they are highly electronegative.

11. Which of the following types of intermolecular forces is the most accurate explanation for why noble gases can liquefy?

A. Van der Waals forces

B. Ion-dipole interactions

C. Dispersion forces

D. Dipole-dipole interactions

12. What is correct electron configuration for elemental chromium?

A. [Ar] 3p⁶

B. [Ar] 3d⁵4s¹

C. [Ar] 3d⁶

D. [Ar] 3d⁴4s²

13. In the structure shown, which atom(s) has/have the most positive charge?

A. The phosphorus atom has the most positive charge.

B. All atoms share the charge equally.

C. The four oxygens share the highest charge.

D. The oxygen at the peak of the trigonal pyramidal geometry has the most positive charge.

14. Which of the following is the best name for the new bond formed in the reaction shown?

A. Polar covalent bond

B. Ionic bond

C. Coordinate covalent bond

D. Hydrogen bond

15. Both BF₃ and NH₃ have three atoms bonded to the central atom. Which of the following is the best explanation of why the geometry of these two molecules is different?

A. BF₃ has three bonded atoms and no lone pairs, which makes its geometry trigonal pyramidal.

B. NH₃ is sp³ hybridized, while BF₃ is sp² hybridized.

C. NH₃ has one lone pair.