AP - Advanced Placement Intermolecular Forces and Properties 1

Question 1

Which of the following noble gases has the highest boiling point?

Accepted Answer

Xe

Answer

Boiling point among noble gases is determined entirely by London dispersion forces, which increase with molar mass and polarizability. Xenon has the greatest number of electrons and the largest, most diffuse electron cloud, producing the strongest temporary induced dipoles and therefore the highest boiling point of the four.

Question 2

Which of the following molecules is capable of forming hydrogen bonds with itself (intermolecular self-association)?

Accepted Answer

CH₃NH₂

Answer

Hydrogen bonding requires a hydrogen atom covalently bonded directly to N, O, or F. In methylamine (CH₃NH₂), hydrogen atoms are bonded directly to nitrogen, allowing intermolecular H-bond formation. Dimethyl ether (CH₃OCH₃) has oxygen but no H on O, so it cannot act as a hydrogen bond donor. CH₃F and CH₄ have hydrogens only on carbon, which is not electronegative enough.

Question 3

Which of the following liquids would be expected to have the highest vapor pressure at 25°C?

Accepted Answer

Diethyl ether (C₂H₅OC₂H₅)

Answer

Vapor pressure is inversely related to the strength of intermolecular forces. Diethyl ether lacks O–H bonds and therefore cannot self-associate through hydrogen bonding; it relies only on weak dipole-dipole and London dispersion forces, giving it a boiling point of ~35°C and the highest vapor pressure at 25°C. Water, ethanol, and glycerol all form hydrogen bonds, making them much harder to vaporize.

Question 4

n-Pentane (C₅H₁₂) boils at 36°C while neopentane (also C₅H₁₂) boils at only 9°C. Which explanation best accounts for this difference?

Accepted Answer

n-Pentane's elongated chain provides greater surface area for London dispersion interactions.

Answer

Both isomers are nonpolar hydrocarbons with identical molar masses, ruling out polarity and molar mass as explanations, and neither has N–H, O–H, or F–H bonds for hydrogen bonding. The key difference is molecular shape: n-pentane's linear chain maximizes surface contact between neighboring molecules, strengthening London dispersion forces. Neopentane's compact, nearly spherical shape minimizes surface area and intermolecular contact, lowering its boiling point.

Question 5

Which of the following correctly ranks the three major types of intermolecular forces from weakest to strongest?

Accepted Answer

London dispersion < dipole-dipole < hydrogen bonding

Answer

London dispersion forces arise from instantaneous temporary dipoles and are present in all molecules, but are generally the weakest. Dipole-dipole forces are stronger, acting between molecules with permanent dipoles. Hydrogen bonding is a special, unusually strong subset of dipole-dipole interaction that occurs specifically when H is bonded to the highly electronegative atoms N, O, or F, giving it the greatest strength of the three types.

AP Practice Test

AP Practice Test

AP - Advanced Placement Intermolecular Forces and Properties 1