AP - Advanced Placement Intermolecular Forces and Properties 1 — Questions and Answers
Question 1: Which of the following noble gases has the highest boiling point?
- He
- Ne
- Ar
- Xe (Correct answer)
Correct answer: Xe
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)?
- CH₃OCH₃
- CH₃F
- CH₃NH₂ (Correct answer)
- CH₄
Correct answer: CH₃NH₂
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?
- Water (H₂O)
- Ethanol (C₂H₅OH)
- Diethyl ether (C₂H₅OC₂H₅) (Correct answer)
- Glycerol (C₃H₈O₃)
Correct answer: Diethyl ether (C₂H₅OC₂H₅)
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?
- n-Pentane has a larger molar mass than neopentane.
- n-Pentane is more polar than neopentane.
- n-Pentane's elongated chain provides greater surface area for London dispersion interactions. (Correct answer)
- Neopentane forms stronger hydrogen bonds than n-pentane.
Correct answer: n-Pentane's elongated chain provides greater surface area for London dispersion interactions.
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?
- London dispersion < dipole-dipole < hydrogen bonding (Correct answer)
- Dipole-dipole < London dispersion < hydrogen bonding
- Hydrogen bonding < dipole-dipole < London dispersion
- London dispersion < hydrogen bonding < dipole-dipole
Correct answer: London dispersion < dipole-dipole < hydrogen bonding
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.
Question 6: Among the Group 16 hydrides, H₂S, H₂Se, and H₂Te show a regular increase in boiling point with molar mass, yet water (H₂O) has an anomalously high boiling point far above the trend. What is the primary reason for this anomaly?
- Water has the greatest molar mass among the Group 16 hydrides.
- Water molecules form an extensive hydrogen-bond network because oxygen is highly electronegative and small enough to concentrate charge density. (Correct answer)
- Water experiences stronger London dispersion forces than H₂Te.
- Water is a linear molecule, allowing maximum molecular contact.
Correct answer: Water molecules form an extensive hydrogen-bond network because oxygen is highly electronegative and small enough to concentrate charge density.
Based on London dispersion forces alone, boiling point should increase down Group 16, making water's boiling point predictably the lowest. Instead it is 100°C because each water molecule can participate in up to four hydrogen bonds (two as donor through its O–H groups, two as acceptor through lone pairs). Oxygen's high electronegativity and small atomic radius make these bonds exceptionally strong. Note also that water is bent (V-shaped), not linear, and has a far lower molar mass than H₂Te.
AP - Advanced Placement Intermolecular Forces and Properties 1
Which of the following noble gases has the highest boiling point?