AP Chemistry Unit 3 — Intermolecular Forces.
This unit covers London dispersion, dipole-dipole, hydrogen bonding and phase diagrams — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers London dispersion, dipole-dipole, hydrogen bonding and phase diagrams — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 London Dispersion Forces
London dispersion forces (LDFs) are temporary, induced dipole interactions that exist in ALL molecules, both polar and nonpolar. Strength increases with molecular size (more electrons = greater polarizability) and surface area. On the AP exam, LDFs explain why larger nonpolar molecules have higher boiling points.
Key Points
- Present in every molecule; the ONLY IMF in nonpolar, symmetric molecules (e.g., Br₂, CH₄, noble gases)
- Strength increases with increasing molar mass / number of electrons
- Branched molecules have weaker LDFs than straight-chain isomers (less surface area contact)
- LDFs account for boiling point trends among nonpolar molecules (e.g., F₂ < Cl₂ < Br₂ < I₂)
Explain why pentane (C₅H₁₂, MW = 72 g/mol) has a higher boiling point (36°C) than neopentane (also C₅H₁₂, MW = 72 g/mol, bp = 9.5°C).
Both isomers have identical molar masses and are nonpolar, so LDFs are the only relevant IMF. Pentane is a straight chain with greater surface area, allowing more extensive LDF contact between molecules. Neopentane's compact, spherical shape reduces surface area and therefore intermolecular contact, resulting in weaker LDFs and a lower boiling point.
2 Dipole-Dipole Forces
Dipole-dipole forces occur between polar molecules — molecules with a permanent net dipole moment due to polar bonds AND asymmetric geometry. The partially positive end of one molecule attracts the partially negative end of a neighboring molecule. On the AP exam, you must be able to identify polarity using both electronegativity differences AND molecular geometry.
Key Points
- Require a permanent dipole; molecule must be both polar-bonded AND geometrically asymmetric
- Stronger than LDFs of comparable size but weaker than hydrogen bonds
- A molecule can be polar-bonded yet nonpolar overall if geometry causes dipoles to cancel (e.g., CO₂, CCl₄, BF₃)
- Higher polarity (larger dipole moment) → stronger dipole-dipole forces → higher boiling/melting point
Which has a higher boiling point: SO₂ or CO₂? Both have similar molar masses (~44–64 g/mol).
CO₂ is linear, so its two C=O bond dipoles point in opposite directions and cancel, making the molecule nonpolar — only LDFs act between CO₂ molecules. SO₂ is bent (lone pair on S), so the bond dipoles do not cancel, giving SO₂ a net dipole and dipole-dipole interactions in addition to LDFs. Therefore SO₂ (bp = −10°C) has a higher boiling point than CO₂ (sublimes at −78.5°C).
3 Hydrogen Bonding
Hydrogen bonding is a special, strong dipole-dipole interaction that occurs when H is covalently bonded directly to N, O, or F, and that H interacts with a lone pair on an N, O, or F of a neighboring molecule. It is the strongest IMF (excluding ionic/metallic), responsible for anomalously high boiling points and unique properties of water. The AP exam tests both identifying H-bonding capability and explaining its macroscopic consequences.
Key Points
- Strict requirement: H must be bonded to N, O, or F — not just present in the molecule
- Explains water's high bp, surface tension, capillary action, and density of ice < liquid water
- H-bonding raises boiling points far above what molar mass alone would predict (e.g., H₂O vs H₂S)
- Intramolecular H-bonds reduce intermolecular H-bonding, lowering boiling point relative to isomers that form intermolecular H-bonds
Rank the following in order of increasing boiling point: CH₄, NH₃, H₂O, HF. Justify your ranking.
CH₄ is nonpolar with only weak LDFs, giving it the lowest boiling point (−161°C). NH₃, HF, and H₂O all exhibit hydrogen bonding, so they are all significantly higher than CH₄. Among the three, the ranking reflects the number of H-bonds each molecule can form: NH₃ and HF can each donate/accept fewer H-bonds per molecule than H₂O, which has two O–H donors and two lone pairs as acceptors, giving it the highest boiling point (100°C) and making the order CH₄ < NH₃ < HF < H₂O.
4 Phase Diagrams
A phase diagram maps the stable phase of a substance as a function of temperature and pressure. Students must identify the triple point (all three phases coexist), critical point (beyond which liquid and gas are indistinguishable), and the slopes of the phase boundary lines. The AP exam frequently asks about phase transitions under changing conditions and the anomalous behavior of water (negative slope of solid-liquid boundary).
Key Points
- Triple point: unique T and P where solid, liquid, and gas coexist in equilibrium
- Critical point: above this T and P, the substance exists as a supercritical fluid; liquid and gas phases are indistinguishable
- Water's solid-liquid boundary has a negative slope (unlike most substances) because ice is less dense than liquid water — applying pressure melts ice
- At pressures below the triple point, the substance cannot exist as a liquid; heating the solid converts it directly to gas (sublimation)
Using a phase diagram for CO₂ (triple point: −56.6°C, 5.11 atm), explain why dry ice sublimes at 1 atm rather than melting.
At 1 atm atmospheric pressure, the pressure is far below CO₂'s triple point pressure of 5.11 atm. Because liquid CO₂ can only exist above 5.11 atm, moving along the 1 atm isobar (horizontal line on the phase diagram) takes the substance directly from solid to gas without ever entering the liquid phase region. This is why dry ice sublimes — transitioning solid → gas — at standard atmospheric conditions, with no liquid intermediate.
Questions, answered.
What is Intermolecular Forces?
Intermolecular Forces is Unit 3 of AP Chemistry, covering London dispersion, dipole-dipole, hydrogen bonding and phase diagrams.
How to study for AP Chemistry Unit 3?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.