AP Chemistry Unit 3: Intermolecular Forces and Properties practice questions

Question 1

What are London dispersion forces and what determines their strength?

Accepted Answer

Temporary, fluctuating dipoles create Coulombic attractions between all molecules. Strength increases with greater contact area and higher polarizability (more electrons, larger electron cloud).

Answer

Forces only present between nonpolar molecules; polar molecules do not experience them

Answer

Strength depends primarily on molecular mass, not electron count or contact area

Answer

Permanent dipoles are required; LDF cannot exist without a net molecular dipole

Question 2

Pentane (C₅H₁₂) has a higher boiling point than neopentane (C₅H₁₂) even though they're isomers. Why?

Accepted Answer

Pentane is linear with greater surface contact area, increasing London dispersion forces. Neopentane is compact/spherical, reducing contact area and therefore LDF strength.

Answer

Pentane has stronger hydrogen bonds because it has more hydrogen atoms

Answer

Neopentane is more polar due to its branched structure, lowering its boiling point

Answer

Boiling points of isomers are always identical since they share the same molecular formula

Question 3

What structural requirement must be met for hydrogen bonding to occur between molecules?

Accepted Answer

An H bonded to N, O, or F must be attracted to a lone pair on N, O, or F nearby. Only N, O, F qualify — not just any hydrogen.

Answer

Any molecule containing hydrogen atoms can participate in hydrogen bonding

Answer

Hydrogen bonding requires two identical molecules; it cannot occur between different species

Answer

Hydrogen bonding occurs when H is bonded to any electronegative atom, including Cl

Question 4

How do dipole-dipole forces differ from dipole-induced dipole forces in terms of which molecules are involved?

Accepted Answer

Dipole-dipole: between two polar molecules; dipole-induced dipole: polar plus nonpolar. The polar molecule distorts the nonpolar molecule's electron cloud.

Answer

Both interactions occur only between polar molecules of the same chemical identity

Answer

Dipole-induced dipole forces are repulsive; only dipole-dipole forces are attractive

Answer

Dipole-dipole forces are weaker than dipole-induced dipole forces for molecules of similar size

Question 5

NaCl dissolves readily in water. Which intermolecular force primarily drives the dissolution of Na⁺ and Cl⁻ ions?

Accepted Answer

Ion-dipole forces — water's δ⁻ oxygen pulls Na⁺ and δ⁺ hydrogen pulls Cl⁻. These are strong enough to overcome the ionic lattice energy.

Answer

Hydrogen bonding between water molecules pulls ions into solution

Answer

London dispersion forces between ions and water molecules drive dissolution

Answer

Ion-ion interactions between Na⁺ and water replace the original ionic bonds

Question 6

Why are ionic solids brittle but metallic solids malleable?

Accepted Answer

In ionic solids, shifting layers brings like charges together, causing repulsion and fracture. In metallic solids, the mobile sea of electrons adjusts to any rearrangement of metal cores without repulsion.

Answer

Ionic solids are brittle because they lack covalent bonds, while metals have strong covalent bonds that flex

Answer

Metallic solids are malleable because metal atoms are smaller than ions, allowing easier sliding

Answer

Both ionic and metallic solids are equally brittle; metals appear malleable only at high temperatures

Question 7

Diamond and graphite are both covalent network solids made of carbon. Why is diamond hard and rigid while graphite is soft?

Accepted Answer

Diamond has a 3D network of covalent bonds with fixed bond angles, making it rigid. Graphite has 2D layers connected by weak LDF — layers slide past each other easily.

Answer

Diamond is harder because it has more electrons available for bonding than graphite

Answer

Graphite is soft because it contains ionic bonds between layers that are easily broken

Answer

Both should be equally hard since both consist entirely of covalent C–C bonds

Question 8

Why do molecular solids like iodine (I₂) have much lower melting points than ionic solids like NaCl?

Accepted Answer

Molecular solids are held together by relatively weak IMFs (LDF for I₂). Ionic solids are held together by strong electrostatic attractions between oppositely charged ions, requiring far more energy to disrupt.

Answer

Molecular solids melt lower because their molecules are smaller in size than ions

Answer

Ionic solids have higher melting points because they contain more atoms per formula unit

Answer

I₂ has a lower melting point because it is a gas at room temperature, not a true solid

AP Chemistry — Unit 3: Intermolecular Forces and Properties

What AP Chemistry Unit 3 covers

AP Chemistry Unit 3 practice questions

Key terms in Unit 3

Drill all of Unit 3 with spaced repetition