AP Intermolecular Forces and Properties 2

Question 1

Which of the following pairs of substances would be expected to be miscible (mix completely) in each other?

Accepted Answer

Ethanol (C₂H₅OH) and water (H₂O)

Answer

Ethanol and water are both polar and can form hydrogen bonds with each other ('like dissolves like'). Hexane, octane, and CCl₄ are nonpolar and immiscible with polar water.

Question 2

Arrange the following substances in order of increasing boiling point: CH₄, NH₃, Ne, HF.

Accepted Answer

Ne < CH₄ < NH₃ < HF

Answer

Ne (bp −246°C): only tiny LDF. CH₄ (bp −161°C): small LDF, no dipole. NH₃ (bp −33°C): strong H-bonding and dipole-dipole. HF (bp +19.5°C): very strong H-bonding (F is most electronegative). Increasing bp: Ne < CH₄ < NH₃ < HF.

Question 3

Why does ice float on liquid water, which is unusual for solids compared to their liquid phases?

Accepted Answer

Ice has a less dense crystal structure due to directional H-bonding that holds molecules in an open hexagonal lattice, making solid water less dense than liquid water

Answer

In ice, each water molecule forms 4 directional hydrogen bonds in a hexagonal lattice with large open spaces. This arrangement (density ~0.917 g/mL) is less dense than liquid water (~1.00 g/mL) where H-bond networks are more disordered and compact.

Question 4

Which of the following has the highest viscosity at room temperature?

Accepted Answer

Glycerol (HOCH₂CHOHCH₂OH)

Answer

Glycerol has three –OH groups that form extensive hydrogen bonds both within and between molecules, creating a highly viscous liquid. The other compounds have fewer H-bonding sites or only weak LDF, resulting in lower viscosity.

Question 5

What is the primary intermolecular force responsible for DNA's double helix structure holding together the two strands?

Accepted Answer

Hydrogen bonding between complementary base pairs (A-T and G-C)

Answer

The two DNA strands are held together by hydrogen bonds between complementary base pairs: adenine (A) forms 2 H-bonds with thymine (T), and guanine (G) forms 3 H-bonds with cytosine (C). These H-bonds allow separation during replication and transcription.

AP Practice Test

AP Practice Test

AP Intermolecular Forces and Properties 2