Intermolecular Forces — Practice · Grade 11 Science

Chapter: States of Matter & Gases

What this practice covers

By the end of Intermolecular Forces you should be able to:

10 questions, multiple choice and written answer, each with a worked explanation.

Vocabulary used: intermolecular forces (IMFs), London dispersion forces, dipole-dipole forces, hydrogen bonding, polarity, polarizability.

Problem 1

Which intermolecular force is present in ALL molecules?
  1. London dispersion forces
  2. dipole-dipole forces
  3. hydrogen bonding
  4. ionic bonding
Answer and explanation

London dispersion forces

London dispersion forces arise from temporary dipoles and act in every molecule, even nonpolar ones.

Problem 2

Hydrogen bonding can occur only when a hydrogen atom is bonded directly to…
  1. another hydrogen
  2. nitrogen, oxygen, or fluorine
  3. carbon
  4. any metal
Answer and explanation

nitrogen, oxygen, or fluorine

Hydrogen bonding requires H bonded to a highly electronegative N, O, or F atom.

Problem 3

Which of these substances can form hydrogen bonds between its molecules?
  1. Cl₂
  2. CH₄
  3. H₂O
  4. CO₂
Answer and explanation

H₂O

Water has H bonded to O, so it can hydrogen-bond; the others lack H attached to N, O, or F.

Problem 4

Two nonpolar molecules differ only in size. Compared with the smaller one, the larger molecule will have…
  1. weaker dispersion forces
  2. no intermolecular forces at all
  3. hydrogen bonding
  4. stronger dispersion forces and a higher boiling point
Answer and explanation

stronger dispersion forces and a higher boiling point

A larger, more polarizable electron cloud produces stronger London dispersion forces, which raises the boiling point.

Problem 5

Water (H₂O) boils at 100°C, but methane (CH₄), which has a similar molar mass, boils at −161°C. Explain the enormous difference.
Answer and explanation

Water molecules hydrogen-bond to one another (H attached to O is attracted to the O of neighbors), a strong intermolecular force, so a great deal of energy is needed to separate them into a gas. Methane is nonpolar with only weak London dispersion forces, so its molecules pull apart at a very low temperature.

Problem 6

The noble gases Ne, Kr, and Xe have only London dispersion forces. Rank them by increasing boiling point.
  1. Ne < Kr < Xe
  2. Xe < Kr < Ne
  3. Kr < Ne < Xe
  4. they all boil at the same temperature
Answer and explanation

Ne < Kr < Xe

Dispersion forces increase with size and number of electrons (Ne < Kr < Xe), so boiling point rises in the same order.

Problem 7

Why do nonpolar molecules still attract each other? Show the reasoning, not just the conclusion.
Hints

Molecular size and shape can change the result; do not use one factor as a universal ranking.

Answer and explanation

Temporary and induced charge fluctuations produce London dispersion forces.

Problem 8

A large nonpolar molecule boils above a small polar one. Does that disprove intermolecular-force reasoning?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Size and polarizability strengthen dispersion; compare the complete set of interactions.

Problem 9

Hydrogen bonding needs a donor and a suitable acceptor contextHydrogen bonding needs a donor and a suitable acceptor contextDonor O–H … acceptor O regionHydrogen bonding is an interaction with a specific structural context.OHHOHH
Original selected water-pair interaction. The dotted line is a qualitative hydrogen-bond connection, not an extra covalent bond or measured angle.
Interpret the supplied visual. Why does simply finding H atoms in CH₄ not establish the same hydrogen-bond donor pattern as water?
Answer and explanation

Methane’s hydrogens are bonded to carbon, not the strongly polar N–H, O–H or F–H donor pattern used in this introductory comparison.

In common introductory examples, hydrogen bonding involves a hydrogen covalently attached to N, O or F interacting with a suitable neighboring electron-rich region. Water can supply O–H donors and oxygen acceptor sites. A molecule containing any hydrogen atom is not automatically a hydrogen-bond donor, and some molecules accept without donating. Hydrogen bonding is an intermolecular interaction here, distinct from the O–H covalent bond inside water; geometry and chemical context affect its strength.

Methane’s hydrogens are bonded to carbon, not the strongly polar N–H, O–H or F–H donor pattern used in this introductory comparison.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A student says a nonpolar molecule has no attractions and boiling always breaks covalent bonds. Correct both claims and identify the scale of the relevant interaction.
Answer and explanation

Nonpolar molecules still experience dispersion attractions between molecules. Ordinary vaporization of a stable molecular substance mainly changes intermolecular arrangement/interactions, while breaking its internal covalent bonds is a chemical change requiring a different account.

Nonpolar molecules still experience dispersion attractions between molecules. Ordinary vaporization of a stable molecular substance mainly changes intermolecular arrangement/interactions, while breaking its internal covalent bonds is a chemical change requiring a different account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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