Intermolecular Forces — Drills · Grade 11 Science

Chapter: States of Matter & Gases

What these drills covers

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

6 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

Intermolecular forces are attractions that act…
  1. between protons in a nucleus
  2. only inside metals
  3. between separate molecules
  4. within a molecule's covalent bonds
Answer and explanation

between separate molecules

Intermolecular forces act between molecules; the bonds within a molecule are much stronger intramolecular forces.

Problem 2

Which molecule has ONLY London dispersion forces?
  1. HF
  2. H₂O
  3. NH₃
  4. CH₄
Answer and explanation

CH₄

CH₄ is nonpolar and cannot hydrogen-bond, so its only intermolecular force is London dispersion; the others can hydrogen-bond.

Problem 3

For molecules of similar size, which list orders intermolecular forces from weakest to strongest?
  1. London dispersion < dipole-dipole < hydrogen bonding
  2. hydrogen bonding < dipole-dipole < London dispersion
  3. dipole-dipole < London dispersion < hydrogen bonding
  4. they are all equal in strength
Answer and explanation

London dispersion < dipole-dipole < hydrogen bonding

For comparably sized molecules, London dispersion is weakest, then dipole-dipole, then hydrogen bonding.

Problem 4

Why does I₂ have a higher melting point than F₂, even though both are nonpolar?
  1. I₂ is larger with more electrons, giving stronger dispersion forces
  2. I₂ is polar while F₂ is not
  3. I₂ can form hydrogen bonds
  4. they actually melt at the same temperature
Answer and explanation

I₂ is larger with more electrons, giving stronger dispersion forces

I₂ has more electrons and a larger, more polarizable electron cloud, so its London dispersion forces are stronger, raising its melting point.

Problem 5

What two conditions must be met for hydrogen bonding to occur between molecules?
Answer and explanation

(1) A hydrogen atom must be bonded directly to a highly electronegative N, O, or F atom, and (2) that hydrogen must be attracted to a lone pair of electrons on an N, O, or F atom of a neighboring molecule.

Problem 6

None of HCl, HBr, or HI can hydrogen-bond. Rank them by increasing boiling point.
  1. they all boil at the same temperature
  2. HCl < HBr < HI
  3. HI < HBr < HCl
  4. HBr < HCl < HI
Answer and explanation

HCl < HBr < HI

Since none can hydrogen-bond, London dispersion forces dominate and increase with size, so boiling point rises HCl < HBr < HI.

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