Intermolecular forces flashcards

Intermolecular forces (IMFs) are the attractive forces between separate molecules or between molecules and ions. Students must understand the three main types: London dispersion forces, dipole-dipole forces, and hydrogen bonding, as well as ion-dipole forces for solutions. The strength of IMFs determines physical properties such as boiling point, melting point, vapor pressure, viscosity, and surface tension. London dispersion forces exist in all substances and increase with molar mass and surface area. Dipole-dipole forces act between polar molecules. Hydrogen bonding, the strongest IMF, requires a hydrogen atom bonded directly to N, O, or F. Students must be able to compare IMF strengths across different substances and explain how those differences account for observed physical properties.

1. Card 1

   What are intermolecular forces?

   Intermolecular forces are attractive forces that act between separate molecules, distinct from the covalent or ionic bonds within a substance.

2. Card 2

   What are the three main types of intermolecular forces?

   The three main types are London dispersion forces, dipole-dipole forces, and hydrogen bonding, listed from weakest to strongest.

3. Card 3

   What causes London dispersion forces?

   London dispersion forces arise from temporary, instantaneous shifts in electron distribution that create momentary dipoles, inducing dipoles in neighboring molecules.

4. Card 4

   How does molar mass affect London dispersion forces?

   Larger molar mass means more electrons and greater polarizability, leading to stronger London dispersion forces and higher boiling points.

5. Card 5

   What conditions are required for hydrogen bonding to occur?

   Hydrogen bonding requires a hydrogen atom covalently bonded directly to N, O, or F, and a nearby electronegative N, O, or F atom on another molecule.

6. Card 6

   Why does water have an unusually high boiling point for its molar mass?

   Water forms an extensive hydrogen bonding network, requiring significantly more energy to separate molecules than its low molar mass would otherwise suggest.

7. Card 7

   What is a dipole-dipole force?

   A dipole-dipole force is the attraction between the positive end of one polar molecule and the negative end of a neighboring polar molecule.

8. Card 8

   Do London dispersion forces exist in polar molecules?

   Yes. London dispersion forces are present in all molecules, polar or nonpolar, because all molecules have electrons that can shift temporarily.

9. Card 9

   How do IMF strengths relate to boiling point?

   Stronger intermolecular forces require more energy to overcome, resulting in higher boiling points and lower vapor pressures.

10. Card 10

    What is an ion-dipole force and when is it important?

    An ion-dipole force is the attraction between an ion and a polar molecule; it is the key force responsible for dissolving ionic compounds in polar solvents like water.

11. Card 11

    How does molecular shape affect London dispersion forces?

    More elongated, linear shapes increase surface area contact between molecules, strengthening London dispersion forces compared to compact, branched shapes of the same formula.

12. Card 12

    What physical property is most directly used to compare IMF strengths between similar molecules?

    Boiling point is the most direct comparison: the substance requiring more energy to vaporize has stronger intermolecular forces holding its molecules together.

• Many students think hydrogen bonding can occur whenever hydrogen is present in a molecule, but it specifically requires hydrogen to be covalently bonded directly to N, O, or F.
• Students often forget that London dispersion forces exist in polar molecules and assume that polar molecules only experience dipole-dipole forces, leading to errors when comparing boiling points.
• A common error is confusing intermolecular forces with intramolecular bonds, mistakenly stating that boiling breaks covalent bonds rather than simply overcoming the weaker attractions between molecules.

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