S2.2.8 Intermolecular forces Notes

Understanding Intermolecular Forces: London Dispersion Forces, Dipole-Dipole Forces, Dipole-Induced Forces and Hydrogen Bonding

London Dispersion Forces: The Universal Attraction

1. Did you know that even noble gases like helium and nonpolar molecules such as nitrogen gas $N_2$ experience forces of attraction?
2. These forces, called London dispersion forces (LDFs), arise from temporary instantaneous dipoles.

What Causes London Dispersion Forces?

1. Electrons within atoms or molecules are always in motion.
2. At any instant, this motion can create an uneven distribution of electron density, forming a temporary dipole, a region of partial positive charge $δ^+$ and partial negative charge $δ^-$.
3. This temporary dipole can induce a similar dipole in a neighboring molecule, leading to a weak electrostatic attraction between the two.

Factors Affecting London Dispersion Forces

1. Number of Electrons:
   1. Molecules with more electrons have greater polarizability, meaning their electron clouds are more easily distorted, resulting in stronger LDFs.
   2. For instance, iodine $I_2$ has stronger dispersion forces than fluorine $f_2$, leading to a higher boiling point.
2. Molecular Shape:
   1. Elongated molecules like pentane have larger surface areas for contact, resulting in stronger LDFs compared to compact molecules like neopentane, even if their molecular masses are similar.

Dipole-Dipole Forces: Permanent Polarity

1. Have you noticed that some molecules, like hydrogen chloride $HCl$, have stronger attractions than others?
2. This is due to dipole-dipole forces, which occur between molecules with permanent dipoles.

How Are Dipole-Dipole Forces Different from LDFs?

1. Unlike the temporary dipoles in LDFs, dipole-dipole forces arise in polar molecules with permanent dipoles.
2. The positive end of one molecule is attracted to the negative end of another.
3. The strength of these forces depends on the magnitude of the dipole moment, which is influenced by the difference in electronegativity between atoms and the molecule's geometry.

Dipole-Induced Dipole Forces: Temporary Polarity Induced by Permanent Dipoles

This happens as the electric field of the polar molecule distorts the electron cloud of the nonpolar molecule, creating a momentary charge imbalance.

How Are They Different from Dipole-Dipole Forces?

1. Dipole-Dipole Forces: These arise between two polar molecules with permanent dipoles.
2. Dipole-Induced Dipole Forces: These involve one polar molecule and one nonpolar molecule, with the polar molecule inducing temporary polarity in the nonpolar molecule.

What Factors Influence Dipole-Induced Dipole Forces?

1. Polarizability of the Nonpolar Molecule: Larger, more loosely held electron clouds are more easily distorted, increasing the strength of the interaction.
2. Dipole Moment of the Polar Molecule: Stronger permanent dipoles create a larger electric field, inducing a more significant dipole in the nonpolar molecule.

Hydrogen Bonding: The Strongest Intermolecular Force

Hydrogen bonding is an especially strong form of dipole-dipole interaction.

Why Is Hydrogen Bonding So Strong?

1. High Polarity: The large electronegativity difference between hydrogen and N, O, or F creates a significant partial positive charge on hydrogen and a partial negative charge on the electronegative atom.
2. Small Size of Hydrogen: Hydrogen’s small size allows it to approach the lone pair of the electronegative atom closely, increasing the strength of the interaction.