S2.2.8 Intermolecular forces

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

Imagine a world where water couldn’t form droplets, where oceans didn’t exist, or where liquids couldn’t evaporate into clouds.

These everyday phenomena are made possible by intermolecular forces—the invisible attractions between molecules.

London Dispersion Forces: The Universal Attraction

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

Definition

London dispersion forces

London dispersion forces are weak intermolecular forces caused by temporary dipoles that arise due to momentary fluctuations in electron distribution within atoms or molecules.

What Causes London Dispersion Forces?

Electrons within atoms or molecules are always in motion. 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 $δ^{-}$ .

This temporary dipole can induce a similar dipole in a neighboring molecule, leading to a weak electrostatic attraction between the two.

Analogy

Think of London dispersion forces as a fleeting handshake between molecules. Even if they aren’t "friends" (polar), they can still briefly connect due to these temporary charges.

Formation of London dispersion forces: An instantaneous dipole in one particle induces a dipole in a neighboring particle, creating a weak attractive interaction.

Factors Affecting London Dispersion Forces

Number of Electrons: Molecules with more electrons have greater polarizability, meaning their electron clouds are more easily distorted, resulting in stronger LDFs. For instance, iodine $I_{2}$ has stronger dispersion forces than fluorine $f_{2}$ , leading to a higher boiling point.
Molecular Shape: 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.

Example

Noble gases exhibit increasing boiling points as you move down Group 18 in the periodic table.

Helium ( $- {268.9}^{\circ}$ C) has weaker LDFs than xenon ( $- {108.1}^{\circ}$ C) because xenon has more electrons, making its electron cloud more polarizable.

Tip

LDFs are the only intermolecular forces present in nonpolar molecules like oxygen $O_{2}$ and noble gases.

Dipole-Dipole Forces: Permanent Polarity

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

Definition

Dipole-dipole forces

Dipole-dipole forces are intermolecular attractions between the oppositely charged ends of permanent dipoles in polar molecules.

How Are Dipole-Dipole Forces Different from LDFs?

Unlike the temporary dipoles in LDFs, dipole-dipole forces arise in polar molecules with permanent dipoles. The positive end of one molecule is attracted to the negative end of another.

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.

Example

Dipole-dipole interactions explain why HCl ( $- {85.05}^{\circ}$ C) has a higher boiling point than the nonpolar molecule $F_{2}$ ( $- {188.1}^{\circ}$ C), despite their similar molecular masses.

Chlorine’s higher electronegativity creates a polar H–Cl bond, resulting in dipole-dipole attractions.

Common Mistake

Common Mistake: Students often confuse dipole-dipole forces with hydrogen bonding. Remember, hydrogen bonding is a special type of dipole-dipole interaction that occurs only when hydrogen is bonded to nitrogen, oxygen, or fluorine.

Dipole-dipole forces between the molecules oh hydrochloric acid.

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

Definition

Dipole-induced dipole forces

Dipole-induced dipole forces occur when a polar molecule with a permanent dipole induces a temporary dipole in a nearby nonpolar molecule.

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?

Dipole-Dipole Forces: These arise between two polar molecules with permanent dipoles.
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?

Polarizability of the Nonpolar Molecule: Larger, more loosely held electron clouds are more easily distorted, increasing the strength of the interaction.
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.

Definition

Hydrogen bonding

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (N, O, or F), creating a highly polar bond.

The partially positive hydrogen atom $δ^{+}$ is then attracted to a lone pair of electrons on another electronegative atom in a nearby molecule.

Why Is Hydrogen Bonding So Strong?

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.
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.

Example

Hydrogen Bonding

Water $H_{2} O$ : Each water molecule can form up to four hydrogen bonds, creating an extensive network that explains water’s high boiling point (100°C).
Ammonia $N H_{3}$ : Hydrogen bonds form between the hydrogen atoms of one ammonia molecule and the lone pair on the nitrogen atom of another.
Hydrogen Fluoride $H F$ : Fluorine’s high electronegativity creates exceptionally strong hydrogen bonds, giving HF a higher boiling point than expected for its size.

Note

Hydrogen bonds are weaker than covalent bonds but stronger than other intermolecular forces, with bond enthalpies ranging from 20–40 $k J m o l^{- 1}$ .

Reflection

Self review

What type of intermolecular force is present in all molecules, regardless of polarity?
Why is hydrogen bonding stronger than dipole-dipole forces?
Explain why water has a higher boiling point than methane, even though methane is a larger molecule.

Theory of Knowledge

How does the ability of water to form hydrogen bonds influence its role as the "universal solvent"?
Can you think of other examples where intermolecular forces affect substance behavior in the natural or industrial world?

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