Intermolecular Forces: Types, Effects, and Examples

Intermolecular Forces (IMFs)

Definition and Importance

Intermolecular forces (IMFs) are the attractions and repulsions between atoms and molecules that determine the physical properties of substances, such as melting point, boiling point, solubility, and the existence of solids and liquids. IMFs are much weaker than chemical bonds but are crucial for understanding condensed phases of matter.

Comparison of Chemical Bonds and IMFs

• Chemical bonds (e.g., covalent bonds) are strong and hold atoms together within molecules.

• IMFs are weaker and act between molecules, influencing their physical behavior.

Types of Intermolecular Forces

Overview

The strength and type of IMFs depend on three main properties:

• Presence of charge (ion or not)

• Polarity (molecular shape, dipoles)

• Molar mass (size of molecule)

Type	Present In	Molecular Perspective	Strength
Dispersion*	All molecules and atoms		0.05–20 kJ/mol
Dipole–Dipole	Polar molecules		3–20 kJ/mol
Hydrogen Bonding	Molecules containing H bonded to F, O, or N		10–40 kJ/mol
Ion–Dipole	Mixtures of ionic compounds and polar compounds		30–100+ kJ/mol

Ion-Dipole Forces

Ion-dipole forces occur when ions from an ionic compound interact with the dipole of polar molecules. This is a key factor in the solubility of ionic compounds in water.

Dipole-Dipole Forces

Dipole-dipole forces exist between polar molecules with permanent dipoles. The positive end of one molecule is attracted to the negative end of another, increasing boiling and melting points compared to nonpolar molecules of similar size.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction, occurring when hydrogen is bonded to N, O, or F. It significantly affects properties such as boiling point, melting point, and the structure of water and biological molecules.

• Hydrogen bonds are not as strong as covalent bonds but are stronger than other IMFs.

• Substances with hydrogen bonding have higher boiling and melting points.

Dispersion Forces (London Forces)

Dispersion forces arise from temporary fluctuations in electron distribution, creating instantaneous dipoles. These forces are present in all molecules and atoms, but are the only IMFs in nonpolar molecules.

• Strength increases with molar mass and polarizability.

• Molecular shape affects the magnitude: straight chains have higher boiling points than branched chains due to greater surface contact.

Summary Table: Intermolecular Forces

Type	Interacting Particles	Polar Molecules?	Ions?	Hydrogen Bonding?
London Forces	All molecules/atoms	No	No	No
Dipole–Dipole	Polar molecules	Yes	No	No
Hydrogen Bonding	H bonded to N, O, F	Yes	No	Yes
Ion–Dipole	Ions and polar molecules	Yes	Yes	No
Ionic Bonding	Ions	No	Yes	No

Effects of IMFs on Physical Properties

Boiling and Melting Points

The strength of IMFs directly affects boiling and melting points. Stronger IMFs result in higher boiling and melting points for substances of similar molar mass.

Solubility and Miscibility

IMFs also determine solubility and miscibility. "Like dissolves like": polar substances dissolve in polar solvents, nonpolar substances in nonpolar solvents. Molecules with both hydrophilic and hydrophobic parts may have complex solubility behavior.

Induced Dipole Forces

Polar molecules can induce dipoles in nonpolar molecules, allowing them to mix. For example, water (polar) can induce a dipole in O2 (nonpolar), enabling oxygen to dissolve in water.

Key Takeaways

• IMFs are essential for understanding the physical properties of substances.

• There are several types of IMFs: dispersion, dipole-dipole, hydrogen bonding, and ion-dipole.

• IMFs influence boiling/melting points, solubility, and miscibility.

• Polar molecules can induce dipoles in nonpolar molecules, enabling mixing.