Chapter 6: Intermolecular Forces

Chapter Overview

Intermolecular forces (IMFs) are the forces of attraction or repulsion between neighboring molecules. These forces play a crucial role in determining the physical properties of substances, such as boiling point, melting point, viscosity, and solubility. Understanding IMFs allows us to predict and explain the behavior of matter in different states and conditions.

• Relate intermolecular forces to states of matter: IMFs influence whether a substance is solid, liquid, or gas at a given temperature and pressure.

• Explain the influence of shape on intermolecular forces: Molecular geometry affects the strength and type of IMFs present.

• Rank strength of intermolecular forces: Different IMFs vary in strength, affecting physical properties.

• Rationalize solubility of different substances: Solubility depends on the compatibility of IMFs between solute and solvent.

• Draw and label a phase diagram: Phase diagrams visually represent the state of matter under varying conditions.

Section 6.2: London Dispersion Forces

Introduction to London Dispersion Forces

London dispersion forces (LDFs) are the weakest type of intermolecular force and exist between all molecules, including non-polar substances such as hydrocarbons. They arise due to temporary fluctuations in electron distribution, which create instantaneous dipoles that induce dipoles in neighboring molecules.

• Definition: London dispersion forces are weak, temporary attractive forces resulting from momentary uneven electron distribution in atoms or molecules.

• Occurrence: Present in all molecules, but most significant in non-polar molecules.

• Mechanism: Instantaneous dipoles induce dipoles in adjacent molecules, leading to attraction.

• Example: Hydrocarbons (e.g., methane, ethane) exhibit only LDFs.

Factors Affecting London Dispersion Forces

The strength of LDFs depends on the ease with which the electron cloud can be distorted (polarizability) and the size of the electron cloud.

• Polarizability: Larger atoms/molecules have more easily distorted electron clouds, leading to stronger LDFs.

• Molecular Size: As molar mass increases, LDF strength increases.

• Shape: Linear molecules have greater surface area for interactions, increasing LDF strength compared to branched molecules.

Comparison of Noble Gases

The table below shows how molar mass and boiling point increase with atomic size, reflecting stronger LDFs.

Additional info: Boiling points inferred for completeness.

Effect of Chain Length and Shape

Boiling point increases with increasing chain length due to greater surface area and stronger LDFs. Shape also matters: straight-chain molecules have higher boiling points than branched isomers.

• Example: Pentane (linear) vs. 2-methylbutane and 2,2-dimethylpropane (branched) — linear pentane has the highest boiling point.

Viscosity and London Dispersion Forces

Viscosity is a measure of a fluid's resistance to flow, often reported in centipoise (cP). Higher LDFs lead to higher viscosity because molecules are more strongly attracted to each other.

• Formula:

Additional info: Viscosity values inferred for completeness.

Section Summary

• London dispersion forces exist between all molecules.

• Strength depends on size, shape, and polarizability.

• Physical properties such as boiling point and viscosity can be predicted by LDF strength.

Section 6.3: Interactions Involving Polar Molecules

Dipole-Dipole Interactions

Polar molecules possess permanent dipoles due to uneven electron sharing. Dipole-dipole interactions occur when the positive end of one molecule is attracted to the negative end of another.

• Definition: Dipole-dipole interactions are attractive forces between polar molecules with permanent dipoles.

• Example: Acetone (CH3COCH3) molecules align so that opposite charges attract.

Hydrogen Bonds

Hydrogen bonds are a special, strong type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms (N, O, or F). These bonds are crucial in biological molecules, such as DNA.

• Definition: Hydrogen bonds are strong dipole-dipole attractions between H and N, O, or F.

• Strength: Typically 4–50 kJ/mol, making them the strongest IMF among neutral molecules.

• Example: Hydrogen bonds hold together the base pairs in DNA.

Ion-Dipole Interactions

Ion-dipole interactions occur between ionic compounds and polar solvents, such as when salt dissolves in water. The positive and negative ions are stabilized by the dipoles of the solvent molecules.

• Definition: Ion-dipole interactions are attractions between ions and polar molecules.

• Example: Na+ and Cl- ions are surrounded by water molecules during dissolution.

Section Summary

• Polar molecules experience dipole-dipole, hydrogen bonding, and ion-dipole interactions.

• These IMFs are stronger than London dispersion forces and significantly affect physical and chemical properties.