Chapter 12: Solids, Liquids, and Intermolecular Forces – Study Notes

Solids, Liquids, and Intermolecular Forces

Introduction

This chapter explores the nature of solids and liquids, focusing on the role of intermolecular forces in determining physical properties and states of matter. Understanding these forces is essential for explaining phenomena such as boiling points, melting points, and the structure of biological molecules like DNA.

Physical Properties and States of Matter

• Physical Properties: Boiling point, melting point, viscosity, and surface tension are influenced by intermolecular forces.

• State of Matter: The physical state (solid, liquid, gas) at room temperature depends on the strength of intermolecular forces.

Compound	Normal Boiling Point, °C
H2O	100
CH3CH2OH	78.3
CH3OH	64.4

Element	State at 25 °C
Cl2	Gas
Br2	Liquid
I2	Solid

Difference Between Intermolecular and Intramolecular Forces

Intermolecular and intramolecular forces are fundamental concepts in chemistry, distinguishing the interactions between molecules from those within a molecule.

Intermolecular Forces	Intramolecular Forces
Occur between one molecule and another neighboring molecule. Affect the physical properties of substances.	Present within each individual molecule. The chemical bonds that join the atoms together. Influence the chemical properties of substances.

• Intermolecular forces involve smaller charges interacting at greater distances.

• Intramolecular forces involve larger charges (electrons and protons) interacting at very close distances.

• Intermolecular forces are weaker than intramolecular forces.

Example: Hydrogen Chloride (HCl)

• Strong intramolecular attraction: covalent bond between H and Cl.

• Weak intermolecular attraction: between neighboring HCl molecules.

Compound	Process	Energy Required (kJ/mol)	Forces Broken
H2O	vaporization	41	Intermolecular
H2O	break covalent bonds	930	Intramolecular
HCl	vaporization	16	Intermolecular
HCl	break covalent bonds	431	Intramolecular

Recall: During a phase change, such as vaporization, molecules remain intact.

Importance of Intermolecular Forces (IMFs)

The strength of IMFs determines many physical properties and the state of matter at room temperature.

• The stronger the IMFs, the higher the boiling and melting points.

• Moderate to strong IMFs result in solids or liquids at room temperature; weak IMFs result in gases.

Application: DNA Structure

• Hydrogen bonds between DNA bases are crucial for the double helix structure.

Types of Intermolecular Forces

There are several types of intermolecular forces, each with distinct characteristics and effects on molecular behavior.

• London Dispersion Forces (Van der Waals Forces)

• Dipole-Dipole Forces

• Hydrogen Bonding (specific atoms involved)

• Ion-Dipole Forces (ion and polar solvent, especially in aqueous solutions)

London Dispersion Forces (LDFs)

London dispersion forces are present in all molecules and atoms due to fluctuations in electron distribution.

• In nonpolar molecules, electron distribution is usually uniform and symmetrical.

• Momentary asymmetry leads to instantaneous dipoles, which induce dipoles in neighboring molecules.

• LDFs are the attractive forces between these instantaneous and induced dipoles.

Example: Helium Atom

• Temporary charge separation creates an instantaneous dipole on one atom, inducing a dipole on a neighboring atom.

• The positive end of one atom is attracted to the negative end of another.

Factors Affecting the Strength of London Dispersion Forces

1. Polarizability: The ease with which the electron cloud can be distorted to produce an instantaneous dipole.

   • Polarizability increases with the number of electrons.

   • Larger electron clouds are more easily polarized, leading to stronger LDFs.

   • Small molecules (e.g., F2) have electrons closer to the nuclei and are less polarizable; large molecules (e.g., Br2) are more polarizable.

2. Molar Mass: The magnitude of LDFs increases with increasing molar mass.

   • More electrons dispersed over a larger volume make the electron cloud easier to distort.

   • Higher molar mass leads to stronger LDFs and higher boiling points.

3. Molecular Shape: For nonpolar compounds with the same molar mass, shape influences LDF strength.

   • Straight-chain isomers (e.g., n-pentane) have more surface contact and stronger LDFs than branched isomers (e.g., neopentane).

Examples and Data

Element	State	BP, °C (1 atm)
F2	gas	-219.6
Br2	liquid	58.8

Noble Gas	Molar Mass (g/mol)	Boiling Point (°C)
Ne	20.18	-246
Ar	39.95	-186
Kr	83.80	-152
Xe	131.30	-108

Summary of London Dispersion Forces

• Operate between all molecules and atoms due to electron movement.

• Strength increases with polarizability, molar mass, and molecular shape.

• Not caused by electronegativity differences.

• Weak in small molecules (e.g., H2, He), but significant in large atoms/molecules (e.g., Xe, I2, CCl4).

Ranking London Dispersion Forces

Molecule	Boiling Point (K)
CH4	90.7
CF4	123
CCl4	250
CBr4	363

Conclusion: The more electrons a molecule has, the stronger its London dispersion forces.

Additional info: These notes cover only the first part of Chapter 12, focusing on London dispersion forces. Other types of intermolecular forces (dipole-dipole, hydrogen bonding, ion-dipole) and their effects on physical properties are typically covered in subsequent sections.