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

Liquids, Solids, and Intermolecular Forces

Introduction

This chapter explores the properties of liquids and solids, focusing on the intermolecular forces that govern their behavior. Understanding these forces is essential for explaining phenomena such as boiling, melting, solubility, and the unique properties of substances like water.

Climbing Geckos: Biological Application of Intermolecular Forces

• Geckos can adhere to almost any surface due to intermolecular attractive forces.

• Millions of tiny hairs (setae) on their feet branch out and flatten (spatulae), allowing close contact with surfaces.

• This close contact permits strong intermolecular forces, enabling geckos to climb walls and ceilings.

• Example: Gecko adhesion is a real-world application of van der Waals forces.

Properties of the Three Phases of Matter

States of Matter

• Gas: Low density, indefinite shape and volume, weak intermolecular forces.

• Liquid: High density, indefinite shape, definite volume, moderate intermolecular forces.

• Solid: High density, definite shape and volume, strong intermolecular forces.

State	Density	Shape	Volume	Strength of Intermolecular Forces
Gas	Low	Indefinite	Indefinite	Weak
Liquid	High	Indefinite	Definite	Moderate
Solid	High	Definite	Definite	Strong

• Definite means the phase keeps its shape in a container.

• Indefinite means the phase takes the shape of its container.

Three Phases of Water

• Density and molar volume of ice and liquid water are much closer to each other than to steam.

• Density of ice is smaller than liquid water, which is unusual and vital for life (ice floats).

Phase	Temperature (°C)	Density (g/mL)	Molar Volume (mL)
Gas (steam)	100	0.0006	30.5
Liquid (water)	20	0.998	18.0
Solid (ice)	0	0.917	19.6

Degrees of Freedom

• Translational freedom: Ability to move from one position to another.

• Rotational freedom: Ability to reorient direction in space.

• Vibrational freedom: Ability to oscillate about a point in space.

Kinetic-Molecular Theory

• The state of a material depends on:

  1. The amount of kinetic energy particles possess.

  2. The strength of attraction between particles.

• These factors compete to determine phase.

States and Degrees of Freedom

• Gas: Complete freedom of motion; kinetic energy overcomes attractive forces.

• Solid: Locked in place; only vibrational motion; attractive forces dominate.

• Liquid: Limited freedom; some kinetic energy overcomes attractive forces, but not enough to escape.

Kinetic Energy

• Increasing kinetic energy increases particle motion.

• More motion energy means more freedom.

• Average kinetic energy is proportional to temperature: $KE_{avg} = 1.5kT$

Attractive Forces

• Particles are attracted by electrostatic forces.

• Strength varies by particle type.

• Stronger attractive forces resist motion.

Kinetic-Molecular Theory of the Phases

• Gases: No attractions; kinetic energy overcomes all forces.

• Solids: No translational/rotational motion; strong attractive forces.

• Liquids: Limited translational/rotational freedom; moderate attractive forces.

Phase Changes

• Changing state requires changing kinetic energy or limiting freedom.

• Melting: Particles gain enough energy to partially overcome attractions.

• Boiling: Particles gain enough energy to completely overcome attractions.

• Condensation: Achieved by decreasing temperature or increasing pressure.

Intermolecular Attractions

• Moderate to strong attractions result in solids or liquids at room temperature.

• Stronger attractions lead to higher boiling and melting points.

Why Are Molecules Attracted to Each Other?

• Attractions are due to opposite charges (ion-ion, dipole-dipole, hydrogen bonding).

• Even nonpolar molecules can have temporary charges.

• Larger charge = stronger attraction

• Longer distance = weaker attraction

Trends in Strength of Intermolecular Attraction

• Stronger attractions require more energy to separate molecules.

• Boiling requires overcoming intermolecular (not covalent) bonds.

• Higher boiling point = stronger intermolecular forces

Kinds of Attractive Forces

• Dispersion forces: Temporary polarity due to electron distribution fluctuations.

• Dipole-dipole attractions: Permanent polarity due to molecular structure.

• Hydrogen bonds: Strong dipole-dipole attraction when H is bonded to O, N, or F.

Dispersion Forces (London Forces)

• Result from temporary dipoles in atoms/molecules.

• Present in all molecules and atoms.

• Weakest type of intermolecular force.

Size of the Induced Dipole

• Magnitude depends on electron polarizability and molecular shape.

• Larger molar mass = more electrons = stronger dispersion forces.

• More surface contact = stronger attraction.

Effect of Molecular Size and Shape on Dispersion Forces

Noble Gas	Molar Mass (g/mol)	Boiling Point (K)
He	4.00	4.2
Ne	20.18	27
Ar	39.95	87
Kr	83.80	120
Xe	131.30	165

• Boiling point increases with molar mass due to stronger dispersion forces.

• Linear molecules (e.g., n-pentane) have more surface area and stronger dispersion forces than spherical molecules (e.g., neopentane).

Dipole-Dipole Attractions

• Polar molecules have permanent dipoles, increasing boiling and melting points.

• Stronger than dispersion forces.

Name	Formula	Molar Mass (g/mol)	Boiling Point (°C)	Melting Point (°C)
Formaldehyde	CH2O	30.03	-19.5	-92
Ethane	C2H6	30.07	-88	-172

• Formaldehyde (polar) has much higher boiling and melting points than ethane (nonpolar).

Hydrogen Bonding

• Occurs when H is bonded to highly electronegative atoms (O, N, F).

• Hydrogen bonds are much stronger than other intermolecular forces, but weaker than covalent bonds.

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

• Example: Water and ethanol both exhibit hydrogen bonding, leading to high boiling points.

Attractive Forces and Solubility

• "Like dissolves like": Polar substances dissolve in polar solvents; nonpolar in nonpolar.

• Hydrophilic groups: OH, CHO, C=O, COOH, NH2, Cl

• Hydrophobic groups: C–H, C–C

• Solubility is a competition between polar and nonpolar group attractions.

Immiscible Liquids

• Pentane (nonpolar) and water (polar) do not mix due to stronger water-water attractions.

Summary Table: Types of Intermolecular Forces

Type	Strength	Present In
Dispersion	Weakest	All molecules/atoms
Dipole-Dipole	Intermediate	Polar molecules
Hydrogen Bond	Strongest (pure substances)	H bonded to O, N, F

Additional info: These notes cover the first half of Chapter 12, focusing on the nature and consequences of intermolecular forces in liquids and solids. Further topics such as phase changes, energetics, and phase diagrams are typically included in the full chapter.