Intermolecular Forces and States of Matter

States of Matter

Phases of Matter

The physical state of a substance—gas, liquid, or solid—is determined by the arrangement and movement of its particles. Each state has distinct properties based on the freedom of movement and the forces between particles.

• Gas: Expands to fill any container; has no definite volume or shape. Particles are widely separated and move randomly.

• Liquid: Has a definite volume but no definite shape; takes the shape of its container. Particles are close together but can move past one another.

• Solid: Has both a fixed volume and a fixed shape. Particles are closely packed in an ordered arrangement and can only vibrate in place.

Example: Water vapor (gas), liquid water, and ice (solid) are all forms of H2O in different states of matter.

What Determines a Substance’s State of Matter

Forces and Kinetic Energy

The state of matter is determined by the competition between the force of attraction between particles and the kinetic energy (related to temperature) of those particles.

• As the force of attraction increases, particles are drawn closer together and their movement becomes more restricted.

• As kinetic energy increases, particles move more freely and are less restricted by attractive forces.

Example: Cooling a gas can cause it to condense into a liquid as particle motion slows and attractions dominate.

Inter- and Intramolecular Forces

Definitions and Effects

• Intermolecular forces are forces of attraction between molecules or between molecules and ions. They influence the physical properties of substances (e.g., boiling point, melting point, solubility).

• Intramolecular forces are forces of attraction within molecules (such as covalent bonds). They influence the chemical properties of substances (e.g., reactivity, bond strength).

Example: The covalent O–H bond in water is an intramolecular force; the attraction between two water molecules is an intermolecular force.

Types of Intermolecular Forces (van der Waals Forces)

Overview

There are four main types of intermolecular forces, collectively known as van der Waals forces:

• Ion-dipole forces

• Dipole-dipole forces

• Hydrogen bonds

• London dispersion forces

Ion-Dipole Forces

Ion-dipole forces are the strongest type of intermolecular force. They occur between an ion and a polar molecule.

• Example: NaCl dissolved in water. The positive end of the water dipole is attracted to Cl− ions, and the negative end to Na+ ions.

$\text{Ion-dipole force} \gg \text{other intermolecular forces}$

Dipole-Dipole Forces

Dipole-dipole forces occur between polar molecules, which have permanent dipole moments due to differences in electronegativity.

• Example: CH2Cl2 molecules attract each other via dipole-dipole forces.

• These forces are weaker than ion-dipole forces because they involve partial (not full) charges.

Sample Question: Pure samples of which of the following exhibit dipole-dipole forces: CO, CO2, or CCl4? Answer: CO (because it is polar; CO2 and CCl4 are nonpolar).

Hydrogen Bonds

Hydrogen bonds are a special, strong type of dipole-dipole force. They occur when hydrogen is bonded to one of the three most electronegative elements: nitrogen (N), oxygen (O), or fluorine (F), and is attracted to a lone pair on another N, O, or F atom.

• Example: Water (H2O) and ammonia (NH3) both exhibit hydrogen bonding.

• Hydrogen bonds are crucial in biological systems (e.g., DNA structure).

Sample Question: Pure samples of which of the following exhibit hydrogen bonds: H2CO, HF, or HCN? Answer: HF (hydrogen bonded to F), HCN (hydrogen bonded to N), but not H2CO (hydrogen not bonded to N, O, or F).

Hydrogen Bond Strength

• Hydrogen bonds are stronger than regular dipole-dipole forces but weaker than ion-dipole forces.

• The small size of H and the high electronegativity of N, O, and F create highly polar bonds and strong attractions.

London Dispersion Forces

London dispersion forces are the only intermolecular forces present in nonpolar molecules and noble gases. They arise from temporary, instantaneous dipoles induced in atoms or molecules.

• Example: CO2 and Br2 are nonpolar and interact via London dispersion forces.

• All atoms and molecules exhibit London dispersion forces, but they are the weakest type.

London Dispersion Force Strength

• The strength of London dispersion forces depends on polarizability—the ease with which a molecule’s electron cloud can be distorted.

• Polarizability increases with molar mass and number of electrons.

• Stronger London forces lead to higher boiling and melting points among nonpolar substances.

$\text{London dispersion force} \propto \text{polarizability} \propto \text{molar mass}$

Sample Question: Rank Br2, F2, and I2 according to increasing London dispersion force strength. Answer: F2 < Br2 < I2 (as molar mass increases, so does dispersion force strength).

Ranking of Intermolecular Forces

Relative Strengths and Physical Properties

The relative strengths of the intermolecular forces are as follows:

• Ion-dipole > Hydrogen bond > Dipole-dipole > London dispersion

Boiling point and melting point increase as the strength of intermolecular forces increases.

Formula	Intermolecular Force	Boiling Point (°C)
O2	London	-183
HCl	Dipole-dipole	-85
NH3	H-bond	-33

Sample Question: Rank the following compounds according to increasing boiling point: CO2, H2O, and SO2. Answer: CO2 < SO2 < H2O (London < dipole-dipole < hydrogen bond).

Summary

• Intermolecular forces are responsible for the physical properties of substances, such as boiling and melting points.

• There are four main types: ion-dipole, dipole-dipole, hydrogen bonds, and London dispersion forces.

• The strength of these forces determines the state of matter and the energy required for phase changes.

• Boiling point is a practical measure of intermolecular force strength.

Additional info: The notes also included sample multiple-choice questions to test understanding of intermolecular forces and their effects on physical properties.