Intermolecular Forces: Liquids, Solids, and Their Properties (Ch. 11.3)

Intermolecular Forces (IMF) and States of Matter

Overview of Intermolecular Forces

Intermolecular forces (IMF) are the attractive forces that exist between molecules, influencing the physical properties of solids, liquids, and gases. The balance between the kinetic energy of particles and the strength of IMFs determines the physical state of a substance.

• Kinetic energy moves particles apart, favoring the gaseous state.

• Intermolecular forces hold particles together, favoring the liquid or solid state.

• Boiling point is an indicator of IMF strength: higher boiling points correspond to stronger IMFs.

Types of Intermolecular Forces

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

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonding

• Ion-dipole forces

IMFs are much weaker than covalent bonds, involving smaller charges acting over longer distances.

London Dispersion Forces

Definition and Origin

London dispersion forces are present in all molecules and atoms, regardless of polarity. They arise from instantaneous fluctuations in electron distribution, creating temporary dipoles that induce dipoles in neighboring particles.

• Polarizability: The ease with which an electron cloud can be distorted. Increases with the number of electrons and molar mass.

• Molecular shape: Molecules with greater surface area have stronger dispersion forces.

Table: Boiling Points and Number of Electrons in Noble Gases

Noble Gas	Number of Electrons	Boiling Point (K)
He	2	4.2
Ne	10	27
Ar	18	87
Kr	36	120
Xe	54	165

Additional info: Higher molar mass and electron count lead to stronger London dispersion forces and higher boiling points.

Example: Ranking London Dispersion Forces

• Br2 > Cl2 > F2 (Br2 has the highest molar mass and strongest LDF)

• C5H12 > C3H8 > CH4 (More electrons and larger surface area increase LDF strength)

Application

• Geckos can stick to surfaces due to London dispersion forces acting between their feet and the wall.

Dipole-Dipole Forces

Definition and Properties

Dipole-dipole forces occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.

• The strength of dipole-dipole interactions increases with the magnitude of the molecular dipole.

Table: Dipole-Dipole Strength and Boiling Points

Name	Formula	Structure	Boiling Point (°C)	Melting Point (°C)
Methanal (Formaldehyde)	CH2O	H2C=O	-19.3	-92
Ethene	C2H4	H2C=CH2	-104	-169.4

Additional info: Methanal has a stronger dipole and higher boiling point than ethene.

Example: Dipole Moment Strengths

• CH3I ≈ CHBr3 < CHCl3 < CHF3

• Trend in melting points can be explained by increasing dipole moment and molecular mass.

Hydrogen Bonding

Definition and Characteristics

Hydrogen bonding is a special, strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms (N, O, or F). The hydrogen atom can interact with N, O, or F on another molecule, forming a hydrogen bond.

• Hydrogen bond donor: N, O, or F covalently bonded to H.

• Hydrogen bond acceptor: Electronegative atom not covalently bonded to H.

• Hydrogen bonds are much stronger than regular dipole-dipole forces.

Table: Hydrogen Bonding and Physical Properties

Name	Formula	Structure	Boiling Point (°C)	Melting Point (°C)
Ethanol	C2H5OH	CH3CH2OH	78.3	-114.1
Dimethyl ether	CH3OCH3	CH3OCH3	-22.0	-138.5

Additional info: Ethanol forms hydrogen bonds, resulting in a much higher boiling point than dimethyl ether.

Biological Importance

• Hydrogen bonding is crucial in protein structure (e.g., alpha helices) and DNA base-pair binding.

Examples

• Hydrogen bonding is important in hydrazine (H2NNH2) and methyl fluoride (CH3F), but not in methane (CH4) or hydrogen sulfide (H2S).

• In HF and H2O, HF is the donor and H2O is the acceptor.

Ion-Dipole Forces

Definition and Application

Ion-dipole forces exist between an ion and a polar molecule. These interactions are responsible for the dissolution of ionic compounds (salts) in polar solvents such as water.

• The strength of ion-dipole interactions enables ionic compounds to dissolve in water.

Comparing Intermolecular Forces

Relative Strengths

When analyzing a sample, it is important to determine the types and relative strengths of the intermolecular forces present. The general order of strength is:

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

Decision Flow for Dominant IMF

• If one molecule is much larger than another, dispersion forces will dominate.

• If two molecules are of comparable size and shape, the strongest IMF present will dictate the properties.

Summary Table: Types of Intermolecular Forces

Type	Occurs Between	Relative Strength	Example
London Dispersion	All molecules/atoms	Weakest	He, CH4
Dipole-Dipole	Polar molecules	Intermediate	CH2O
Hydrogen Bond	H bonded to N, O, or F	Strong	H2O, NH3
Ion-Dipole	Ion and polar molecule	Strongest	Na+ in H2O

Key Equations

• There are no specific equations for IMF strength, but boiling point ($T_b$) is often used as an indicator:

$\text{Stronger IMF} \rightarrow \text{Higher } T_b$

Additional info: Understanding IMFs is essential for predicting solubility, boiling/melting points, and biological function.