3.4 Intermolecular Forces

Intermolecular Forces

1. Understanding Molecular Changes during Physical State Transitions

• Boiling Water: When water boils, bubbles form and vapor rises, indicating that molecules are gaining kinetic energy from thermal sources (e.g., stovetop).

• Sublimation of Carbon Dioxide: Similarly, carbon dioxide sublimates in a fog machine, where heat causes the solid to transition to gas, highlighting the gain of kinetic energy by the molecules.

• Physical Changes: Boiling and sublimation are physical changes, meaning no new chemical substances are formed (e.g., water remains the same in both liquid and gas states).

2. Intermolecular Forces Explained

• Molecular Attraction: Molecules are held close together by weak forces known as intermolecular forces, which allow them to transition into gaseous states when sufficient kinetic energy is acquired.

• Comparison with Covalent Bonds: While covalent bonds are strong connections within molecules, intermolecular forces operate between molecules and are considerably weaker.

3. Ionic Compounds and Intermolecular Forces

• Absence of Intermolecular Forces in Ionic Compounds: Ionic compounds consist of ions in a solid state and do not have intermolecular forces acting between them due to the ionic bonding that holds the ionic crystal lattice together.

• Strength of Ionic Bonds: Ionic bonds are strong, leading to high boiling and melting points (e.g., sodium chloride melts at 801 °C).

Intermolecular Forces and Physical Properties

1. Relationship of Intermolecular Forces and Physical Properties

• Intermolecular forces in compounds influence key physical properties, including:

  • Physical state at given temperature and pressure

  • Melting point

  • Boiling point

  • Surface tension

  • Hardness and texture

  • Solubility in solvents

2. General Observations

• As intermolecular forces increase, melting points, boiling points, and surface tensions also tend to rise.

3. Types of Intermolecular Forces

• Johannes van der Waals' Theory: Three primary types of intermolecular forces exist:

  • Dipole–Dipole Forces: Attraction between polar molecules with partial positive and negative charges.

  • London Dispersion Forces: Weak forces that arise from temporary shifts in electron clouds in atoms/molecules.

  • Hydrogen Bonds: Strong type of dipole force occurring specifically between molecules that contain hydrogen bonded to highly electronegative atoms (N, O, F).

Dipole–Dipole Forces

• Polar vs Non-Polar Molecules: Polar molecules exhibit stronger intermolecular forces due to their charged ends (e.g., hydrogen fluoride).

• Example Properties: Hydrogen fluoride (HF) has higher melting and boiling points compared to non-polar counterparts, indicating the strength of dipole-dipole interactions.

• Characteristics: Dipole-dipole forces occur specifically between polar molecules and are generally stronger than London forces.

London Dispersion Forces

• Nature of Attraction: Exist in all molecules, especially notable in non-polar substances with low boiling points (e.g., hydrogen, helium).

• Molecule Size Relation: Larger molecules have greater London dispersion forces due to increased electron cloud potential, correlating with higher melting points.

Hydrogen Bonds in Biological Context

• Significance in Molecules: Hydrogen bonds contribute crucially to the structure of biomolecules like proteins and DNA.

• Protein Functionality: The shape of proteins (enzymes, structural materials) is influenced by hydrogen bonds, allowing them to fold properly.

• DNA Structure: Hydrogen bonds help maintain the double helix structure, with bases pairing between strands.

Conclusions on Intermolecular Forces

• Intermolecular forces are substantially weaker than covalent bonds, having a variety of strengths that dictate the physical properties of molecular substances.

• Strength Hierarchy: Arranged from weaker to stronger: London dispersion forces < dipole-dipole forces < hydrogen bonds.

Summary of Intermolecular Forces

• Types of Forces: There are three types of intermolecular forces: dipole–dipole forces, London dispersion forces, and hydrogen bonds.

• Physical Properties Relation: As the strength of intermolecular forces increases, so too do the melting point, boiling point, and surface tension of compounds.

• Comparison with Bonds: Intermolecular forces vary significantly in strength compared to covalent or ionic bonds, influencing molecular states and behaviors.