Intermolecular Forces and Condensed States: Comprehensive Notes

Intermolecular Forces & Solids & Liquids

Objectives

• Describe and differentiate the types of intermolecular forces.

• Describe the following properties of liquid and explain the effect of intermolecular forces on these properties: surface tension, viscosity, vapor pressure, boiling point, and molar heat of vaporization.

• Describe the difference in the structure of crystalline and amorphous solids.

Intermolecular Forces

• Forces that hold condensed states together.

• Generally attractive forces based on electrostatic attraction (positive and negative charges).

• Intramolecular: "within" a molecule.

  

  • Can hold together identical particles.

  • Not strongly affected by physical changes.

  • Chemical bonds.

  • Determine the chemical behavior of a substance.

• Intermolecular: "between" molecules.

  • Can hold particles together.

  • Strongly affected by physical changes.

  • Attractive forces.

  • Determine the state of matter.

Coulomb's Law

• The potential energy (E) of two oppositely charged particles decreases (becomes more negative) with increasing magnitude of charge and with decreasing separation (r) between them.

• Bonding forces result from large charges interacting at close distances.

• Intermolecular forces result from smaller charges interacting at greater distances.

Types of Intermolecular Forces

• Dispersion Forces (London Dispersion Forces)

• Dipole-dipole Forces

• Hydrogen Bonds

• Ion-dipole Forces

Dispersion Forces (London Dispersion Forces)

• Present in all molecules and atoms.

• Result from fluctuations in electron distribution within molecules or atoms.

• Create instantaneous or temporary dipoles.

• Magnitude depends on how easily electrons polarize, which depends on the size/volume of the electron cloud.

Dispersion Force and Molecular Shape

• Example:

  • n-Pentane: molar mass = 72.15 g/mol, boiling point = 36.1 °C

  • Neopentane: molar mass = 72.15 g/mol, boiling point = 9.5 °C

• n-Pentane has a larger area for interaction compared to Neopentane, leading to a higher boiling point.

  

Boiling Points of n-Alkanes

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

  • Example: n-Nonane (C9H20) has a higher boiling point than n-Pentane (C5H12).

Quick Exercise

• Which halogen has the highest boiling point?

  • Answer: Iodine (I2)

Dipole-Dipole Forces

• Exist in all polar molecules.

• Polar molecules have electron-rich (partial negative charge) and electron-deficient regions (partial positive charge).

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

• How to determine if a molecule has dipole-dipole forces:

  1. Determine if the molecule contains polar bonds.

  2. Determine if the polar bonds add together to form a net dipole moment.

• Bond Polarity:

  • Depends on the electronegativity difference between the bonding atoms.

  • The greater the difference, the more polar the bond.

• Electronegativity values (Pauling scale) are provided in a table.

Bond Polarity

• Small (0 - 0.4): Non-polar covalent bond.

• Intermediate (0.4 - 2.0): Polar covalent bond.

• Two atoms with identical electronegativities form a nonpolar covalent bond (e.g., Cl-Cl).

• Large electronegativity difference leads to electron transfer, forming an ionic bond.

• Continuum of Bond Types:

  • Pure covalent: Electrons shared equally.

  • Polar covalent: Electrons shared unequally.

  • Ionic: Electrons transferred.

Molecular Shape & Polarity

• Geometric shapes influence polarity.

• Examples:

  • Linear: Can be nonpolar if symmetrical.

  • Trigonal planar: Can be nonpolar if symmetrical.

  • Bent: Usually polar.

  • Tetrahedral: Can be nonpolar if symmetrical.

  • Trigonal pyramidal: Usually polar.

• Net dipole moment is the vector sum of individual bond dipoles.

• Polar molecules have higher melting and boiling points than nonpolar molecules of similar molar mass.

Dipole Moment and Boiling Point

• Boiling points increase with increasing dipole moment.

Polar and Nonpolar Compounds

• Miscibility:

  • Polar + Polar: Miscible (forms a solution).

  • Nonpolar + Nonpolar: Miscible.

  • Polar + Nonpolar: Non-miscible.

• Water (polar) and pentane (nonpolar) do not mix.

Hydrogen Bonding

• Occurs in polar molecules containing hydrogen atoms bonded directly to small, electronegative atoms (F, O, N).

• A "super" dipole-dipole force.

• Example: Hydrogen bonding in HF.

• Ethanol (C2H5OH) has a higher boiling point than dimethyl ether (CH3OCH3) due to hydrogen bonding.
  *Hydrogen bonding between antiparallel double stranded DNA is a vital function to its stability

Hydrogen Bonding in Water

• Anomalously high boiling point compared to other hydrogen-containing compounds due to hydrogen bonding.

Quick Exercise:

• One of the compounds shown here is a liquid at room temperature. 30.03g/mol, 34.03g/mol, 34.02g/mol. Which one and why?

Ion-Dipole Force

• Occurs when an ionic compound is mixed with a polar compound.

• Important in aqueous solutions of ionic compounds.

• Responsible for the ability of ionic substances to form solutions with water.

Summary of Intermolecular Forces

Intermolecular Forces in Action

• Surface Tension

• Viscosity

• Capillary Action

Surface Tension

• The energy required to increase the surface area of a liquid by a unit amount.

Viscosity

• The resistance of a liquid to flow.

Viscosity of Several Hydrocarbons at 20 °C

• Viscosity decreases as temperature increases.

Capillary Action

• The ability of a liquid to flow against gravity up a narrow tube.

• Results from:

  • Cohesive forces: Between molecules in a liquid.

  • Adhesive forces: Between liquid molecules and the surface of the tube.

• Meniscuses: curvature of water and mercury.

Vaporization and Vapor Pressure

• Vaporization: The process by which thermal energy overcomes intermolecular forces, causing a state change from liquid to gas.

• The higher the temperature, the greater the average energy of the molecules.

• Factors affecting the rate of vaporization:

  • Increasing temperature increases the rate.

  • Increasing surface area increases the rate.

  • Decreasing strength of intermolecular forces increases the rate.

Types of Solids

• Amorphous solids:

  • Ex: rubber, cotton candy, plastics.

• Crystalline solids:

  • Ex: ice, table salt, metals.

Crystalline Solids: Unit Cells & Basic Structures

• The crystalline lattice is represented by a small collection of atoms, ions, or molecules called the unit cell.

Fundamental Types of Crystalline Solids

• Molecular: Composite units are molecules, held together by intermolecular forces, low to moderately low melting points.

• Ionic: Composite units are ions, held together by coulombic interactions, high melting points.

• Atomic: Composite units are individual atoms.

  • Nonbonding atomic solids: Noble gases in solid form, held together by weak dispersion forces.

  • Metallic atomic solids: Held together by metallic bonds (interaction of metal cations with a "sea" of electrons).

  • Network covalent atomic solids: Crystal structures restricted by geometrical constraints of covalent bonds.