Intermolecular Forces (IMFs)

• Intermolecular forces (IMFs) include both attractive and repulsive forces between molecules.
• IMFs dictate several physical properties, including boiling and melting points, viscosity, surface tension, and a substance's solubility.
• IMFs are weaker than intramolecular forces that constitute chemical bonds.

Types of Intermolecular Forces

• IMFs include ion-dipole forces, hydrogen bonding, dipole-dipole forces, and London dispersion forces (van der Waals forces).
• The strength of these forces varies; ion-dipole forces are strongest, and London dispersion forces are weakest.

Ion-Dipole Forces

• These occur between an ion and a polar molecule.
• They are relatively strong, due to the interaction between a full charge (ion) and a partial charge (dipole).
• Example: dissolving NaCl in water; Na+ interacts with the negative oxygen of water, while Cl- interacts with the positive hydrogen.

Hydrogen Bonding

• This is a specific dipole-dipole interaction when hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.
• The hydrogen atom carries a significant positive charge and is drawn to the lone pair of electrons on another electronegative atom in a different molecule.
• While substantial, hydrogen bonds are weaker than covalent bonds.
• Many of water's properties arise from hydrogen bonding; these characteristics include high boiling point and surface tension.
• Example: hydrogen bonds forming between water molecules.

Dipole-Dipole Forces

• These forces occur between polar molecules.
• Polar molecules have positive and negative ends due to unequal electron distribution.
• Oppositely charged ends of separate polar molecules attract.
• Dipole-dipole forces are stronger than London dispersion forces, but weaker than hydrogen bonds.
• Example: interaction between two carbonyl sulfide (OCS) molecules.

London Dispersion Forces

• These are the weakest IMF.
• They are present in all molecules, regardless of polarity.
• London dispersion forces are the result of temporary shifts in electron distribution, creating temporary dipoles.
• These temporary dipoles induce dipoles in neighboring molecules, causing attraction.
• Strength increases with molecular size and surface area. Larger molecules with more electrons exhibit greater dispersion forces.
• Example: interactions between methane (CH4) molecules.

Factors Affecting the Strength of London Dispersion Forces

• Molecular size (number of electrons): Larger molecules have stronger London dispersion forces due to a higher degree of polarizability.
• Molecular shape: Greater surface area yields stronger London dispersion forces as it increases the interaction between temporary dipoles; linear molecules have stronger dispersion forces than branched molecules of similar molecular weights.

Impact of Intermolecular Forces on Physical Properties

• IMFs impact substance's physical properties significantly.
• Boiling Point: Higher IMFs result in higher boiling points, due to the energy required to overcome intermolecular attractions.
• Melting Point: Stronger IMFs typically mean higher melting points.
• Viscosity: Greater resistance to flow in liquids results from stronger IMFs which hinder molecular movement.
• Surface Tension: Liquids with high IMFs have relatively high surface tension because molecules at the surface attract strongly to the bulk liquid, reducing surface area.
• Solubility: "Like dissolves like"; polar substances dissolve in polar solvents due to dipole-dipole interactions and hydrogen bonding, and vice versa for nonpolar substances and London dispersion forces.

Examples of Physical Properties and IMFs

• Water (H2O) has a high boiling point due to extensive hydrogen bonding.
• Methane (CH4) is a gas at room temperature due to weak London dispersion forces.
• Ethanol (C2H5OH) has a higher boiling point than ethane (C2H6) because ethanol can form hydrogen bonds, unlike ethane.
• Larger alkanes (octane) have higher boiling points than smaller alkanes (butane) due to stronger London dispersion forces.

Polarity

• Polarity has a direct impact on intermolecular forces.
• Nonpolar molecules experience only London dispersion forces.
• Polar molecules exhibit dipole-dipole forces, in addition to London dispersion forces.
• Hydrogen bonding creates particularly strong intermolecular interactions.