London Dispersion Forces (LDFs)

• Intermolecular forces are attractive forces between molecules.

Van der Waals Forces

• The first two intermolecular forces are collectively known as Van der Waals forces of attraction, named after the Dutch scientist Johannes van der Waals.

London Dispersion Forces (LDFs)

• LDFs are the weakest type of intermolecular force.
• LDFs are present between all electrically neutral molecules, including polar and nonpolar molecules.
• This intermolecular force was named after the German American physicist Fritz London, who proposed its origin.
• LDFs result from temporary dipoles.
• LDFs are present in all molecules.
• LDFs increase with molecular size.
• LDFs' fundamental cause is temporary fluctuations in electron distribution.
• They do not occur only in ionic compounds.
• They are not stronger than hydrogen bonds.
• The more electrons present in a molecule, the stronger the LDFs. Larger molecules are more polarized.

Polarizability

• Polarizability is the measure of how easily the electron distribution of a molecule can be distorted. In large molecules, electrons are less tightly held, easing the formation of temporary dipoles.

Fluctuations

• LDFs are caused by fluctuations in the electron distribution within atoms or molecules.
• A usually nonpolar atom becomes polar due to the motion of its electrons, resulting in a temporary dipole.
• A molecule's temporary dipole induces instantaneous dipoles in neighboring molecules.

Dipole-Dipole Forces

• Dipole-dipole forces influence physical properties such as boiling point.
• They increase the boiling point of a substance.
• These forces occur between certain polar molecules.
• Dipole-dipole forces are stronger than LDFs in polar molecules.
• Dipole-dipole forces occur between molecules with permanent dipoles.
• They do not only affect melting point.

Hydrogen Bonding

• Hydrogen bonding is a particularly strong dipole-dipole force, contributed by the high electronegativity of N, O, or F.
• HF has a higher boiling point that HCl due to hydrogen bonding.

Ion-Dipole Forces

• Ion-dipole forces are between ions and water molecules.
• They are stronger than dipole-dipole forces.
• Ion-dipole forces are strongest when ions are highly charged.

Ranking of Intermolecular Forces

• Ranking from weakest to strongest: London disperison forces < dipole-dipole forces < hydrogen bonding < ion-dipole forces

Properties of Liquids

• Intermolecular forces influence properties of liquids.

Surface Tension

• Surface tension allows a water strider insect to stay on top of the water.

Viscosity

• Viscosity is the resistance of a liquid to flow, and it can be changed by adding a starch.

Liquid Droplets

• Water droplets form nearly spherical beads on a waxed car surface.

Cohesion and Adhesion

• A combination of high adhesion and high cohesion best explains water rising on a paper towel, even against gravity

Mercury

• Mercury has stronger cohesion than adhesion to glass.

Adhesion between Water and Grass

• Adhesion between water and the grass is responsible for water droplets clinging to the grass.

Soap

• When Soap is added to water, and the surface tension of the water decreases
• Surface tension decreases, and bubbles will be easier to form but break more easily.

Ethanol

• Ethanol has a lower boiling point than water, indicating it has weaker intermolecular forces.

Liquids vs. Honey

• A metal ball will reach the bottom of the water beaker first compared to honey.

Capillary Action

• Capillary action facilitates water transport in plants.
• A plant absorbs water from the soil through its roots, which then travels up to the leaves.

Polar vs. Non-Polar Solvents

• A polar solvent is likely to have a higher boiling point compared to a non-polar solvent.