Intermolecular forcesIntermolecular forces

• Why do some solids dissolve in water but others do not?

• Why are some substances gases at room temperature, but others are liquid or solid?

• The answers have to do with …

Questions

Intermolecular ForcesIntermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule.

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force - boiling point- melting point- Surface tension- evaporation rate

There are three main types of IMFs

•London Dispersion Forces (IMF)

•Dipole-Dipole Forces (DD)

•Hydrogen Bonding Forces (HB)

Types Of Intermolecular Forces:

London Dispersion Forces•The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors.Fritz London

1900-1954

•London forces increase with the size of the molecules.

London Dispersion ForcesLondon Dispersion Forces• Non-polar molecules do not have dipoles

like polar molecules. How, then, can non-polar compounds form solids or liquids?

• London forces are attractive forces caused due to small temporary dipoles that exist in non-polar molecules

• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical

• The resulting tiny dipoles cause attractions between molecules

London forcesLondon forces

Instantaneous dipole: Induced dipole:

Eventually electrons shift so that tiny

dipoles form

A dipole forms in one atom or molecule,

inducing a dipole in the other

London Dispersion ForcesLondon Dispersion Forces

Dipole-Dipole ForcesAttractive forces between polar molecules

Orientation of Polar Molecules in a Solid

Hydrogen BondThe hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond in one molecule and an electronegative O, N, or F atom.

Hydrogen BondingHydrogen BondingBonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen

Hydrogen bonding between ammonia and water

Maximum Density40C

Ice is less dense than water

Density of Water

Water is a Unique Substance

The hexagonal structure of ice

The H-bonding abilitiy of the

water molecule

Forces & Properties

• Bigger size = Stronger force = higher melting and boiling point because more energy is needed to separate the molecules

Ex. H2 Vs. Cl2

• Heavier the molecule = stronger force• Ex. F2 Vs. Cl2

Molecular shape and boiling pointLonger molecule = stronger force

C5H12 (straight structure) Vs. C5H8 (branched structure)

Properties of LiquidsSurface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.

Strong intermolecular forces

High surface tension(liquid beads up)

=

(polar molecules, (polar molecules, liquid metals). liquid metals).

Properties of Liquids

Cohesion is the intermolecular attraction between like molecules

Adhesion is an attraction between unlike molecules

Adhesion

Cohesion

Figure 12.20 Shape of water or mercury meniscus in glass

adhesive forces

stronger cohesive forces

• This is why oil and water will not mix! Oil is nonpolar, and water is polar.

• The two will repel each other, and so you can not dissolve one in the other

Like Dissolves Like

•Polar substances dissolves Polar substances

•Non-Polar Dissolves Non-polar sbubstances

Bond Polarity• “Like Dissolves Like”

– Polar dissolves Polar– Nonpolar dissolves

Nonpolar