Intermolecular forces Questions 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 … Intermolecular forces

Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. “Measure” of intermolecular force - boiling point - melting point - Surface tension - evaporation rate Generally, intermolecular forces are much weaker than intramolecular forces.

Types Of Intermolecular Forces: There are three main types of IMFs London Dispersion Forces (IMF) Dipole-Dipole Forces (DD) Hydrogen Bonding Forces (HB)

London Dispersion Forces The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors. London forces increase with the size of the molecules. Fritz London 1900-1954

London 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 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 Forces

Orientation of Polar Molecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid

Hydrogen Bond The 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 Bonding Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen Hydrogen bonding between ammonia and water

Ice is less dense than water Water is a Unique Substance Maximum Density 40C Density of Water Ice is less dense than water

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 point Longer molecule = stronger force C5H12 (straight structure) Vs. C5H8 (branched structure)

Strong intermolecular forces Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. (polar molecules, liquid metals). Strong intermolecular forces = High surface tension (liquid beads up)

Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Adhesion Cohesion

stronger cohesive forces Figure 12.20 Shape of water or mercury meniscus in glass stronger cohesive forces adhesive forces

Like Dissolves Like 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 Polar substances dissolves Polar substances Non-Polar Dissolves Non-polar sbubstances

Bond Polarity “Like Dissolves Like” Polar dissolves Polar Nonpolar dissolves Nonpolar