Why do some substances dissolve and others not? Intermolecular Forces

Inter- molecular Forces Have studied INTRA molecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces. Forces between molecules, between ions, or between molecules and ions.

Recall: Intramolecular Forces IonicCovalent Metal + non-metalBetween non-metals Transfer of electronsSingle, double, triple In H 2 O, dissolves to form ions (dissociates) Share electrons Conduct electricityDissolves as molecules Form crystal latticeDo not conduct electricity

Intermolecular Forces Forces between molecules, between ions, or between molecules and ions. Forces between molecules, between ions, or between molecules and ions. Not as strong as intramolecular forces Not as strong as intramolecular forces Strength of intramolecular forces increases the melting point, boiling point and surface tension of a molecular compound Strength of intramolecular forces increases the melting point, boiling point and surface tension of a molecular compound

1. Dipole-Dipole Forces Weak force between polar molecules Weak force between polar molecules Dipoles in molecules attract each other Dipoles in molecules attract each other

Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. CompdMol. Wt.Boil Point N o C CO o C Br o C ICl16297 o C

2. London Dispersion Forces Weak force that exists between all molecules Weak force that exists between all molecules Unlike polar molecules with a permanent dipole, nonpolar molecules have instantaneous dipole moments Unlike polar molecules with a permanent dipole, nonpolar molecules have instantaneous dipole moments

2. London Dispersion Forces Because electrons are moving around in atoms, there will be instances when charge around the atom is not symmetrical Because electrons are moving around in atoms, there will be instances when charge around the atom is not symmetrical Unlike polar molecules with a permanent dipole, nonpolar molecules have instantaneous dipole moments Unlike polar molecules with a permanent dipole, nonpolar molecules have instantaneous dipole moments

London forces Instantaneous dipole: Induced dipole: Eventually electrons are situated so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

3. Hydrogen Bonds A particularly strong dipole-dipole force that occurs between an H atom bonded to an electronegative (EN) atom (therefore with a δ + charge) an the unshared electron pair of another small EN atom A particularly strong dipole-dipole force that occurs between an H atom bonded to an electronegative (EN) atom (therefore with a δ + charge) an the unshared electron pair of another small EN atom

Hydrogen Bonding in H 2 O H-bonding is especially strong in water because the O—H bond is very polar the O—H bond is very polar there are 2 lone pairs on the O atom there are 2 lone pairs on the O atom Accounts for many of water’s unique properties.

Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water.

Hydrogen Bonding H bonds leads to abnormally high boiling point of water. See Screen 13.7

Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —adenine with thymine —guanine with cytosine —guanine with cytosine

Portion of a DNA chain Double helix of DNA

Base-Pairing through H-Bonds

Intermolecular Forces Figure 13.13

Dissolving Solutes dissolve when the attractive forces between the solute and solvent are stronger than the attractive forces within either the solute or the solvent E.g. NaCl dissociation in water

Dissolution of NaCl in Water

Dissolving of Molecular Compounds Polar molecular compounds are also miscible in water (e.g. NH 3 ) Polar molecular compounds are also miscible in water (e.g. NH 3 ) Non-polar molecules are NOT miscible in polar solvents Non-polar molecules are NOT miscible in polar solvents “Like dissolves like” “Like dissolves like”