Intermolecular Forces Chapter 11 Part I

States of matter Gas Gases are compressible with no definite shape nor volume. Particles are in constant rapid motion.

Liquid Liquids have a constant volume (at a constant temperature) but not a constant shape. Molecules move in relationship to one another.

Solid Solids have a constant shape and a constant volume. Molecules vibrate in place.

Bonding versus Attractions Molecules form stable states by sharing electrons in covalent bonds. This is called intramolecular bonding or bonding within the molecule.

Forces Molecules may be held together as a liquid or a solid by strong covalent or ionic bonds

Intermolecular Forces: IMF Attractions (not bonds) between different molecules are called intermolecular forces. When these forces are “broken” the molecules remain intact. For example: when water is vaporized, the water molecules remain as water, but the attractions between the molecules are interrupted.

IMF’s summary

Dipole-Dipole Molecules with a dipole moment (align in a magnetic field) line up so the positive ends are in close proximity to the negative ends.

Dipole-Dipole In a condensed state such as a liquid, the molecules find the best compromise between attraction and repulsion Dipole-dipole Forces are typically only 1% as strong as ionic or covalent bonds.

Hydrogen Bonding The strongest of the dipole-dipole attractions are seen in molecules in which hydrogen is bound to a highly electronegative element such as oxygen, nitrogen or fluorine.

Hydrogen Bonding and Physical Properties The boiling points of covalent hydrides ranging in group 4A are in the next slide. Note that there is a steady increase in boiling point with an increase in molar mass of these non-polar tetrahedral hydrides.

Boiling points of Covalent Hydrides

Boiling points of Hydrides from groups 4A, 5A, 6A and 7A Compare the boiling points of all the groups 4A to 7A and note the difference in the boiling point of the highly electronegative elements relative to their molar mass. Why? Hydrogen bonding.

Hydrogen Bonding These unusually strong hydrogen bonds are due to two factors. 1. the relatively high electronegative value of the lightest elements in these groups. This leads to especially polar ends. 2. The small size of the first element in each group allows close approach of the dipoles.

London Dispersion Forces The weakest of the IMF’s Is relative to the “squishiness” of the molecule’s electrons. Larger molecules have more area to move electrons about, creating larger partial charges. Larger London!!