Overview of Chapter 13 • Intermolecular Forces • Liquids and their Properties Questions to consider: • Why is water usually a liquid? • Why does water boil at 100°C and not 30°C? • Solids and Their Properties • Why does ice float? • Phase diagrams • Do all snowflakes have 6 sides? Dipole-Dipole Forces Intermolecular Forces • Dipole-Dipole Dipole-dipole forces occur between molecules having permanent dipoles. – Hydrogen bonding • Dipole-Induced dipole • Induced dipole-Induced dipole • Ionic Bonds • Network Bonds Hydrogen Bonding Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compound N2 CO Br2 ICl Mol. Wt. 28 28 160 162 A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. Boiling Point -196°C -192°C 59°C 97°C H-bonding is strongest when X and Y are N, O, or F 1 Hydrogen Bonding Between Methanol and Water -δ H-bond +δ Hydrogen Bonding in H2O H-bonding is especially strong in water because • the O— O—H bond is very polar • there are 2 lone pairs on the O atom H-bonding accounts for many of water’ water’s unique properties. -δ Hydrogen Bonding in H2O Ice has open lattice-like structure. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid, so ice floats on water. One of the VERY few substances where solid is LESS DENSE than the liquid. Hydrogen Bonding Boiling Points of Simple Hydrogen-Containing Compounds Hydrogen bonds leads to abnormally high boiling point of water. 2 Boiling Points of Hydrocarbons C4H10 C3H8 C2H6 Forces Involving Induced Dipoles How can non-polar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. CH4 Note linear relation between boiling point and molar mass. Dipole-induced dipole Forces Involving Induced Dipoles Solubility increases with mass of the gas 3
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