Tuesday, February 19, 2008 Section 14.1 Forces Water and Its Unique Properties Vocabulary (Definitions Due Wednesday, Quiz Friday) Intermolecular ForcesIntramolecular Forces Dipole-Dipole MomentHydrogen Bonding London Dispersion ForcesNormal Boiling Point Heating/Cooling CurveNormal Freezing Point Molar Heat of FusionMolar Heat of Vaporization VaporizationCondensation Vapor PressureCrystalline Solids Ionic SolidsMolecular Solids Atomic SolidsElectron Sea Model AlloySubstitutional Alloy Interstitial Alloy

3 Physical States Describe the following for solids, liquids, and gases. Shape Volume Movement of molecules Density

3 States Which physical states are most similar? Why? 6 kJ/mol + H 2 O(s)  H 2 O(l) 41 kJ/mol + H 2 O(l)  H 2 O(g) Liquid is most similar to solid illustrated by the energy required to go from one state to the next.

Representations of the gas, liquid, and solid states.

Forces Intramolecular forces- between atoms (aka bonds) hold molecules together Intermolecular forces- between molecules; not ionic or covalent bonds! –much weaker If they are relatively strong, will the molecule be more likely to be a liquid or gas? liquid

Intermolecular Forces – Between separate molecules (a) Interaction of two polar molecules. (b) Interaction of many dipoles in a liquid.

Dipole-dipole interactions These dipole-dipole attractions are about 1% the strength of a covalent bond. Is water polar? Why? Electronegativities and shape. Water dipole attraction is stronger than most (large difference in electronegativity). Close approach of the dipoles because of the small H atom. We give it a special name. What is it?

Hydrogen bonding among water molecules. A H-bond is really a very strong dipole attraction that occurs when H is bonded to one of the following very electronegative atoms: H-F H-O H-N

Non-dipoles Is there any attractions between molecules with no polarity? There have to be some attractions because even the noble gases can be liquid or solid at low temperatures. Some attractions when they move slow enough to feel it.

Atoms with spherical electron probability. London dispersion forces – temporary distortions of the electron cloud around the molecule create temporary dipoles

The atom on the left develops an instantaneous dipole. London forces are most important with large molecules because the e- are so far from the nucleus.

Water’s Unique Properties Water is less dense as a solid than liquid.

Water’s Unique Properties Ice (solid) is less dense than the liquid. This is due to more permanent H-bonding. LiquidSolid

Water’s Unique Properties Water has very high surface tension. Under the surface, molecules are attracted in all directions so there is no net force. On the surface there is a net downward or inward force that will tend to minimize surface area.

Water’s Unique Properties Water has a very high boiling and melting point. The boiling points of covalent hydrides.

Boiling Points of Group 6 Strong H-bonding makes H 2 O have high boiling points compared to other group 6 elements with hydrogen Hard to overcome Water molecules remain together even at high temps

Review Vocab Dipole-dipole attraction – when molecules with dipole moments attract each other by lining up so that the positive and negative ends are close to each other.

Review Vocab Hydrogen bonding – unusually strong dipole-dipole attractions that occur among molecules in which hydrogen is bonded to a highly electronegative atom.

Review Vocab London Dispersion Forces – the relatively weak forces, which exist among noble gas atoms and nonpolar molecules, that involve an accidental dipole that induces a momentary dipole in a neighbor.