What Holds Molecules Together Intermolecular Forces of Attraction

What Holds Molecules Together? Intermolecular Forces of Attraction Called “Van der Waals Forces” They arise from weak “electrostatic attractions” between molecules. l Different from “Intramolecular Forces” or “covalent bonds” that exist between atoms!

Shows electrostatic attractions between opposite charged areas on molecules

Forces Between Polar Molecules l Dipole-Dipole Attractions: l Occurs between polar molecules l Polar Molecule = “Permanent Dipole” l asymmetrical molecule with polar bonds

Forces Between Polar Molecules l Hydrogen Bonding l Super strong Dipole-Dipole Attraction l Occurs when molecules have hydrogen atoms bonded to very small, highly electronegative atoms like F, O or N l Creates a very polar bond an “extra polar” molecule.

l Hydrogen Bonding between molecules results in unusually high MP/BP temps. l Water has hydrogen bonding between its molecules!

l Ethanol can also form hydrogen bonds to itself and also with water that is why bartenders can make mixed drinks!

Remember: l Hydrogen Bonding is “FON”!! l

Hydrogen bonds are responsible for ice floating on liquid water l Ice is ordered with open structure to optimize H-bonding. l Therefore, solid ice is less dense than liquid water. l

l Also it is responsible for water’s l Surface tension l How it beads Properties of Water: https: //www. youtube. com/watch? v=3 jw. AG Wky 98 c

l DNA has Hydrogen Bonds between the 2 strands of the Double Helix https: //www. youtube. com/watch? v=a. H 2 Ib. Ys_Xj. Y

Forces Between Nonpolar Molecules l Dispersion Forces (“weak force”) l Only attractive forces between nonpolar molecules and noble gas atoms. l Weakest of all intermolecular forces.

l l Instantaneous or “momentary” dipoles are formed as electrons move around the atoms. Also called an “induced dipole” force.

https: //www. youtube. com/watch? v=kv. TRL 8 uy. M 8 o

l How can you increase the chance for Dispersion forces to be felt between NONPOLARS? High pressure/low temp. conditions l Nonpolar gas particles are so far apart, weak dispersion forces cannot be felt. l Molecules must be close and moving slowly so that the momentary dipoles can be created.

l Dispersion Forces have different strengths. l The more total electrons in a molecule, the greater the force can get. l Ex: Cl 2 has 34 electrons (it’s a gas) Br 2 has 70 electrons (it’s a liquid) I 2 has 106 electrons (it’s a solid)

l Permanent Dipoles can “induce” nonpolars

Why Do Ionics Dissolve in Water? l Molecule-Ion Attractions: l Attraction between polar solvent molecules and ions in an ionic crystal. l Allows polar solvent (like water) to “pull” the ions into solution, creating “hydrated” ions. Remember which side of water is negative!

Ionic compounds are generally soluble in polar solvents like water. http: //www. chem. iastate. edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/solution. Salt. html

What Effects do IMF Have?

Melting and Boiling Point Temperature: l Stronger forces make it harder to separate molecules and undergo a phase change. l H-Bonding = highest MP/BP l Dipole-Dipole = middle MP/BP l London Dispersion Forces (only) = lowest MP/BP

Solubility: solutes must form attractions to solvent to dissolve in it. l Polar solvents attract polar solutes l Nonpolar solutes only dissolve in nonpolar solvents. l Polar solvents attract ions

l A brown and a white bear jump into the water at the same time, which one dissolves faster?

Viscosity: measure of resistance of a fluid to flow. l Stronger IMF = more viscosity l Since there are cohesive forces between the molecules of liquid, like intermolecular forces, these forces create an "internal friction" which reduces the rate of flow of that fluid, so when a substance has greater IMF, these frictional forces are stronger. Therefore, it has more resistance toward moving. l http: //www. youtube. com/watch? v=f 6 sp. Bk. Ve. Q 4 w

Vapor Pressure: l Pressure exerted by collisions of vapor particles above a liquid l The weaker the attractions between molecules in a liquid, the easier to become a gas. Therefore: l Nonpolars with weaker attractions have higher VP than polars

Vapor Pressure l Measured in a closed system at a specific temperature. http: //www. mhhe. com/physsci/chemistry/essentialchemistry/flash/vaporv 3. swf

Vapor Pressure l If the temperature of a liquid increases, what happens to VP?

Vapor Pressure Boiling Point: l occurs when VP equals the atmospheric pressure pushing down on the liquid. l

l Normal Boiling Point: l The temp. a liquid boils at standard pressure l 1 atm l 101. 3 KPa l 760 mm. Hg (torr) For water it is 100°C.

l Can you boil water at different Temperatures? l It depends more on the amount of air pressure pushing down on the liquid. http: //www. youtube. com/watch? v=8 lyq. Fk. Fs. H 28&safe=active

Using Reference Table H

Significant Figures Click for Powerpoint: l http: //www. slideshare. net/crespiryan/signifi cant-figures-550789 l Sig Figs Practice: l http: //www. sciencegeek. net/Chemistry/tate rs/Unit 0 Sigfigs. htm l

Accuracy vs. Precision l https: //www. youtube. com/watch? v=8 Cl 5 C ei. T 7 h. U