Intermolecular Attractions

What is the difference?  What is the difference between:  Inter-molecular?  Intra-molecular?  More solid  Moves less  These are the forces that hold our world together!

Intermolecular Forces  Macro-covalent  Ionic  Metallic  Hydrogen  Dipole-Dipole  Dispersion

Macrocovalent  The strongest of them all.  Diamond (all Carbon atoms)  Rocks (Silicon and Oxygen)  See Toy Examples

Covalent Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity ExamplesExamplesExamples diamond graphite carbon atoms

Crystalline quartz (SiO 2 ) Non-crystalline quartz glass

Ionic  Very strong  Permanent strong charge interactions  Usually crystal in formation  Very high melting points  See toy example.  Example of Lattice Example

Metallic Bonding  Solids at room temperature  This means STRONG Intermolecular Forces  Metal Atoms give up electrons  This creates many dipoles throughout the atoms, they SHIFT but are always there  “SEA of electrons”

Hydrogen Bonding A strong intermolecular attractions Caused by very strong dipole-dipole attraction between molecules with N-H, O-H, and F-H bonds. Responsible for many of water’s special properties.

Hydrogen Bonding

Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND. A H … B A H … A or A & B are N, O, or F

Dipole-Dipole (can you think of an example of this?)

Dipole-Dipole Attraction Caused by attraction of   end of one polar molecule to   of another polar molecule. For a molecule to have dipoles it must be polar: A. Asymmetrical in shape and B. With polar bonds

Molecular Polarity In order for a molecule to be polar it must have polar bonds and be asymmetrical in shape. Lone pairs always lead to asymmetry. Linear, trigonal planar, and tetrahedral are symmetrical shapes and dipoles will cancel if all bonds are equal.

Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid

Dispersion Forces (non polar molecules)  “Moving the Fat Around”  Caused by momentary (temporary) dipoles (pg. 444)  Get stronger with increase in mass  Only attractive force between non-polar molecules (symmetrical or those with only non-polar bonds) or single atoms.

London Dispersion Forces Force that exits among noble gas atoms and nonpolar molecules Can create a temporary dipole moment

Momentary Dipoles

Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights SubstanceFW (g/mol)mp (°C)bp (°C) F2F Cl Br I2I Boiling point is a true measure of the strength of intermolecular attractions. Why? Let’s look at the effect of size on nonpolar molecules. Conclusion?

Boiling Point of Different Hydrocarbons

Melting Points and Boiling Points of Substances with Similar Formula Weights SubstanceFW (g/mol)mp (°C)bp (°C) F2F NO CH 3 OH Ca NaF Let’s compare strength of intermolecular attractions in molecules of the same size. F 2 = nonpolar, NO = dipole-dipole, CH 3 OH = hydrogen bonding, Ca = metal, NaF = ionic. What does that say about the relative strength of these attractions?

State of Matter-(at room temp) Ionic Compounds = Always Solid Covalent Compounds-Solid, liquid or gas Depends on size and polarity. Gas  small and non-polar or dipole-dipole Liquid  small & H-bonding, or larger with non-polar or d/d Solid  large and polar, or very large and non-polar.

Practice – Label each with Phase and type of IMF SiO 2 Fe CCl 4 NO SO 2 C 2 H 6 C 10 H 22 Hg

O 3 CO 2 N 2 C (diamond) C 2 H 5 OH C 2 H 21 OH SO 3 CI 4

Practice – Label each with Phase and type of IMF Answers SiO 2 Solid, macrocovalent Fe Solid, metallic CCl 4 liquid, nonpolar NO gas, dipole-dipole SO 2 gas, dipole-dipole C 2 H 6 gas, non polar C 10 H 22 liquid, non-polar Hg liquid, metallic

O 3 gas, polar (assymetrical design) CO 2 gas, non polar (bent) N 2 gas, non-polar C (diamond) solid, macrocovalent C 2 H 5 OH liquid, H-bonding C 2 H 21 OH liquid, H-bonding + dispersion SO 3 Gas, non polar (symmetrical) CI 4 gas, non polar

Boiling Point of Different Hydrides

Boiling Points of Halogen Hydrides

Boiling Point Differences

Volatility Ability to evaporate High when attractions are weak. Why? Compare volatility of alcohol and water. Capillarity Ability to climb up a tube or surface High when liquid molecules are attracted to the tube or surface. Ex. Meniscus, paper towels, blood test

Capillary Action

Incredibly Tacky Household Products

Surface Tension A “skin” develops on the surface of liquids. Water’s is very strong.  High when molecules are attracted to each other  cohesion. Ex. Paper clip “floating” Water striders

Basilisk Lizard

More Surface Tension Examples “Don’t touch the tent” Rainx Surfactants  Soaps and detergentsSurfactants  Soaps and detergents

Surfactants Molecules that act to disrupt a liquid’s surface tension  “wetting agent” Structure-long non- polar hydrocarbon tail and a polar or ionic head

How do they work? Disrupt surface tension by acting like a “wedge”

Surfactants as Cleaning Agents Non-polar hydrophobic (water-hating) tails and polar or ionic hydrophilic (water-loving) heads form micelles.

Soap dissolves grease by taking grease molecules into the non-polar interior of the micelle. Micelles are carried away during rinsing.

Shampoo/Conditioners

Changes of State and Attractive Forces

Heating Curves Water Why is the plateau for boiling so much longer than for melting?

Solubility What dissolves in what? I. Ionic substances dissolve in water II. Covalent compounds: A. Non-polar dissolves in non-polar solvents. B. Polar solutes dissolve in polar solvents. C. Partially polar (only polar in a small part of the molecule) solutes dissolve in partially polar solvents. Like Dissolves Like!

The solute and the solvent have to be attracted to each other in order to dissolve. Ionic solid dissolving in water