Polar or Non-Polar That is the question…

Review: Bond Polarity What do we mean when we say a bond between two atoms is polar? Non-polar? Non-polar: Electronegativity difference = 0 Electrons are shared equally Polar: Electronegativity difference > 0 One atom pulls electrons toward it more

Molecule Polarity Bond Polarity leads to Molecule Polarity H2 vs. HF - draw vector arrows - can you separate + and -? Non-polar bond, Polar bond Non-polar molecule Polar molecule

What if the molecule is more complicated? Examples: BF3 and CH4 To determine polarity, you must first know 3D shape. Then, draw vector arrows for each bond. Ask yourself, can I separate + and - with a straight line? Polar bonds BUT Molecules are Non-polar overall.

What if the molecule has lone pairs? Lone pairs are negative! Draw arrows for each bond and lone pair. Can you separate + from - ? Polar bonds Polar molecules

Polar or Non-Polar? SiF4 Non-polar molecule, polar bonds AlCl3 H2S Non-polar molecule, non-polar bond CS2 CBr4 PH3

Summary So… It is possible for molecules to have: Non-polar bonds and be non-polar overall Polar bonds and be non-polar overall Polar bonds and be polar overall Can there be molecules with non-polar bonds that are polar overall?

Intermolecular Forces Intra vs. Inter Attractions within a molecule (Intra) vs. Attractions between molecules (Inter). Not as strong as chemical bonds. Also play a role in macroscopic properties we can observe.

Dipole-Dipole Force Dipole = two poles or POLAR attraction between two or more polar molecules most common IMF oppositely charged sides of molecules attracting each other

Dipole-Dipole Cont’d Hydrogen bonding – type of dipole-dipole IMF Special, stronger dipole attraction Made of a H of one molecule and F,O, or N of another molecule Primary ex. – H2O, HF, NH3 “Chemistry is FON” Causes intermolecular attraction to be stronger than predicted – raises b. pt. higher than predicted

Hydrogen Bonding

Remember this? Plateaus are where IMFs are broken.

Ice… it floats!

It’s in your DNA…

Dipole-Dipole Forces Ethanol and Water - demo

What about non-polar molecules? Van der Waals forces OR London dispersion forces *Caused by momentary dipole attraction* Weakest intermolecular attraction lowest m. pt., b. pt., insoluble, nonconductors Allow gases to become liquids or solids under conditions of low temperature and high pressure Strength affected by: number of electrons, mass of molecule, distance between the molecules Only form of attraction between the noble gases

How does it work? Momentary Dipoles are formed. They attract other molecules.

Dry Ice Solid CO2 Sublimation http://www.youtube.com/watch?v=pP_lZaOchE0

Molecule-Ion Attraction Attraction between ions and molecules Most common when ionic solids dissolve in water (+) ion will be attracted by negative side of water molecules and (-) ion will be surrounded by the positive side of water molecule Allows for electrical conductivity of solutions – mobile ions: increase number of ions, increase conductivity

Solutions of ionic compounds Video