Polar or NonPolar That is the question Review

Polar or Non-Polar That is the question…

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

Molecule Polarity Ø Bond Polarity leads to Molecule Polarity Ø H 2 vs. HF - draw vector arrows - can you separate + and -? Non-polar bond, Non-polar molecule Polar bond Polar molecule

What if the molecule is more complicated? Ø Examples: BF 3 and CH 4 To determine polarity, you must first know 3 D shape. Ø Then, draw vector arrows for each bond. Ø Ask yourself, can I separate + and - with a straight line? Ø Polar bonds BUT Molecules are Non-polar overall.

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

Polar or Non-Polar? Ø Ø Ø Ø Si. F 4 l Non-polar molecule, polar bonds Al. Cl 3 l Non-polar molecule, polar bonds H 2 S l Polar molecule, polar bonds Cl 2 l Non-polar molecule, non-polar bond CS 2 l Non-polar molecule, polar bonds CBr 4 l Non-polar molecule, polar bonds PH 3 l Polar molecule, polar bonds

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

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

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

Dipole-Dipole Cont’d l Hydrogen bonding – • type of dipole-dipole IMF l l Special, stronger dipole attraction Made of a H of one molecule and F, O, or N of another molecule • Primary ex. – H 2 O, HF, NH 3 “Chemistry is FON” Causes intermolecular attraction to be stronger than predicted – l • raises b. pt. higher than predicted

Hydrogen Bonding

Remember this? Ø Plateaus are where IMFs are broken.

Ice… it floats!

It’s in your DNA…

Dipole-Dipole Forces Ø Ethanol and Water - demo

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

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

Dry Ice Ø Solid CO 2 Ø Sublimation Ø http: //www. youtube. com/watch? v=p. P_l. Za Och. E 0

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

Ø Solutions of ionic compounds Video