Warm-up: Polar molecules

• What has to be true for a molecule to be polar? (2 things)

• What is one example of a polar molecule from your lab?

• What is one example of a nonpolar molecule?

Good Morning!We have 3 goals today

1. Review how you know if a molecule is polar or not

2. Learn about the reasons for and strengths of intermolecular forces

3. See evidence of those forces in the boiling points of different substances

Intramolecular Forces vs. Intermolecular Forces

• Intramolecular Forces

• --forces within a molecule.

• --tend to be very strong.

• --hold the atoms in molecules/formula units together.

• --include: ionic bonding, covalent bonding and metallic bonding.

• --been there…done that!

Intramolecular Forces vs. Intermolecular Forces

• Intermolecular Forces • --forces between molecules. • --tend to be weaker than intramolecular forces. • --matter has entropy (the tendency to be

disordered) • --therefore, a force must be present to keep

the individual atoms, molecules, or ions of a solid or liquid, in place, organized.

Intermolecular Forces are also called Van der Waals Forces

• There are four types: • 1. London/dispersion (non-polar molecules)

• 2. Dipole-dipole forces (polar molecules)

• 3. Hydrogen bonds (special case for polar molecules)

• 4. Molecule-ion attractions (as the name implies, interactions between ions and polar molecules, such as dissolving salt in water)

London Dispersion Forces

• occurs btw molecules that are non-polar

• Due to attractive forces between e-s of one atom and the nucleus of another

• stronger for atoms/molecules with more electrons

Intermolecular Forces!(this is the information from your notes)

Dispersion forces

Usually this Sometimes this

When they are unevenly distributed, there is a temporary dipole. This can induce new temporary dipoles in neighboring molecules.

How dispersion forces are created

Temporary RANDOM dipole Induced dipole Induced dipole

Induced dipoles

Partial positive charges are attracted to partial negative charges!

First random dipole can induce a neighbor to become a dipole.

And that dipole can induce the next dipole…

And then the next…and so on…

Dispersion forces – remember these are temporary!

• When the original random dipole changes back to normal, all other dipoles go back to normal…

• until the next random event.

Another Dispersion Graphic

1. Evenly distributed electrical charge

2. Uh-oh, (random event!!) uneven distribution in one molecule (temporarily)

3. Uneven distribution in one causes uneven distribution in the other…then they have charged ends (dipoles) that stick together!

4. Things return to normal, (random event, again…) and we start the process over

Dipole-Dipole Forces

• due to attractive forces between the positive end of one molecule and the negative end of another

• occurs between polar molecules (molecules have a permanent dipole)

Dipole-Dipole Forces

• Stronger than dispersion forces – molecules are permanently polar, so they always want to stick together!

Hydrogen Bonding• A specific and EXTRA STRONG dipole-

dipole interaction…

• between Hydrogen and three other elements with very high electronegativities and small radii (Oxygen, Nitrogen and Fluorine)

• explains high boiling point of water

these atoms are electronegative enough, to make a BIG enough dipole…

…to count as a different kind of force

Hydrogen bonding

H-bonding animation

• http://bcs.whfreeman.com/thelifewire/content/chp02/02020.html

Molecule-ion Attractions

• ionic compounds dissolve in water and other polar liquids because of attraction between the dipoles and the ions

Molecule-Ion attractions

Please take out a piece of paper

• Divide it into 4 sections, label each with one kind of intermolecular force

• A’s draw pictures for Dispersion forces and dipole-dipole forces

• B’s draw pictures for Hydrogen bonding and Molecule-ion interactions

• In 2 minutes turn and share with your neighbor

Now please take out…

• “Intermolecular bonding and boiling points”

• Use the data to complete each graph, then start working on the questions

• When you have finished the questions decide which type of intermolecular force is present for EACH compound listed

• There are 8 total compounds…

I leave you with this!

Both graphs are similar