Day 29

Context

Knowing about polar and nonpolar molecules puts us in a position to think about interactions between molecules.

Explanation

Not all polar molecules have the same amount of polarity. The interactions between molecules, whether they be polar or nonpolar, influence many of our everyday observations. We will not consider these interactions and look at example from the world around us.

After this class you should be able to:

• Predict the relative strength of interactions between nonpolar-nonpolar, nonpolar-polar, and polar-polar molecules.
• Define the term intermolecular force.
• See the effect of intermolecular forces and mass on a variety of physical circumstances.

Three General Interactions

There are three general categories of interactions: nonpolar-nonpolar, nonpolar-polar, and polar-polar.

nonpolar-nonpolar

When there are two nonpolar molecules interacting there is a weak attractive force due to instantaneous dipoles. These dipoles are formed instantaneously due to the random movement of the electrons. Once in a while the electrons will end up on one side of the molecule forming a temporary dipole. They will repel each other so the electrons won't stay that way for long and we say it is instantaneous. This is the weakest interaction of the three we are considering.

nonpolar-polar

When a nonpolar molecule interacts with a polar molecule, the permanent dipole of the polar molecule can induce a temporary dipole in the nonpolar molecule. It will always do it in such a way as to have opposite charges together and so it will always be an attractive force. It is stronger than the instantaneous dipole interactions, but weaker than the polar-polar interactions.

Polar-Polar

These molecules will rotate their permanent dipoles so that the opposite charges are next to each other. This is the strongest of the three interactions. Remember that all polar molecules don't have the same amount of polarity. The really polar ones will have strong attractions, while the less polar molecules will have weaker interactions. The rule is that the more polar a molecule is, the stonger the attractions will be.

The attractions between molecules that we have been talking about are called intermolecular forces. The attraction between molecules are the intermolecular forces between molecules. I emphasize that these are interactions between molecules, not between atoms.

Hydrogen Bonds

The most electronegative elements will form the biggest dipoles and the most polar molecules. Three cases are particularly polar and very important. These are bonds between hydrogen and nitrogen, between hydrogen and oxygen, and between hydrogen and flourine. An example would be H₂O. When molecules with these very polar bonds exist the intermolecular forces are strong. Because they each involve hydrogen they are called hydrogen bonds. These are not as strong as ionic or covalent bonds, in fact they are not really bonds at all, they are strong intermolecular forces. They are important, however. They keep the strands of DNA from coming apart!

Examples for Today and Monday

1. Water over top of container
2. Boiling point
3. Evaporation/boiling of polar liquid vs nonpolar liquid
4. Evaporation of water vs alcohol
5. Space marbles
6. Polar/Nonpolar students
7. Water and oil
8. * Density and polarity expt
9. Insects walking on water
10. Detergents - Oil off of your hands
11. Spicy food
12. * Painters drink milk
13. Meniscus up or down
14. Water beading on waxed car
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15. Soap bubbles
16. Why doesn't a duck get wet?
17. Hair spray for spots
18. Paper chromatography
19. Boat races
20. Tape sticking
21. Gecko toys
22. Vitamin C
23. Dudley R. Herschbach

Homework

No homework associated with Day 29.