1. Warmup 11/16/15 Water bends up at the edges of a graduated cylinder. What do you think causes this? Objective Tonight’s Homework To learn about how we define polar molecules pp 355: 3, 4, 5

2. Notes on Polar Molecules What They Are Let’s take a closer look at a water molecule. H O H

3. Notes on Polar Molecules What They Are Let’s take a closer look at a water molecule. See the unshared electron pairs? These have an effect on the molecule. Electrons have negative charge. Since they spend most of their time around the oxygen, this gives the oxygen a little bit of overall negative charge. Similarly, the hydrogens get a little bit of overall positive charge. H O H

4. Notes on Polar Molecules What They Are Let’s take a closer look at a water molecule. See the unshared electron pairs? These have an effect on the molecule. Electrons have negative charge. Since they spend most of their time around the oxygen, this gives the oxygen a little bit of negative charge. Similarly, the hydrogens get a little bit of positive charge. We show this with a “δ” symbol. H O H δ-δ- δ+δ+ δ+δ+

5. Notes on Polar Molecules Other Polar Molecules There are many other molecules that exhibit this behavior: NH 3 HFCH 3 Cl

6. Notes on Polar Molecules Other Polar Molecules There are many other molecules that exhibit this behavior: NH 3 HFCH 3 Cl The polarity of each of these molecules is different. Some are stronger than others. Most aren’t polar at all. Polarity is usually determined by if the molecule is lopsided or not. So what does this mean?

7. Notes on Polar Molecules Behavior of Polar Molecules Polar molecules have a pull on each other. The partially positive end of one pulls on the partially negative end of another.

8. Notes on Polar Molecules Behavior of Polar Molecules Polar molecules have a pull on each other. The partially positive end of one pulls on the partially negative end of another. This results in many effects you’ve seen. The meniscus is one. This happens because the molecules pull together a little. This creates a surface tension that helps hold the water together.

9. Notes on Polar Molecules This results in many other effects you’ve seen. Water forming into droplets:

10. Notes on Polar Molecules This results in many other effects you’ve seen. Water forming into droplets: Surface tension holding up objects:

11. Notes on Polar Molecules This results in many other effects you’ve seen. Water forming into droplets: Surface tension holding up objects: Water bending near electric charge:

12. Notes on Polar Molecules This results in many other effects you’ve seen. Water forming into droplets: Surface tension holding up objects: Water bending near electric charge: Water pulling up through drinking straws:

13. Notes on Polar Molecules Interaction Forces There are a few other forces within and between molecules. These are usually called weak forces because they aren’t nearly as strong as the forces we’ve discussed so far.

14. Notes on Polar Molecules Interaction Forces There are a few other forces within and between molecules. These are usually called weak forces because they aren’t nearly as strong as the forces we’ve discussed so far. The first type are called dipole-dipole forces. These are the forces between polar molecules. This is a pretty weak attraction. In water, it can create surface tension, but it doesn’t physically bond molecules.

15. Notes on Polar Molecules But we can have one even weaker. If we bring a polar molecule near a nonpolar molecule, it can induce a dipole.

16. Notes on Polar Molecules But we can have one even weaker. If we bring a polar molecule near a nonpolar molecule, it can induce a dipole. For example, when water gets near chlorine, the chlorine gains a bit of a dipole. This helps to pull the 2 molecules together, but it’s a weak force. H O Cl Cl H This is an induced dipole force. δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+

17. Notes on Polar Molecules There’s one last one, even weaker than these. If we bring two nonpolar molecules together, they make each other just a little polar as well.

18. Notes on Polar Molecules There’s one last one, even weaker than these. If we bring two nonpolar molecules together, they make each other just a little polar as well. Example: If we bring 2 Br 2 molecules together, they both become a little polar. BrBr Br Br δ-δ- δ+δ+ δ-δ- δ+δ+

19. Notes on Polar Molecules There’s one last one, even weaker than these. If we bring two nonpolar molecules together, they make each other just a little polar as well. Example: If we bring 2 Br 2 molecules together, they both become a little polar. BrBr Br Br This is an extremely weak force and is called a dispersion force or London force. δ-δ- δ+δ+ δ-δ- δ+δ+

20. Notes on Polar Molecules All of these weak forces together are called van der Waals forces and contribute in many small ways to molecular interactions.

21. Exit Question Which end of a water molecule is the partially positive end? a) The hydrogen end b) The oxygen end c) Halfway between the two d) None of the above e) Not enough information