1. 14.1 A: Intermolecular Forces

2. In the last few chapters…
In Chapter 12, we learned about how chemicals bond together; how the atoms stay together in a molecule.
This chapter discusses how molecules stay together.
Then we learned about gases in Chapter 13.
In this chapter, we will learn about liquids and solids.
Gases are molecules that don’t stay together; the molecules are not attracted to each other.
This chapter discusses why groups of molecules are attracted to each other.

3. Today…
We are going to learn about the forces that are responsible for keep molecules together.

4. The Phases of Matter
The three phases of matter are gas, liquid, and solid.
Gases = low density, highly compressible and fill the container they are in
Liquids = high density, slightly compressible and variable shape
Solids = high density, very slightly compressible, have a rigid shape

5. The Phases of Matter
When you compare these properties of the different phases, which two phases are the most similar?
-Liquids and solids have more in common than they do with gases.
Gas
Liquid
Solid

6. The Phases of Matter
The best way to picture solids is in terms of closely packed, highly ordered particles.
The best way to picture liquids is in terms of particles that are generally quite close together, but with a more disordered arrangement than the solid state and with some empty spaces.

7. Intermolecular Forces
Most substances consisting of small molecules are gases at normal temperatures and pressures. (Like O2)
However, water is an exception.
Even though water molecules are small (smaller than ozone molecules), they can remain a liquid.
Why is this?

8. Intermolecular Forces
The reason why water can remain a liquid is due to “intermolecular forces.”
Intermolecular forces are the forces that occur between the molecules.
These forces are different from bonds (intramolecular forces) - which keep the molecule together.
“Inter-” between
“Intra-” inside
Chapter 12 really focused on intramolecular forces.
This chapter focuses on three intermolecular forces.

9. Intermolecular Forces
The main reason why molecules are attracted to other molecules is because of electric charges.
They will come together if a negative charge can get close to a positive charge.
We are going to look at three ways that this can happen:
Dipole-Dipole Attraction, Hydrogen Bonding and London Dispersion Forces

10. The Dipole-Dipole Attraction
Remembering dipole moments (or “dipoles” for short)…
Dipole moments are represented by arrows and show polarity.
Polarity means “uneven sharing” of electrons.
They really point to where all the electrons want to go.
So if we illustrate what a polar molecule looks like, it would have a positively charged side and a negatively charged side.

11. The Dipole-Dipole Attraction
Also, remember that “like-charges” repel and “unlike-charges” attract. So the negatively charged side of a molecule will be attracted to the positively charged side of another molecule.
Molecules with dipole moments can attract each other by lining up so that the positive and negative ends are close to each other.
This is called a dipole-dipole attraction.

12. The Dipole-Dipole Attraction
In a liquid, there is no guarantee that every positively charged side will find a negatively charged side to be next to.
So in liquids, the dipoles find the best compromise between attraction and repulsion.

13. The Dipole-Dipole Attraction
How strong are dipole-dipole attractions?
They are only about 1% as strong as covalent or ionic bonds, and they become weaker as the distance between the dipoles increases.
In gases, the molecules are usually very far apart, so the dipole-dipole forces are virtually nonexistent.

14. Hydrogen Bonding
This is really just a special name for unusually strong dipole-dipole attractions that occur among molecules in which hydrogen is bonded to a highly electronegative atom.
Why can there be a very strong dipole-dipole attraction when hydrogen is involved?
-Due to the small size of hydrogen, other molecules can get incredibly close to the proton of hydrogen, which is what they want.

15. London Dispersion Forces
So far, we looked at how molecules stick together when they have dipoles.
Now we are going to try to explain why molecules without dipoles can stick together.

16. London Dispersion Forces
Scientists have found that…
Even molecules without dipole moments exert forces on each other.
Even noble gases (which do not want to share electrons) will be attracted to each other at cold enough temperatures (they are condensed enough to become a liquid).

17. London Dispersion Forces
These are the forces that exist among noble gas atoms and nonpolar molecules.
But if these molecules are nonpolar (their electrons are evenly shared), and molecules are only attracted to each other if they have a unlike charges, how can they attract each other?

18. London Dispersion Forces
Even though we consider nonpolar molecules to share their electrons evenly, that doesn’t mean that, at a certain point in time, there are more electrons around one side of the molecule than the other.

19. London Dispersion Forces
If there are more electrons at one side, the atoms can then develop a temporary dipole!
This instantaneous dipole can then induce a similar dipole in the neighboring molecule.
The attraction that is formed is weak and short-lived.

20. London Dispersion Forces
Even though they are short-lived, they can still be very significant.
As a matter of fact, the larger the atom / molecule, the more significant the London forces become.
Larger size means there are more electrons available to form the dipoles.

21. Summing Up the Forces
We have just learned about the three “different” intermolecular forces:
-Dipole-Dipole
-Hydrogen Bonding (a strong dipole formed by hydrogen)
-London Dispersion (a weak, short-lived dipole in nonpolar molecules)
As we can see, all of these forces have to do with the fact that unlike charges attract.