Intermolecular forces are attractive and repulsive forces between molecules. Intermolecular forces are weaker than intramolecular forces but are strong enough to influence physical properties of substances. There are several different types of intermolecular forces. Most fall under the category of "van der Waals forces." In class, I learned of four important van der Waals forces: dipole-dipole interactions, dipole induced dipole interactions, induced dipole-induced dipole interactions (a.k.a. London dispersion forces), and Hydrogen bonding.
Dipole-dipole interactions are attractive forces between the partially negative end of a polar molecule and the partially positive end of another molecule. Although the strength of dipole-dipole interactions are dependent on the magnitude of a substance's dipole moment and the proximity of its molecules, in general dipole-dipole attractions are relatively weak. Dipole-dipole interactions are only present in polar substances.
Hydrogen bonds are closely related to dipole-dipole interactions. Hydrogen bonds occur when partially positive hydrogen atoms are attracted to very electronegative partially negative atoms like oxygen, fluorine, and nitrogen. Although in essence they are the same as dipole-dipole interactions, they have their own classification because they are much stronger than typical dipole-dipole forces. Hydrogen bonds are the strongest of all of the van der Waals forces.
When a polar molecule approaches a nonpolar molecule, the electrons in the nonpolar molecule, usually shared evenly, are attracted to the polar molecule. This results in an induced dipole in the nonpolar molecule. This interaction is aptly named a dipole-induced dipole interaction. Much like the dipole-dipole interaction, it too is relatively weak.
Induced dipole-induced dipole interactions are the most common. Electrons are constantly in motion. Even in nonpolar molecules, there is a probability that at some point there will be an imbalance of electrons (this distortion in electron clouds is called polarizability). When such an imbalance occurs, a temporary dipole is produced. This temporary dipole induces a temporary dipole in other molecules. This intermolecular force, also called the London dispersion force, is present in all substances.
Additionally, I learned this week about ion-dipole interactions. Ion-dipole interactions are forces between ions and molecules. They are not van der Waals forces. Ion-dipole interactions make it possible for some ionic substances to dissolve in water. If the forces of attraction between atoms in an ionic compound are greater than the ion-dipole force between the ionic compound and water, the substance will not dissolve in water. If the forces of attraction between atoms in an ionic compound are lesser than the ion-dipole force between the ionic compound and water, the substance will dissolve in water.
Intermolecular forces have a great effect on the physical properties of substances. For example, the stronger the intermolecular forces, the higher the boiling and melting point of a substance. Intermolecular forces also affect the vapor pressures and viscosity. To help learn the concepts associated with intermolecular forces, I completed three lectures (one on dipoles and induced dipoles, one on hydrogen bonds, and one on ion-dipole interactions) as well as a the Intermolecular Forces POGIL and the Water POGIL. In addition, we went over the Intermolecular Forces I worksheet. Initially I found the concept of intermolecular forces to be confusing. I found the scanned textbook chapter you gave us in class on Tuesday to be challenging reading. I did not comprehend the text well (the fact that I was running on a minuscule amount of sleep certainly did not help in this regard). However, I found that the lectures brought me to understand these topics well.
 |
| A demonstration from class this past week. Butane is lit on fire. In the can, butane is in liquid form because it is under very high pressure. However, when it was sprayed into a test tube, it immediately started to boil, as butane normally has a very low boiling point. When lit on fire, the butane gas burned. |
No comments:
Post a Comment