8/13/2019 Chem 1AL Lab Exam 2013

1/3

Dr. Pete Marsden Fall 2013 Chem 1AL

Page 1of 3

Intermolecular Forces

1. In this class (Chem 1AL), we will focus mainly on the way molecules interact witheach other in solution. It is not that their interactions in the gas phase or in thesolid phase are unimportant, only that for what we are learning in Chem 1AL,their behavior in solution is more relevant.

2. There are three major classes of interactions:a. Dipole-dipole (polar molecule with polar molecule)b. Induced dipole-induced dipole (nonpolar molecule with nonpolar molecule)c. Dipole-induced dipole (polar molecule with nonpolar molecule)

3. Dipole-Dipole interactionsa. First, well look at two identical molecules interacting with each other.

b. When these molecules are in solution, their molecular dipoles align suchthat the positive end of one molecule is close to the negative end ofanother. This is the same way that magnets align. The physical law thatgoverns this is Coulombs Law.

c. A particularly strong dipole-dipole interaction is the hydrogen bond. Thisparticular type of interaction is very important to both chemistry andbiology as youll see in the coming semesters here at Cal.

i. The hydrogen bond is not a real bond, but is still very strong. Togive an idea for how strong these interactions are, think of a glass

of water filled to the brim. The water will create a dome beforespilling over the edge.ii. Hydrogen bonds are formed with the lone pair of one neutral or

negatively charged electronegative atom on a molecule (H-BondAcceptor) with a hydrogen atom (H-Bond Donor) bound to a veryelectronegative atom (such as N, O, or F).

iii. The dashed lines drawn above are the hydrogen bonds.iv. It is not necessary for a molecule to possess both a hydrogen bond

donor and a hydrogen bond acceptor to participate in hydrogenbonding.

v. Label the molecules on the following page as donors, acceptors, orboth. Draw in the lone pairs and circle the hydrogen capable ofhydrogen bonding.

8/13/2019 Chem 1AL Lab Exam 2013

2/3

Dr. Pete Marsden Fall 2013 Chem 1AL

Page 2of 3

4. Induced Dipole-Induced Dipole interactionsa. These interactions are caused by the London dispersion force which is a

type of van der Waals force. This particular force is responsible for theextremely strong adhesion of geckos feet.

b. When nonpolar molecules are isolated, they have random, minutemolecular dipoles that are constantly changing, and always sum to zero.When these molecules are brought near each other, those random shiftsin electron density cause similar shifts in nearby molecules, resulting in anoverall attractive force.

5. Dipole-Induced Dipole interactionsa. These interactions occur when there are two different molecules, one

polar, and the other is nonpolar. The dipole of one molecule can cause theelectrons of another molecule to realign in order to form a strongerinteraction, similar to the way nonpolar molecules spontaneously aligntheir momentary dipoles.

6. Intermolecular forces applied to solubilitya. Unlike charges, where opposites attract, solutions tend to prefer similar

conditions. By this, I mean that like dissolves like, and unlikes repel eachother.

b. This is something youve undoubtedly seen while cooking. Water repelsoil. The dipole-induced dipole interactions are nowhere near strongenough to break up the hydrogen-bonding of water.

c. However, if you were to take nail polish remover (acetone) and mix it withwater, youd see that they are quite soluble with each other. The dipole ofthe acetone, as well as its ability to be a hydrogen bond acceptor, allow forthe two molecules to mix.

8/13/2019 Chem 1AL Lab Exam 2013

3/3

Dr. Pete Marsden Fall 2013 Chem 1AL

Page 3of 3

d. Solubility in water is a very tricky subject, and fairly difficult to predictwithout doing the experiment. Since water prefers hydrogen bonding withitself so much, some polar organic compounds are quite insoluble in it. For

example, CHCl3(chloroform) has a very strong molecular dipole moment,but is very insoluble with water. This is because the chloroform cannotoffer new hydrogen bonds to the water.