Intermolecular forces: Background

Transcription

1 Intermolecular forces: Background Electrostatics Up until now, we have just discussed attractions between molecules in the area of the covalent bond. Here, atoms within a molecule are attracted to one another by the sharing of electrons. This is called an intramolecular force. We know how the atoms in a molecule are held together, but why do molecules in a liquid or solid stick around each other? What makes the molecules attracted to one another? These forces are called intermolecular forces, and are in general much weaker than the intramolecular forces. We have, however, already discussed a very strong type of force that is responsible for much of chemistry - electrostatics. The attraction of a positive charge with a negative charge is the force that allows for the structure of the atom, causes atoms to stick together to form molecules; both ionic and covalent, and ultimately is responsible for the formation of liquids, solids and solutions. London dispersion forces The forces that hold molecules together in the liquid, solid and solution phases are quite weak. They are generally called London dispersion forces. We already know that the electrons in the orbitals of molecules are free to move around. As such, if you would compare a "snapshots" of a molecule at an instant in time, you would see that there would be slightly different charge distributions caused by the different positions of the electrons in the orbitals. Just how much difference one sees as a function of time is based on the polarizability of the molecule, which is a measure of how well electrons can move about in their orbitals. In general, the polarizability increases as the size of the orbital increases; since the electrons are further out from the nucleus they are less strongly bound and can move about the molecule more easily. Given that two molecules can come close together, these variations in charge can create a situation where one end of a molecule might be slightly negative and the near end of the other molecule could be slightly positive. This would result in a slight attraction of the two molecules (until the charges moved around again) but is responsible for the attractive London dispersion forces all molecules have.

2 However, these London dispersion forces are weak, the weakest of all the intermolecular forces. Their strength increases with increasing total electrons. Dipole-dipole attractions What would happen if we had a beaker of polar molecules, like ClF, In addition to the attractive London dispersion forces, we now have a situation where the molecule is polar. We say that the molecule has a permanent dipole. Now, the molecules line up. The positive ends end up near to another molecule's negative end: Since this dipole is permanent, the attraction is stronger. However, we only see this sort of attraction between molecules that are polar. It is usually referred to as dipole - dipole interaction. The strength of this attraction increases with increasing total number of electrons. Hydrogen bond Hydrogen is a special element. Because its nucleus is really just a proton, it turns out that it can form a special type intermolecular interaction called the hydrogen bond. If the hydrogen in a molecule is bonded to a highly electronegative atom in the second

3 period (N, O, or F only) with a lone pair of electrons, a hydrogen bond will be formed. In essence the three elements listed above will grab the electrons for itself, and leave the hydrogen atom with virtually no electron density (since it had only the one). Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back. This ends up forming a partial bond, which we describe as the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces (except for the ionic bond). Hydrogen bonding interactions in water : Could the ClF molecule exhibit hydrogen bonding? The answer is no, since the hydrogen must be bound to either N, O, or F. Just having one of those species in the molecule is not enough. Trends in the forces While the intramolecular forces keep the atoms in a molecule together and are the basis for the chemical properties, the intermolecular forces are those that keep the molecules themselves together and are virtually responsible for all the physical

4 properties of a material. For small molecules, the intermolecular forces increase in strength according to the following: London dispersion < dipole-dipole < H-bonding < ion-ion Now, as these things increase in strength it becomes harder to remove the molecules from each other. Therefore, one would expect the melting and boiling points to be higher for those substances which have strong intermolecular forces. We know that it takes energy to go from a solid to a liquid to a gas. This energy is directly related to the strength of attraction between molecules in the condensed phases. Since energy is directly proportional to the temperature, the above trends ought to hold true. In addition, there are energies associated with making these phase transitions: Each of these processes are endothermic, and scale with the magnitude of the intermolecular forces. Thus, as these intermolecular forces increase, so do the energies requires to melt, vaporize, or sublime (go from solid to a gas) a species. Every substance also has an associated vapor pressure with it. The vapor pressure is defined to be the amount of gas of a compound that is in equilibrium with the liquid or solid. If the intermolecular forces are weak, then molecules can break out of the solid or liquid more easily into the gas phase. Consider two different liquids, one polar one not, contained in two separate boxes. We would expect the molecules to more easily break away from the bulk for the non-polar case. This would mean that, proportionately, there are more molecules in the gas phase for the non-polar liquid. This would increase the vapor pressure. Thus, unlike the physical properties listed above, the vapor pressure of a substance decreases with increasing intermolecular forces.

5 Name: Pre-Laboratory Questions 1. Read about intermolecular forces in your textbook. Construct a table comparing the following intermolecular forces: dipole-dipole interactions, hydrogen bonds and London Forces. Force Origins Occurs between (polar or non polar Molecules) Strength Example 2. Show the hydrogen bonding interactions in the following a) ammonia b) A mixture of water and methanol, CH 3 OH

6 Procedure Part 1 Assemble at you table a dropper and a little bit of water, alcohol and acetone in small test tubes (~ 1 cm high). Place one drop of each liquid on the table, in a place where the drops won t be disrupted, and observe which evaporates first. (They will evaporate even faster if you place them in the hood.) Make sure your drops are not in your way as you will proceed with part 2 while you wait for your drops to evaporate. You may blow lightly over the tops of the drops to speed the process as long as you try to blow equally across all three. Write down your results later on. Part 2 1. Rub a penny clean with a paper towel to remove any oil that might be on the surface. After this, try not to handle the penny with your bare hands as this will deposit additional oil on the surface. 2. Place the penny on a paper towel. Predict how many drops of water you will be able to place on the penny without getting the paper towel wet. Record your prediction in the table below. 3. Now place drops of water on top of the penny, one drop at a time, using a disposable pipet. Note the shape of the water on the penny as the water increases in volume. Continue adding and counting water drops until the water spills onto the paper towel. Record the number of drops in the table. Describe the shape of the water on top of the penny or draw a picture. 4. To repeat for a 2 nd trial, what variables should you keep constant so that you could get reproducible results? 5. Do the 2 nd trial and record the drops. Find an average of the two values. (If they are fairly different, you may want to do and record a third trial before averaging). 6. Repeat the procedure above with alcohol, C 2 H 5 OH, and then with acetone, CH 3 COCH 3, after you have made predictions. Make sure the drops of each are about the same size as the drops of water you used before. Note the shapes of each. Record your results in the table. 7. Now add 2 drops of liquid soap from the cart to your test tube with water and stir. Determine how many drops of the soap/water mixture can be piled on top of the penny before it spills on to the paper towel. Record your results of at least 2 trials in the table.

7 Data Sheet Part 1 Data: (record these observations after you complete part 2) Observations:

8 Part 2 Data: Prediction # of drops Trial 1 Trial 2 Trial 3 # of drops # of drops # of drops Average # of drops Water H 2 O Alcohol* C 2 H 5 OH Acetone CH 3 COCH 3 Soap water (as explained in step 6) *ethanol

9 Post Lab Questions Critical Thinking Skill: Constructing convincing arguments. Argument: A train of reasoning in which claims and supported reasons are linked to establish a position. 1. State your position first: Which liquid had the strongest intermolecular forces and which had the weakest: 2. When you build your argument, what data (results from three liquids in the evaporation experiment and results from three liquids in the penny experiment) supports your position? What data does not support your position? Supports- Does not support 3. Now construct your argument. Write a full paragraph conclusion in which you rank the strength of the intermolecular forces of these three liquids and explain how your data supports that ranking. Your argument will be evaluated on three criteria: Is it complete? Is it correct? Is it clear?

10 4. If you put your three liquids into beakers with a thermometer and heated them on a hotplate, one liquid would start to boil at 56 C, one would start to boil at 78 C and one would start to boil at 100 C. Considering this boiling point data in connection with your evaporation data: which liquid is water? ethanol? acetone?