Intermolecular+Forces

Intermolecular Forces: What Are Intermolecular Forces (IMF)? Intermolecular forces are the attractive forces that hold molecules together. They are weaker than intramolecular forces (the forces in between different atoms within a molecule ex. the bond between hydrogen and fluorine in hydrogen fluoride), weaker still than an ionic bond, yet intermolecular forces are often responsible for what holds together solids, liquids, and to some extent gases.

Dipole-Dipole forces: Dipole – dipole attractive forces are the result of unequal sharing of electrons. It is useful to check the electronegativity numbers of different atoms as dipole – dipole forces only occur on polar molecules (molecules that have great differences in electro negativity, have lone pairs on the central atom, and/or are not symmetrical in design). Consider hydrogen and chlorine. Chlorine has far more electrons, creating a larger cloud of electrons around the nucleolus of the chlorine atom and pulling the shared electron pare closer as well. This creates a partial negative charge towards the chlorine atom (represented by sigma negative: -) and partially positive charge near the hydrogen atom (represented by sigma positive: +). Dipole – dipole forces typically increase in strength as they decrease in mass. This is due to the ability for molecules to get closer together in correctly orient themselves and create a stronger pull. Dipole – dipole forces are only about 1% as strong as a covalent or ionic bond. Test answer tip: Remember intermolecular forces are still not as strong as an ionic bond.

Hydrogen Bonding: Hydrogen bonding is a type of strong dipole – dipole attractive force with a particularly strong bond brought on by high electronegativity differences an atom and hydrogen. When it comes to hydrogen bonding think FON. Hydrogen bonding occurs between H-F, H-O, and H-N bonds. A hydrogen bond is 4 to 5 times as strong as a typical dipole – dipole bond. A typically seen chart is a chart of homologue series that lists the boiling points of the elements in the same group bonded to hydrogen. The trend is that the boiling points of H2O, HF, and HN3 are much higher than any other in their group.

Test answer tip: Whenever you see hydrogen bonded with fluorine, oxygen, or nitrogen on an IMF question immediately know that hydrogen bonding should be mentioned somewhere in your answer. Fun Activity: Take a dry piece of thick string or yarn. Place it underneath a faucet at an angle and turn on the water. The string should cut through the stream of water and break it into two. Now wet the string. Place it again at an angle under the faucet once again. Much of the water should now travel down the length of the string rather than falling to the bottom of the sink. This is due to the hydrogen bonds of the water molecules that pull grab hold of the other water molecules and pull them down the course of the string. Van Der Waals (London Dispersion) Forces: Although there is not a large difference in electronegativity there may at a point in time be more electrons on one side of a molecule as the electrons move in there orbit. This creates a momentary charge in the molecule causing temporary polarization and what we call Van Der Waal or London Dispersion forces. Van Der Waals forces are present in ALL molecules. They are the only IMF present between nonpolar molecules and noble gases. Strength of Van Der Waals forces increases with molecular mass. This is causes by a large electron cloud where the electrons and become temporarily polarized. However, these are the weakest type of intermolecular force. Test answer tip: You can never go wrong mentioning Van Der Waals forces in an intermolecular forces question. When comparing two molecules both with Van Der Waals forces make sure to use the test favorite word //polarizability// (typically referring to one molecule with greater having greater polarizability because of its large electron cloud) and state, the ability for an electron to be temporarily distorted through to the unequal spread of electrons causing it to be momentarily polarized. Intermolecular Forces in States of Matter: To change from solid stage to a liquid stage, or a liquid to a gaseous phase, molecules have to overcome the strength of their intermolecular forces (discussed later). Gases: Gases have very weak intermolecular forces. The average kinetic energy of their molecules is much greater than the energy of the attractive forces. Gases are the least dense of these phases. Liquids: Liquids have stronger intermolecular forces that hold molecules together, yet do not hold them in place. Liquids are denser then gases. Solids: Solids have the strongest IMF. They keep the molecules closely compacted and rigid in shape. Solids are typically denser than liquid, the exception being water. Water is unique in its form because its solid form is actually denser than the liquid, allowing ice to float, which is a lucky thing. If not lakes would not freeze over but rather, freeze all the way through, killing marine life. Affects of Intermolecular Forces: · __ Surface tension __ is the inward amount of force that pulls liquid molecules towards one another. The higher the IMF the higher the surface tension. · __ Viscosity __ is a liquids resistance to flow. The stronger the intermolecular forces the more viscous a liquid. · The __Boiling Point__ is the temperature at which a liquid begins to change into gas form. The __Melting Point__ is the temperature at which a solid begins to change into a liquid. The stronger the IMF the higher the boiling point and melting point (higher amount of energy needed to break the bonds). · Some solids have such weak intermolecular forces that they will __sublime__; go straight from solid to gas. · __ Vapor Pressure __ is the pressure within a liquid the causes particals to escape the liquid and rise into the air. The increase in strength of IMF the decrease of vapor pressure. Overcoming Intermolecular Forces: There are two main ways to overcome IMF. The first is adding heat. The stronger the forces the more heat that needs to be added. The second is removing pressure. The more pressure that is added the stronger IMF. The loser the pressure, the weaker they become. Sample Questions: Test answer tip: Best to always discuss both molecules in your answer for ful credit. 1) Explain why ethyl alcohol (C2H5OH) has a higher boiling point (78.40 C) than methyl alcohol (CH3OH; 64.70 C). At first glance we can see that both ethyl alcohol and methyl alcohol have one O-H bond which means that there is some amount of hydrogen bonding involved in both. The next thing to check would be the Van Der Waal forces. Ethyl alcohol has a higher molecular weight which gives it stronger London Dispersion forces because it has a larger electron cloud and it has more polarizablilty, accounting for its higher boiling point. 2)  Predict which compound in each pair will have the higher melting point.

(a) CS2 or CCl4 The first thing to realize is that neither of these are polar. After realizing this we check the London dispersion forces. The molecular weight of CCl4 is much greater and makes it more polarizable.

(b) CH4 or NH3 The first thing to always jump to mind the moment you see H-N bond is hydrogen bonding. Hydrogen bonds are the strongest type of intermolecular forces so it will obviously have the higher melting point.

(c) CHCl3 or CF4 The fact that CHCl3 is polar is the first thing to notice, which means it has dipole – dipole forces. CF4 we know is nonpolar. Remember to state that it is so and can therefore only have Van Der Waal forces. 3)    5 balloons are filled with different gases: CO2, O2, He, N2, and CH4. Which would you expect to deviate most from the expected behaviors of an ideal gas? Now this question requires understanding of a bit of gas characteristics as well and intermolecular forces. Intermolecular forces interfere with the behavior of gases, which is why ideal gas formulas work best at high temperatures and low pressures when IMF are weak. So whichever has the strongest intermolecular forces should vary the most. Firstly we check for polarity. All balloons are nonpolar. Next we move to the weight. Because CO2 has the largest molecular weight its london dispersion forces will be the strongest. So, we can predict that CO2 will deviate most from the behavior of an idea gas.

Animations and Videos: Dipole – dipole forces: []

Dipole – dipole forces and London dispersion forces: []

Van Der Waal Forces: []

OverView: []

Water IMF: []

Overview Video: [] References and Useful Websites for More Information: Websites: [] *very useful [] [] [] [] Books: Barron’s: AP Chemisty 2008, 4th Edition Chemistry: The Central Science AP* Edition Worksheet: K. Arno MnKinney HS, IM Forces pg 1-6