Learn about intermolecular vs. intramolecular forces. Learn the different intermolecular bonds (including hydrogen bonding and dipole-dipole and ion-dipole forces), their strengths, and their effects on properties, such as boiling and melting points, solubility, and evaporation.
Intermolecular Forces
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Intermolecular forces are the forces that exist between molecules. Don’t confuse these with intramolecular forces, which are the strong forces that keep a molecule together. ‘Intra’ means inside, so these are the inside forces in a molecule.
‘Inter’ means between, so these are the forces between molecules. To remember the difference between inter and intra, I always think of ‘interstate.’ In every state, there is an interstate road that goes across more than one state, like Interstate 80 that runs from the Pacific Ocean in California to the Atlantic Ocean in New York.
Intermolecular forces are weaker than intramolecular forces but still very important. The two intermolecular forces we are talking about in this video include dipole-dipole and ion-dipole.
Dipole-Dipole Forces
A dipole is a molecule that has both positive and negative regions. Although we talk as though electrons distribute their time evenly among all atoms in a molecule, some elements have more affinity for the electrons than others, and they hang out around that atom more.
We also talk about these molecules being polar. A polar molecule is a molecule with a slightly positive side and a slightly negative side. I always think about the North and South Poles of the earth to help me remember what a polar molecule is. A dipole-dipole force is when the positive side of a polar molecule attracts the negative side of another polar molecule. In order for this kind of bond to work, the molecules need to be very close to each other, like they are in a liquid.
Ion-Dipole Forces
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Simply put, a hydrogen bond is an attraction between a slightly positive hydrogen on one molecule and a slightly negative atom on another molecule. Hydrogen bonds are dipole-dipole forces.
The large electronegativity difference between hydrogen atoms and several other atoms, such as fluorine, oxygen and nitrogen, causes the bond between them to be polar. The other atoms have more affinity for the shared electrons, so they become slightly negatively charged and hydrogen becomes slightly positively charged. Hydrogen atoms are small, so they can cozy up close to other atoms. This allows them to come very close to the slightly negatively charged unshared electron pair of a nearby atom and create a bond with it. A hydrogen bond is usually represented as a dotted line between the hydrogen and the unshared electron pair of the other electronegative atom.
It looks like this:
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Hydrogen bonds are the strongest of all intermolecular forces. They are extremely important in affecting the properties of water and biological molecules, such as protein. Water is a great example of hydrogen bonding.
Water is a polar molecule composed of two hydrogens and one oxygen. The oxygen is slightly negative, while the hydrogens are slightly positive. The hydrogens form hydrogen bonds with the oxygen of adjacent molecules. Then, those molecules form more bonds with other adjacent water molecules.
It looks like this:
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These bonds may be relatively weak by themselves, but when you have a lot of them together, as in the case of water, they become very strong. These bonds help keep water in a liquid state for a wide range of temperatures.
Hydrogen bonds make water a good solvent because it easily forms hydrogen bonds with other substances. Hydrogen bonding is also important in biology because, among other things, it affects the way a protein molecule folds up, which affects its functional properties.
Effects of Intermolecular Forces
Intermolecular forces cause molecules to behave in ways we would not predict just from their molecular structures. These forces affect the boiling point, evaporation and solubility of certain molecules. The boiling point of certain liquids increases because of the intermolecular forces. In order for a substance to boil, the molecules that were close together in the liquid have to move farther apart. The intermolecular forces make it difficult for the molecules to move apart because they are so attracted to each other, so more energy is needed, which in turn makes the temperature at which something boils much higher.
This is the same idea, only opposite, for changing the melting point of solids. The intermolecular forces of a solid keep it in the solid state longer than would be expected because it doesn’t want to let go of the bonds and have the molecules move farther apart.
Lesson SummaryIntermolecular forces are the forces that exist between molecules. Although weaker than intramolecular forces, they are still strong enough to have effects on boiling point, melting point, evaporation, and solubility of substances. A dipole-dipole force is when the positive side of a polar molecule attracts the negative side of another polar molecule. An ion-dipole force is a force between an ion and a polar molecule. A hydrogen bond is a dipole-dipole force and is an attraction between a slightly positive hydrogen on one molecule and a slightly negative atom on another molecule. Hydrogen bonds are important in the properties of water and in certain biological molecules, such as proteins. Learning OutcomesFollowing this video lesson, you’ll be able to:
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