What are Intermolecular Forces?
Topics Covered: Intermolecular forces, London-Dispersion forces, dipole-dipole attraction, Hydrogen bond
Have you ever wondered why acetone evaporates faster than water when both are dropped to the ground at the same time? In this post, we will cover the definition and types of intermolecular forces (aka IMF), a common chemical concept that helps explain a variety of physical properties that are relevant in our everyday lives.
Intermolecular forces, as you can guess from the name, are forces that exist between the molecules. The stronger the IMFs, the stronger the molecules are attracted to each other. They are not to be confused with intramolecular forces (which we’ll cover in later posts). There are three types of intermolecular forces, and we will cover them all in this blog!
First is London-dispersion forces. They are the weakest of all three IMFs, and all molecules and atoms, whether they are polar, nonpolar, ionic or covalent, will have them. London dispersion forces are caused by the fluctuations in electron distribution in atoms and molecules, which will result in a temporary dipole (positive and negative charges are separated). Sounds complicated? Think about this. Every molecule with electrons will have a certain region that has excess electron density and depleted electron density. Then, the region with excess electrons would have partial negative charge while the region with depleted electrons would have partial positive charge. If that happens, that molecule would “induce” a dipole from another molecule and disturb the electron arrangement of that molecule, which is why London-dispersion forces (aka LDFs) are also known as induced dipoles.
One thing to note here. Remember I said LDFs are caused by fluctuations in electron distributions? Therefore, if a molecule has a larger molar mass, it will have more electrons and larger electron clouds, so more electrons would be “induced” and LDF will be stronger.
The next is dipole-dipole attraction. Unlike LDFs which exist in every molecule, dipole-dipole attraction only exists in polar molecules. Since polar molecules have a permanent dipole, they would always have partial positive and negative regions, which would attract neighboring molecules. And because these dipoles are permanent, not temporary, the force of dipole-dipole attraction is much stronger than the LDFs.
Last, but certainly not least, is hydrogen bonding. Hydrogen bonding is a type of dipole-dipole attraction, but we give it a different name because this force is so strong. Hydrogen bonding occurs when a very electronegative atom (Fluorine, Oxygen, or Nitrogen) is bonded to hydrogen. When that happens, because the atom is so electronegative, it will strongly pull the bonding electrons of hydrogen away, and since hydrogen has only one electron, when its only electron is pulled away, its nucleus will become de-shielded, exposing the proton of the nucleus. Then, the exposed proton will act as a very strong positive charge, which would attract all the electron clouds from neighboring molecules.
It must not be surprising to hear that hydrogen bonding is the strongest bond out of all three IMF. One thing to note again, hydrogen bond only occurs when both sides of hydrogen are bonded to Fluorine, Oxygen, or Nitrogen (aka FON!). If one side is bonded to Nitrogen but the other is bonded to Carbon, it will NOT be considered as hydrogen bonding.
Now here’s the question: why are these IMFs so important? Intermolecular forces govern different physical properties of molecules. Generally, molecules that have greater IMFs have higher boiling point and melting point. If you think about it, this makes sense because having greater IMFs means the molecules are more strongly attracted to nearby molecules, so it will be harder to separate them!
Finally, let’s go back to our original question: why does alcohol evaporate faster than water? The answer lies in their IMFs. While both acetone (chemically called propanone) and water molecules experience dipole-dipole and London dispersion forces, water also experiences Hydrogen bonding, which is a stronger intermolecular force!* Therefore, the IMFs between water molecules are stronger, so water has a higher boiling point and will not evaporate as quickly as alcohol!
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*It is important to note that a molecule that has hydrogen bonding will NOT ALWAYS have the stronger intermolecular forces. Sometimes, if the other molecule has much greater molar mass, its London-dispersion forces could outweigh the strength of the Hydrogen bonding of the other molecule. Just remember, chemistry is always context-dependent and full of exceptions!
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