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Intermolecular Forces

What are intermolecular forces?

Intermolecular forces are electrostatic interactions between permanently or transiently (temporarily) charged chemical species. The term is usually used to refer only to attractive interactions, which hold molecules and ions together in condensed phases (liquid and solid). These forces govern many of the bulk physical properties of substances and mixtures, such as melting point, boiling point, and surface tension.

Related Topics

Ionic bonding basics

Ion-ion forces, also known as ionic bonding, are the simplest to understand. These forces arise from the electrostatic attraction between two ions with opposite charges. They are not technically considered intermolecular forces, but are a helpful starting point for understanding the true IMFs (intermolecular forces). Ionic bonds are also generally stronger than the forces discussed below, which is why most pure salts are solids except at extremely high temperatures.

A perfect example of this is table salt, NaCl, which has a melting point of 800 degrees Celsius.

Types of intermolecular forces

Ion-dipole forces

Ion-dipole forces result from the interaction of a charged species with a polar molecule. They are very similar to ionic bonds, but tend to be weaker because polar molecules only possess partial electric charge, which generate less electrostatic attraction. Because of these forces, polar solvents are better able to dissolve ionic solids such as NaCl, compared with nonpolar solvents.

Ion-induced dipole interactions

Closely related to ion-dipole forces are ion-induced dipole forces. In this case, there is no permanent dipole on the molecule. Instead, the ion generates a transient dipole from a nonpolar molecule by attracting or repelling its electrons. An attraction then forms between the ion and transient partial charge.

Dipole-dipole forces

When two polar molecules interact, opposite partial charges attract, similarly to ionic bonding, but generally weaker, because of the smaller charge magnitude. Because of these forces, polar compounds tend to have higher melting and boiling points than nonpolar compounds.

Two simple polar molecules with a dipole-dipole interaction shown as a dotted line. Note that the molecules do not need to be oriented in any special way, as long as the positive (green) end of one is interacting with the negative (orange) end of another.

Hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction. It can only occur when the molecules in question have a highly electronegative atom directly bonded to a hydrogen atom, leading to an unusually extreme dipole. For most purposes, these highly electronegative atoms are limited to only nitrogen, oxygen and fluorine.

Hydrogen bonding involves a “donor” molecule and an “acceptor” molecule. The donor provides the hydrogen atom for the bond, while the acceptor provides the electronegative atom. In the image below, the top two water molecules are both acting as donors, while the bottom molecule is acting as an acceptor. Some molecules can only act as acceptors.

Water molecules participate in hydrogen bonding. This gives water its characteristic high boiling point as well as low density in the solid state, which is why ice floats on liquid water.

Hydrogen bonding is also directional – a bond can only qualify as a hydrogen bond if the three participating atoms are in roughly a straight line (180-degree angle). This sets it apart further from ordinary dipole-dipole bonding, which has no directionality.

London dispersion forces (LDF)

London dispersion forces, often abbreviated to LDF, are attractive forces between two transient dipoles. This may seem non-intuitive, but when two nonpolar molecules are near each other, the oscillations in their electron clouds can cause them both to acquire some polarity. We refer to the resulting attraction between transient charges as London dispersion forces.

On an individual basis, LDF are generally the weakest of the intermolecular forces. The cumulative effect of many LDF interactions, however, can result in quite high overall attraction. The number of interactions is closely related to the surface contact area of the molecules, so a large nonpolar molecule may experience quite a large amount of attraction from LDF, while a small, compact one may experience very little. When comparing two molecules of a similar shape (e.g. two noble gases), the one with the higher molar mass will have stronger LDF.

Smaller, more compact molecules (left) have fewer opportunities for LDF than larger molecules with long chains (right). These larger molecules can have much higher boiling points than compact molecules with the same molar mass.

It is important to note that although London dispersion forces are the only IMFs present in nonpolar molecules, they also exist in all other types of substances. Polar molecules also participate in LDF, but this is sometimes not mentioned because they are less important than the other IMFs in those cases.

Intermolecular Forces: Strongest to Weakest

  1. Ionic (not technically an IMF)
  2. Ion-dipole
  3. Hydrogen
  4. Dipole-dipole
  5. LDF

Examples: Ranking IMF Strength

1. Rank the following mixtures from strongest to weakest IMFs:

  • Ethanol and ammonia
  • Water and potassium chloride
  • Octane and methane
  • Chloroform and acetone

2. Rank the following pure substances from highest to lowest boiling point:

  • Propane
  • Water
  • Dichloromethane

Solutions:

1. Look for the strongest interactions between each pair of compounds.

  1. Water and potassium chloride
    These two are a polar molecule and an ionic compound, so ion-dipole forces exist between them. These are the strongest intermolecular forces, generally.
  2. Ethanol and ammonia
    These are both polar molecules, so they have dipole-dipole forces, but more importantly they are both capable of hydrogen bonding, which is stronger than ordinary dipole-dipole interactions.
  3. Chloroform and acetone
    These are both polar molecules that cannot engage in hydrogen bonding. They have dipole-dipole interactions.
  4. Octane and methane are both nonpolar molecules, so they have only London dispersion forces. These are the weakest IMFs.
    NOTE: remember that ALL substances have London dispersion forces in addition to whatever other IMFs they have!

2. Recall that boiling points are related to IMF strength. The stronger the IMFs, the higher the boiling point.

  1. Water has the highest boiling point, because it participates in dipole-dipole forces and hydrogen bonding.
  2. Dichloromethane is next highest because it participates in dipole-dipole forces.
  3. Propane has the lowest boiling point because it participates only in London dispersion forces.

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