Effect of Intermolecular Forces on Physical Properties

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will review what intermolecular forces are and how they will affect physical properties such as boiling point, freezing point, melting point, and vapor pressure.

Intermolecular Forces

Imagine you just broke your favorite lamp. You have several different types of glue to put it back together. If you choose a weaker glue, it won't take much force for the lamp to fall apart again, while using a stronger glue would require a lot more force to break that bond.

Intermolecular forces are like the glue, only instead of holding a lamp together, intermolecular forces hold molecules together. There are strong and weak forces; the stronger the force, the more energy is required to break those molecules apart from each other.

Intermolecular forces include (listed from weakest to strongest):

  • Van der Waals dispersion forces
  • Van der Waals dipole-dipole interactions
  • Hydrogen bonding
  • Ionic bonds

So, if two molecules are only connected using van der Waals dispersion forces, then it would require very little energy to break those molecules apart from each other. On the other hand, if two molecules are connected using ionic bonds, it takes a whole lot more energy to break those two apart.

Intermolecular Forces and Physical Properties

Stronger intermolecular forces will also result in a higher physical properties such as higher melting or boiling points, which require breaking molecules apart. Higher intermolecular forces also leads to a higher freezing point, but since we are talking about lowering the temperature for freezing points, we often say that lower intermolecular forces requires lowering the temperature more.

Since a higher vapor pressure means that it is easier to vaporize a compound, this means that lower intermolecular forces leads to a higher vapor pressure.

Van der Waals Dispersion Forces

Van der Waals dispersion forces, also called London forces, occur due to instantaneous dipoles. At any given moment the electrons in a molecule or atom may not be evenly distributed around the molecule. If more electrons are on the left side of the molecule than on the right side, then there will be a slight negative charge on the left side of the molecule. The side with fewer electrons will have a slight positive charge.

These momentary, slight, positive and negative charges are attracted to each other (like the positive and negative ends on a magnet). This causes momentary bonds between molecules. We can already see why these bonds would be so weak, because they only last for a little while.

Van der Waals dispersion forces increase as the atomic size increases. This means that larger molecules will feel more force, thus increasing the intermolecular forces. So if we have two molecules that are exactly the same except that one is bigger than the other (such as methane and ethane), then the intermolecular forces of the bigger one will be stronger than for the smaller one.

So, in van der Waals dispersion forces:

  • Boiling point is higher for larger compounds
  • Melting point is higher for larger compounds
  • Freezing point is lower for smaller compounds
  • Vapor pressure is higher for smaller compounds

Van der Waals Dipole-Dipole Interactions

A partial positive charge and a partial negative charge can be created between two atoms when there is a difference in electronegativity. These interactions are called van der Waals dipole-dipole interactions.

For example, carbon is less electronegative than oxygen, creating a partial positive on carbon and a partial negative on oxygen. The dipole interactions are stronger than the dispersion forces because the oxygen will almost always have slightly more electrons than the carbon, instead of constantly changing. There still isn't a full negative charge on the oxygen, or a full positive charge on the carbon. But the partial positive and negative charges are still enough to attract opposite charges together.

The higher the difference in electronegativity, the strong the dipole-dipole interactions will be. So compounds with a higher electronegativity difference will have strong intermolecular forces.

Hydrogen Bonding

Under certain circumstances the dipole bonds created from hydrogen can create very strong bonds. Let's say that you have an atom that is attached to several hydrogen atoms: such as the carbon in methane attached to 4 hydrogen atoms.

The hydrogen atoms will surround the carbon atom, putting a partial positive charge all around the methane. But, if you have a cup full of methane, then all of the molecules will be surrounded by a partial positive charge, causing the molecules to push away from each other.

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