Strength of Intermolecular Forces

Strength of Intermolecular Forces in Solids, Liquids, and Gases

Intermolecular forces are attractive forces that hold neighboring molecules together. Intermolecular forces affect the physical properties of molecules.

There are four types of intermolecular forces you should be familiar with, as you will most likely see them in your AP exam!

1. Ion-dipole forces: attractive forces that occur between an ion and a polar (dipole) molecule.
2. Hydrogen Bonding: forces of attraction between a hydrogen atom covalently bonded to a highly electronegative atom (F, N or O) and the F, N or O of another molecule.
3. Dipole-dipole forces: attractive forces that occur between the positive end of a polar molecule and the negative end of another polar molecule. In dipole-dipole forces, the larger the dipole moment, the greater the force.
4. London dispersion forces: weak, attractive forces that is present in all molecules. It is also the only intermolecular force present in non-polar molecules. LDF is dependent on size and surface area. Heavier molecules (higher molecular weight) and also molecules with a larger surface area all result in higher London dispersion forces.

The relative strength of these intermolecular forces is shown below.

Fig. 1: Relative strength of intermolecular forces, Isadora Santos - StudySmarter Originals.

The state of matter of a substance is dependent on both the strength of intermolecular forces and the amount of kinetic energy a substance has. In general, Intermolecular forces decrease when you go from solids to liquids to gases. So, solids have strong intermolecular forces that hold particles together in place. Liquids have intermediate forces that are able to keep particles close while allowing them to move. Gases have the smallest amount of intermolecular forces present and these forces are said to be negligible.

Effects of Intermolecular Forces on Physical Properties

Higher intermolecular forces result in:

• Greater viscosity
• Greater Surface Tension
• Increased solubility
• Higher melting point
• Higher boiling point
• Lower vapor pressure

First, let's talk about viscosity. Viscosity is a property seen in liquids, and it measures the resistance of a liquid to flow. Liquids that are considered polar or that are able to form hydrogen bonds have higher viscosity. The stronger the intermolecular force, the higher the viscosity of a liquid. So, liquids possessing strong intermolecular forces are said to be highly viscous.

For example, think about the structure of water and glycerol. Glycerol has three OH^- groups that are able to undergo hydrogen bonding, compared to water which only has one OH^- group that can form hydrogen bonding. Therefore, we can say that glycerol has a higher viscosity, and also a stronger intermolecular force.

Fig. 3: The structures of glycerol and water, Isadora Santos - StudySmarter Originals.

Next, we have surface tension. This property can be easily understood if we think about water molecules. Hydrogen bonding is present between neighboring water molecules, and this force exerts a downward force at the surface of the liquid, causing surface tension. The stronger the intermolecular force, the higher the surface tension of liquids.

Let's solve an example!

Another property that is affected by the strength of intermolecular forces is solubility. The solubility of solids is greatly affected by temperature. So, if temperature increases, the solubility of solids also increases. The solubility of gases in water is the opposite. It decreases with an increase in temperature.

When it comes to relating solubility to intermolecular forces, we can say that As the intermolecular force between solvent and solute increases in strenght, solubility also increases!

Let's look at an example!

Effect of Intermolecular Forces on Melting Point

The melting points of substances depend on the strength of the intermolecular forces present between molecules. The general relationship between IMF and melting point is that the stronger the intermolecular force, the higher the melting point.

For example, a non-polar compound such as Br₂ that only has London dispersion forces tend to have a low melting point because only a very small amount of energy is required to break its molecules apart. On the other hand, a high amount of energy is needed to melt a compound containing ion-dipole forces because these forces are very strong.

The strength of London dispersion forces is also affected by how heavy a substance is. This can be seen when we compare Br₂ and F₂. Br₂ has a greater molar mass compared to F₂ so Br₂ will have a higher melting point and also a stronger London dispersion force than that of F_2.

Strength of Intermolecular Forces and Boiling point

When molecules change from a liquid to a gas phase, the temperature at which this occurs is known as the boiling point. The general rule relating IMF and boiling point is that the stronger the intermolecular force present, the greater the amount of energy needed to break them, so the higher the boiling point will be.

Let's look at an example!

Until now, we learned that increasing melting point, surface tension, viscosity, boiling point, and solubility lead to an increase in the strength of intermolecular forces of attraction. But, did you know that higher intermolecular forces result in lower vapor pressures?

Vapor pressure occurs when liquid molecules have enough kinetic energy to escape from the intermolecular forces and turn into a gas inside a closed container. Vapor pressure is inversely proportional to the strength of intermolecular forces. So, molecules with strong intermolecular forces have low vapor pressures!

Let's look at an example!

Strength of Intermolecular Forces on Acetone

A common question that you might encounter in your exam or while studying for AP chemistry is to analyze the strength of intermolecular forces on acetone, C₃H₆O. You have probably seen acetone before since acetone (also known as propanone or dimethyl ketone) is an organic compound widely used to remove nail polish and paint!

Fig. 7: Structure of Acetone, Isadora Santos - StudySmarter Originals

Acetone is a polar molecule so it contains dipole moments that do not cancel out due to symmetry. In polar molecules, the intermolecular forces present are dipole-dipole forces and London dispersion forces (remember that London dispersion forces are present in all molecules!). So, the strongest type of intermolecular interaction present in acetone is dipole-dipole forces.

Determining the Strength of Intermolecular Forces

In AP chemistry exams, you might come across different problems asking you to determine the highest type of intermolecular force present in a molecule.

To be able to figure out the intermolecular forces present in a molecule, we can use the following rules:

• Ion-dipole forces will only be present if an ion and a dipole molecule are present.
• Hydrogen bonding will only be present if: no ions are present, the molecules involved are polar, and the hydrogen atoms are bonded to nitrogen (N), oxygen (O), or Fluorine (F).
• Dipole-dipole forces are only present if no ions are present and the molecules involved are polar. Also, if hydrogen atoms are present, they will not be bonded to N, O, or F.
• London dispersion forces are present in all molecules. But, LDF is the only intermolecular force present in non-polar and non-polarizable molecules.

Now I hope that you are feeling more confident about the factors that increase and decrease the strength of intermolecular forces! And if you are still struggling with the basics of intermolecular forces, you should definitely take a look at "Intermolecular Forces" and "Dipoles".

_References:

_{Hill, J. C., Brown, T. L., LeMay, H. E., Bursten, B. E., Murphy, C. J., Woodward, P. M., & Stoltzfus, M. (2015). Chemistry: The Central Science, 13th edition. Boston: Pearson.}

_{Timberlake, K. C., & Orgill, M. (2020). General, organic, and Biological Chemistry: Structures Of Life. Upper Saddle River: Pearson.}

_{Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.}

I