3.1: Intermolecular Forces

Learning Objectives

I can explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when:

The molecules are of the same species.
The molecules are of two different species.

I can apply my knowledge of IMFs to explain the different properties of molecules.

Essential knowledge

What are intermolecular forces?

Intermolecular forces, IMF, are the attractions between atoms, ions or molecules that hold them together. There are six different types of IMF. They differ in the arrangement of the electrons. Intermolecular forces explain why carbon dioxide, CO2, is a gas, water, H2O is a liquid and silicon dioxide, SiO2, is a solid, even though they are all only made from three atoms.

To determine which type of IMF a substance will exhibit you need to look at the types of atoms that are present in the sample. Start by identifying the types of atoms in the sample as either a metal or a nonmetal.

The table below is a summary of all possible types of intermolecular forces

Types of Intermolecular Forces

London Dispersion Forces (LDF):

(FYI: LDF is also called induced dipole-induced dipole interaction)

Are forces that exist between nonmetals only. These forces are the weakest intermolecular forces and are the only type present in non-polar molecules. LDF exist in all molecules that are covalently bonded and in noble gases.

At any given time, one non-polar molecule might have more electrons on one side than another side, making it polar. For that instant, the molecule would have a partial negative side and a partial positive side and it becomes a temporary dipole.

As the molecules get closer together, that temporary dipole (molecule) then induces a dipole on its neighboring molecule and there are LDF forces between the partial negative side of one molecular and the partial positive side of another molecule.

Here is a quick visual to sum up this idea of an LDF:

Important piece of information that is usually asked on the Ap exam:

The strength of LDFs increase as the size of a molecule increases. This is because more electrons yield a stronger instantaneous dipole due to a more polarizable electron cloud. Polarizability is the ease at which an electron cloud could be distorted to give a dipole charge distribution.

2. Dipole-Dipole Interactions:

Dipole-Dipole attractions occur due to the opposite partial charges that exist on the opposite ends of a dipole (polar molecule). Dipole-dipole attractions only occur in a sample of polar molecules and are slightly stronger than LDFs.

As you decrease the distance between the two dipoles, you strengthen the attraction and dipole-dipole interaction. Keep in mind, since these involve polar molecules, the element with the higher electronegativity (in this case Cl-) has the partial negative (δ−).

Dipole-dipole interactions are stronger for more polar molecules and that leads to higher boiling and melting points.

3. Hydrogen Bonding

Hydrogen bonding (which is NOT a bond) is actually an unusually strong dipole-dipole attraction and only occurs when hydrogen is directly bonded to F, O, or N in a molecule. It occurs between these molecules because of their high electronegativity difference and small sizes, which leads to really really strong attractions.

The hydrogen bonded to the F, O or N is partially positively charged and is attracted to the neighboring unshared electrons on the F, O, or N.

Since these attractions are so so strong, the boiling points of molecules with hydrogen bonds are very high!

An example of a molecule that has hydrogen bonding is water (H2O). It is also seen in DNA 🧬.

Let's clear a quick misconception before moving on:

On the AP Exam, you may be asked to draw a molecule with the proper orientation and create hydrogen bonding. Make sure you don't confuse the difference between intermolecular and intramolecular, intermolecular is in between two molecules, however intramolecular forces are the bonds that exist between atoms in a molecule!

4. Ion-Dipole Forces:

Ion dipole forces exist in solutions of ionic compounds where there are ions dissolved in water or in mixtures of ionic compounds and polar molecules. These forces exist when there are ions attracted to dipoles ( an example could be NaCl dissolved in water). In general, Ion-dipole forces are stronger than dipole-dipole and Hydrogen bonding. The picture below shows some example of ion dipole interactions.

Ion-dipole forces increase with increasing polarity of molecule and increasing ion charge.

S2- has a higher charge than Cl-, therefore it has a stronger ion-dipole forces.

5. Ion-Ion Interaction:

Ion-ion attractions are electrostatic forces that take place in ionic compounds. This is the strongest type of intermolecular forces and that's why most ionic compounds are solids at room temperature and they have higher melting and boiling points.

Coulomb's law is used to explain strengths of ion-ion forces.

IMFs and properties of substances

In general, molecules with stronger intermolecular forces are more strongly attracted to one another than molecules with weaker Intermolecular forces.

The following properties increase with stronger intermolecular forces:

Boiling point (molecules with stronger IMFs tend to have higher boiling points)
Melting points (molecules with stronger IMFs tend to have higher melting points)
Viscosity increases with stronger IMFs (example honey has higher viscosity than water and that means honey has stronger IMFs than water)

The following properties decrease with stronger intermolecular forces:

Vapor Pressure (substances with stronger IMFs tend to have lower vapor pressures than molecules with weak IMFs)

On the AP exam, you might be asked to compare boiling points, melting points, viscosity, vapor pressure and other properties of molecules using intermolecular forces. In this case, you have to list all IMFs in the given molecules and try to come up with a reasoning that justify the data given to you in a problem. (make sure to go along with the data and not to contradict it)

IMF lesson starts from the 18:23 minutes

This video explains the different types of Intermolecular forces

Check for Understanding:

What type of intermolecular forces exist in CH4?

CH4 is a non-polar molecule, so the only IMFs that exist in CH4 are London dispersion forces.

2. What type of intermolecular forces exist in CH2O?

CH2O is a trigonal planar shape with different surrounding atoms that makes it a polar molecule and because it is a polar molecule CH2O has Dipole-dipole forces and London dispersion forces.

3. What type of intermolecular forces exist in H2O?

H2O is polar molecule that has a Hydrogen atom connected to an Oxygen therefore H2O has the following forces:

London dispersion forces, dipole dipole forces and Hydrogen Bonding.

4.What type of intermolecular forces exist in CuS?

CuS is an ionic compound where ions are held together by ion-ion

5. Based on the data given in the table above, explain why is the boiling point of CS2 higher than that of COS.

Sample Response: CS2 and COS both have London Dispersion Forces, but since COS is a polar molecule, it also exhibits dipole-dipole forces. However, the London Dispersion Forces in CS2 are so strong that they overpower the strength of both the LDFs and the dipole-dipole forces in COS. Therefore, CS2 has a higher boiling point.

(Keep in mind that even though LDFs are the weakest forces, when they are strong and there are lots of them, they can overpower dipole-dipole forces. It all depends on the size of the molecule!)

6. List all the intermolecular forces that exist in an aqueous solution of KCl?

A solution of KCl includes dissolved ions of K+ and Cl-. The forces that exist in this solution are:

Ion-dipole forces between K+ and H2O molecules
Ion dipole forces between Cl- and H2O molecules
Dipole-dipole forces between each H2O and H2O molecule
Ion-ion interactions between Cl- and K+ ions

7. Which molecule would have a higher boiling point H2O or H2S? Explain

First let's start by listing the IMFs in each molecule

H2S molecules have London dispersion forces and dipole-dipole forces only. However, H2O molecules have London dispersion forces, dipole-dipole forces and hydrogen bonding. Therefore, since hydrogen bonding exist in H2O and not in H2S then, H2O would have a higher boiling point.

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