Types of Intermolecular Forces

Intermolecular Forces

Have you ever wondered why solids are sturdy, or why do liquids flow? How is it that we can liquefy gases? To understand these concepts, we need to learn about intermolecular forces of attraction. So, we need to know what are intermolecular forces, and precisely what is intermolecular force of attraction? Solids are sturdy, and liquid flows because of the forces of attraction between them. We define intermolecular forces of attraction as the electrostatic forces between the molecule and atoms.


Every matter around us constitutes atoms and molecules. There always exists a force of attraction or repulsion between the atoms and molecules. The forces which exist in the molecule are responsible for the properties of the molecule. This can also be considered as an intermolecular forces definition. Now, that you are familiar with what are intermolecular forces, we will look at the different types of intermolecular forces.


Types of Intermolecular Forces

Intermolecular forces are mainly of two types, repulsive forces and attractive forces. The intermolecular forces of attraction are also known as Van der Waals forces. The attraction is primarily a result of the electrostatic forces. However, there can be other causes of attraction between two or more constituents of a substance. Therefore, we can have many types of intermolecular forces depending on their bonds.


Intermolecular forces of attraction are caused between the electron of one molecule and the proton of another molecule. These forces are responsible for all the physical and chemical properties of the molecules. Intermolecular forces are directly proportional to the boiling point of the molecule. Which means that the stronger is the force, the higher will be the boiling point. We will now look at the 3 types of intermolecular forces.


Dipole-Dipole Interaction

Dipole-dipole interaction has the strongest intermolecular forces. They occur only in polar molecules such as HCl. When two dipole molecules interact with each other, the negative portion of a polar molecule is attracted to the positive part of another molecule. As a result, we get a force of attraction called dipole-dipole interaction. 


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The image shows the dipole-dipole interaction between the hydrogen and chlorine atoms.

For Example, HCl shows the best intermolecular forces examples for a dipole-dipole interaction. In HCl, chlorine has a negative charge, and hydrogen has a positive charge. The attractive forces between the two opposite charges give rise to dipole-dipole forces. The magnitude of these forces can be predicted by the polarity of the molecules.


Ion - Dipole Interaction

The ion-dipole interaction is quite similar to dipole-dipole interaction. The only difference is that the bonds are formed between ions and polar molecules. 


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The image shows the ion-dipole interaction between sodium ion and chloride ion when immersed in water.

For example, consider a sodium chloride molecule. When NaCl is dissolved in water, H20 has polar molecules, and this polar molecule is attracted towards Cl- and Na+ ions. The strength of the forces between them depends on the size of the polar molecule, and the strength of the dipole moment.


Ion Induced Dipole Interaction

When an ion is near a nonpolar molecule, it has the ability to polarize it. The non-polar molecule becomes an induced dipole, due to the presence of an ion. This interaction between the ion and nonpolar molecules is called an ion-induced dipole interaction. The intermolecular forces strength depends upon the ease with which the non-polar molecule gets polarized.


Solved Problems

Question 1) Arrange isobutane, ethyl methyl ether, and acetone in the order of increasing boiling points.

Answer 1) We know that if the intermolecular forces are stronger in a molecule, then it will have a higher boiling point. In isobutane, there is only one C-H bond, as both carbon and hydrogen have similar electronegativities, it has a small polar bond. We can say that isobutane has a very small intermolecular force between the molecules. Therefore, isobutane will have a lower boiling point compared to other compounds. Ethyl methyl ether is quite similar to H2O. It has two pairs of polar C-O single bonds. This bond partially strengths the molecules, thus having a higher boiling point than isobutane. Acetone has a stronger C=O double bond, which gives it the highest boiling point.


Question 2) Arrange NaCl, Ar, He, N2O, and C60 in order of their boiling points.

Answer 2) When we look at all the molecules, we can identify that NaCl is the only one with ionic bonds. Hence, NaCl will have the highest boiling point compared to all other molecules. To find the boiling points of the different molecules, we will look at their polarity, molar mass, and the ability to form hydrogen bonds. We know that helium is non-polar; therefore, it should have the lowest boiling point. C60 is nonpolar and has the highest molar mass. Hence, it should have a higher boiling point compared to the rest. N2O will have a higher boiling point than Argon as N2O is polar. Therefore, we get the order as follows.

NaCl > C60 > N2O > Ar > He.

FAQ (Frequently Asked Questions)

Question 1) What are the Intermolecular Forces?

Answer 1) Intermolecular forces definition is as follows. Intermolecular Forces are the forces which act at an atomic level. These forces keep the molecules and atoms bonded together. These forces are responsible for the interaction between the molecules.


Intermolecular forces examples: - Ion-dipole forces, ion-induced dipole forces, and hydrogen bonding.

Question 2) What are the Effects of Intermolecular Forces on the Properties of Substances?

Answer 2) They affect the boiling point and freezing points of compounds. The greater the intermolecular forces, the higher the boiling points and freezing points. The lower the intermolecular forces, the lower the boiling points and freezing points. It also affects the fluidity of the liquid, the higher these forces, the slower the liquid flows.