Intermolecular Forces: Definition, Explanation, Types and Important Questions

Intermolecular forces are the forces of attraction and repulsion that arise between the molecules or atoms of a substance. These forces hold together the molecules of solid and liquid and are responsible for several physical properties of matter. Here in this article, we will go through the Intermolecular forces, their types and some important questions related to intermolecular forces. 

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Intermolecular Forces

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The attractive and repulsive forces that arise between the molecules of a substance are termed as the intermolecular forces. These forces are responsible for the physical and chemical properties of the matter. Intermolecular forces or IMF are also known as the electrostatic forces between molecules and atoms. 

Intermolecular forces exist between the molecules and affect the physical properties of a substance. The intermolecular forces of attraction are the result of the reaction between the protons or positive compounds and the electrons or negative compounds of a molecule. 

Also Read: Intermolecular Force

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Types of Intermolecular Forces

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There are several types of intermolecular forces based on the bonds. Repulsive and interactive forces are the two kinds of forces where electrostatic forces cause attraction. Van der Waals forces are another name for the intermolecular forces of attraction.

The boiling point of different substances is directly proportional to the intermolecular forces. If the boiling point of a substance increases, the strength of intermolecular forces also increases.

The boiling point of the different substances can be determined by the strength of their intermolecular forces. The intermolecular forces break due to the absorbed heat while converting the liquid to vapor. There are various types of interaction that affects the intermolecular forces such as:

Ion dipole interaction

The bond formed in ion-dipole interaction is between polar molecules and ions. The intensity of the interaction depends on:

• Charge and size of an ion
• The proportion of the dipole moment
• The magnitude of the polar molecule

Sodium chloride molecules dissolved in water are examples of intermolecular forces. Chlorine has negative ions (Cl-) and sodium has positive ions (Na+). Water possessing polar molecules is attracted towards the oppositely charged ions.

Dipole-dipole interaction

Dipole-dipole interaction occurs in polar molecules. The difference in the electronegativity of the atom which happens due to covalent bonds, gives rise to permanent dipoles in the polar molecules. The positive part of the molecule is attracted to the negative part of another molecule when two molecules interact with each other. Thus the force created by the attraction is known as the dipole-dipole interaction.

For Example: In HCI, chlorine is partially negatively charged as it is more electronegative than hydrogen whereas hydrogen is positively charged. The opposite charges cause the attractive forces which give rise to dipole-dipole interaction. The polarity of the molecules helps in detecting the intensity of these forces.

Ion induced dipole interaction

A molecule that is non-polar is made polar by placing an ion near it. When charged, the non-polar molecules behave like induced dipoles. The force created between an induced dipole and an ion is known as ion-induced interaction. The strength of the intermolecular forces depends upon the tendency with which the non-polar molecules get charged.

Dipole-induced dipole interaction

The presence of polar molecules changes the non-polar molecules into induced molecules and results in the dipole-induced dipole interaction. The interaction takes place between the molecules without dipoles and the polar moles with dipoles. The other molecule becomes a dipole as the permanent dipole of the polar molecules makes the electrically neutral molecule's dipole by damaging its electron cloud.

Dispersion forces or London forces

Dispersion forces or London forces create temporary positively and negatively charged regions because of the movement of the electrons. The electrically charged cloud of atoms and non-polar molecules is symmetrically distributed, which is why they exist without a dipole moment. But atoms and molecules may show dipole properties temporarily.

These forces are the weakest and travel only across a limited distance.

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Intermolecular Forces: Formulae

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• In solids, potential energy of dipole-dipole interaction is = 1/r³

Where r = distance between polar molecules

• Dispersion force = 1/r⁶

Where r = distance between two particles

• The potential energy between the non-polar identical molecules v= - ¾ α2I /r⁶

Where, α= Polarizability of non-polar molecules

r = Distance between two molecules

I = First ionization energy of the molecule

• The potential energy of interaction between the ion and polar molecules: 

V = - qµ / (4π?0) r²

Where, r = Distance of separation

q = Charge of ion

µ= Dipole moment

• Electrostatic interaction between two dipoles: V = -(2/3) (µ12 µ22)/(4π?0)2r⁶

Where, µ = permanent dipole moment of the molecules

• Potential Energy of Dipole forces:

V = - (3/2) [(I_AI_B)/ (I_A + I_B)] α_A α_B / r⁶

• Van der waals equation :

(P + an²/v²) (v- nb) = nRT

Where n = number of moles of the gas

Constant a and b depend on the characteristics of the gas

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Intermolecular Forces: Things to Remember

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• Intermolecular forces are the forces of attraction and repulsion that arise between the molecules/atoms of a substance.
• Intermolecular forces or IMF are also referred to as the electrostatic forces between molecules and atoms. 
• The boiling point of different substances is directly proportional to the intermolecular forces.
• London forces or dispersion forces are the weakest forces.
• Ion-dipole interactions are similar to dipole-dipole interactions but they arise between ions and polar molecules.

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Intermolecular Forces: Important Questions

Ques.1: (a) Define Van der waals forces. 
(b) Give an example to show dipole-dipole forces. (2 Marks) 

Ques.2: Why does viscosity of liquids decrease as the temperature is raised? (2 Marks) 

Ques.3: The stability of a crystal is reflected in the magnitude of its melting point. Explain (2 Marks) 

Ques.4: Ionic solids are good conductors in molten state and in aqueous solutions but not in solid state. Why? (2 Marks) 

Ques.5: Stability of a crystal is reflected in the magnitude of its melting point'. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules? (3 Marks) 

Ques.6: What are the forces which are responsible for the viscosity of a liquid? Why is glycerol more viscous than water? (3 Marks) 

Ques.7: Leaving electrostatic forces that exist between oppositely charged ions, which other intermolecular attractive forces exist between atoms or ions. Mention them. (2 Marks) 

Ques.8: Give an expression for the van der Waals equation. Give the significance of the constants used in the equation. What are their units? (3 Marks)