Intermolecular Forces and Thermal Energy: Intermolecular forces are the forces of attraction that exist between the molecules of a substance (gaseous, liquid, or solid). These forces are responsible for the substance’s structural characteristics and physical form. On the other hand, intramolecular forces alter the chemical characteristics of a substance because they occur within the same molecule or polyatomic ion.

The boiling point rises as the intermolecular attraction increases. Conversely, the strengths of different substances’ intermolecular forces can be compared by comparing their boiling temperatures. This is because the heat received at the boiling point breaks the intermolecular interactions and turns the liquid to vapour. Similarly, the melting point rises with the intensity of intermolecular interactions.

Any of the following interactions can cause intermolecular forces:

1. Dipole-dipole interactions
2. Ion-dipole interactions
3. Ion-induced dipole interactions
4. Dipole-induced dipole interactions
5. London dispersion forces
6. Hydrogen bonding

The Van der Waals Forces are a group of dipole-dipole, dipole-induced dipole, and dispersion forces named after the Dutch physicist van der Waals, who studied them.

It’s important to note that ion-induced dipole and ion-dipole forces are not van der Waals forces. Furthermore, hydrogen bonding is a specific sort of dipole-dipole attraction that only a few molecules exhibit.

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Types of Intermolecular Forces

There are various types of intermolecular forces, as discussed below:

Dipole-Dipole Interactions

These attraction forces exist between polar molecules. The cause for the emergence of these forces is self-evident. Permanent dipoles exist in polar compounds. As a result, the positive pole of one molecule is attracted to the negative pole of the other. A basic example is \({\rm{HCl}}\), in which the chlorine end obtains a tiny negative charge, and the hydrogen end becomes slightly positively charged due to their electronegativity difference. The \({\rm{HCl}}\) molecules then engage in dipole-dipole interactions.  

The amount of dipole-dipole forces in various polar compounds may be anticipated based on the polarity of the molecules, which is determined by the electronegativities of the atoms present in the molecule as well as the molecule’s geometry (in the case of polyatomic molecules, containing more than two atoms in a molecule) The dipole moments of molecules are commonly used to express the polarities of molecules. For example, the dipole values of \({\rm{P}}{{\rm{H}}_3}\) and \({{\rm{H}}_{\rm{2}}}{\rm{S}}\) are \({\rm{0}}{\rm{.58D}}\) and \({\rm{0}}{\rm{.95D}}\), respectively, indicating that \({{\rm{H}}_{\rm{2}}}{\rm{S}}\) has a higher dipole moment than that of \({\rm{P}}{{\rm{H}}_3}\). Despite the fact that their molecular masses are virtually identical, \({{\rm{H}}_{\rm{2}}}{\rm{S}}\) has a greater melting and boiling point than \({\rm{P}}{{\rm{H}}_3}\).

Ion-Dipole Interactions

This is the attraction between a polar molecule and an ion (cation or anion). When \({\rm{NaCl}}\) is dissolved in water, the polar water molecules are attracted to the \({\rm{N}}{{\rm{a}}^ + }\) and \({\rm{C}}{{\rm{l}}^ – }\) ions (a process known as ion hydration), as seen in the diagram below. The strength of this contact is determined by the polar molecule’s dipole moment and size, as well as the charge and size of the ion. Because the cation has a higher charge density than the same-charged anion, this contact is usually stronger with the cation. Furthermore, because \({\rm{CC}}{{\rm{l}}_{\rm{4}}}\) is nonpolar, it cannot interact with the cations \({\rm{N}}{{\rm{a}}^ + }\) and \({\rm{C}}{{\rm{l}}^ – }\). As a result, \({\rm{NaCl}}\) is insoluble in \({\rm{CC}}{{\rm{l}}_{\rm{4}}}\). Because the charge of any ion is substantially more than the charge of a dipole moment, ion-dipole attraction forces are stronger than dipole-dipole interactions.

Ion-Induced Dipole Interactions

The presence of an ion near a nonpolar molecule can lead it to polarise, resulting in an induced dipole. The interactions between them are known as ion-induced dipole interactions. The charge of the ion and the situation in which the nonpolar molecule gets polarised dictate the strength of these interactions. An anion polarises the molecule through repulsion, whereas a cation polarises it by electron cloud attraction.

Red blood cells, for example, contain haemoglobin (RBC). It is centred on a \({\rm{F}}{{\rm{e}}^{2 + }}\) ion, which attracts the \({{\rm{O}}_{\rm{2}}}\) ion via the ion-induced dipole force.

Dipole-Induced Dipole Interactions

An induced dipole is a nonpolar molecule that has been polarised by the presence of a polar molecule (dipole) near it. Dipole-induced dipole interactions are the result of their interactions. Their strength will be determined by the dipole’s strength and the nonpolar molecule’s polarizability.

In the presence of polar molecules, noble gases, for example, become polarised, as seen in the diagram below:

London Forces or Dispersion Forces

A new force, known as the London Force, was developed to examine intermolecular forces that attract nonpolar molecules such as \({{\rm{H}}_{\rm{2}}}{\rm{,}}{{\rm{O}}_{\rm{2}}}{\rm{,}}{{\rm{N}}_{\rm{2}}}\, and monoatomic gases such as He, Ne, and Ar to each other in the liquid and solid states. Fritz London suggested the genesis of these forces in 1930. As a result, they are known as the London forces. The mobility of electrons is assumed to be the source of these forces. It’s thought that the electron cloud of a molecule can be deformed at any point in time, resulting in an instantaneous dipole or momentary dipole (that is, a dipole for a short time) in which one section of the molecule is slightly more negative than the rest.

It’s worth noting that after a momentary dipole is generated, the orientation of the momentary dipole will be different the next instant since the electrons have shifted. Electrons travel fast over time (a very short time), and the effects of these momentary dipoles cancel out, leaving a nonpolar molecule with no permanent dipole moment. Dipoles in nearby molecules are induced by the momentary dipoles. These are then attracted to each other in the same manner as permanent dipoles are attracted to each other. London Dispersion Forces are the forces of attraction between the produced momentary dipoles. In the case of helium, the genesis of these forces is as follows:

Intermolecular Forces Versus Thermal Energy

A substance’s ability to exist as a solid, liquid, or gas is determined by competition between intermolecular forces and thermal energy.

Intermolecular Forces, or forces of interaction between molecules in a substance that attempt to pull them close together,

Thermal energy is the energy possessed by a molecule as a result of temperature, which causes the molecules to move and so seeks to keep them apart.

Intermolecular forces of attraction are smallest in gases, whereas thermal energy is largest (manifested as the random translatory motion of molecules). Intermolecular forces of attraction are strongest in solids, while thermal energy is lowest. In liquids, the two types of energies are intermediate between those of gases and solids.

On the basis of their interaction energy and thermal energy, some of the features of solids, liquids, and gases can be explained as follows:

Because thermal motion is too weak to overcome the strong intermolecular forces of attraction, a solid has rigidity. The thermal energy in a gas is so high that the molecules are unable to approach close together. As a result, there are vast gaps between them. In a liquid, the attractive intermolecular interactions and thermal energy are in a suitable equilibrium. As a result, molecules in a liquid reside in close proximity, forming a condensed state of matter with no stiffness. As a result, they have no distinct shape.

When a solid is heated, the thermal motion increases, and the solid melts.

Their molecules live together in both liquids and solids, indicating that they are both condensed states of matter with sufficiently strong intermolecular forces of attraction. As a result, they have extremely low compressibility. On the other hand, gases have enormous empty spaces between their molecules and so have high compressibility.

Summary

There are many types of intermolecular forces such as dipole-dipole, ion-dipole, ion-induced dipole, dipole-induced dipole, dispersion forces and hydrogen bonding. Thermal energy is the energy possessed by a molecule because of temperature, which causes the molecules to move and so seeks to keep them apart. Intermolecular forces of attraction are smallest in gases, whereas thermal energy is largest (manifested as the random translatory motion of molecules). Intermolecular forces of attraction are strongest in solids, while thermal energy is lowest. In liquids, the two types of energies are intermediate between those of gases and solids.

FAQs on Intermolecular Forces and Thermal Energy

Q.1. What are the different types of intermolecular forces?
Ans: Different types of intermolecular forces are given below:
1. Dipole-dipole interactions
2. Ion-dipole interactions
3. Ion-induced dipole interactions
4. Dipole-induced dipole interactions
5. London dispersion forces
6. Hydrogen bonding

Q.2. What is thermal energy?
Ans: Thermal energy is the energy possessed by a molecule as a result of temperature, which causes the molecules to move and so seeks to keep them apart

Q.3. What are the factors which decide a substance’s ability to exist as solid, liquid and gas?
Ans: Intermolecular forces and thermal energy.

Q.4. Discuss the thermal energy and intermolecular forces in solid, liquid and gas.
Ans: The thermal motion is too weak to overcome the strong intermolecular forces of attraction in solids, so a solid has rigidity. The thermal energy in a gas is so high that the molecules are unable to approach close together. As a result, there are vast gaps between them. In a liquid, the attractive intermolecular interactions and thermal energy are in a suitable equilibrium. As a result, molecules in a liquid reside in close proximity, forming a condensed state of matter with no stiffness. As a result, they have no distinct shape.

Q.5. What is the effect of an increase in temperature on thermal energy and intermolecular forces?
Ans: On increasing the temperature, intermolecular forces decreases, and thermal energy increases.

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