Intermolecular Forces vs Thermal Interactions

Intermolecular forces and thermal interactions are two of the most important themes underlying the study of physical chemistry and materials science. Intermolecular forces describe the attractive or repulsive forces between molecules or particles that affect properties such as boiling and melting points, viscosity, and solubility. These forces include van der Waals forces, hydrogen bonds, and ion-dipole interactions—very significant in determining how molecules will interact with one another. The other side involves thermal interactions, which refer to all those effects that temperature has on molecular behavior. As temperature increases, molecules gain kinetic energy and develop an enhanced state of motion with an increased collision rate. This kinetic energy—directly proportional to the temperature—gives rise to phenomena such as phase transitions like melting or boiling and thermal expansion.

Thermal Energy
Thermal energy is the energy of a body due to the motion or movement of its atoms or molecules. As the temperature increases, thermal energy increases so the kinetic energy of the atoms and molecules also increases. As the movement of particles increases, the molecules move far apart from each other. Thus thermal energy is the measure of the average kinetic energy of the particles of matter and is responsible for the movement of particles. This movement of particles is also called thermal motion.

Intermolecular Forces vs Thermal Interactions
Intermolecular forces and thermal energy have an opposite effect on the motion of particles. Intermolecular forces are responsible for keeping the molecules together whereas thermal energy tends to keep the molecules apart and in the state of motion.

The net effect of intermolecular forces and thermal energy decides the state of the matter.

• Gas
• Liquid
• Solid
• 

(i) Intermolecular interactions: The strength of intermolecular interactions is highest in solids, then in liquids, and least in gases.

Gas $\rightarrow$ Liquid $\rightarrow$ Solid

(ii) Thermal Energy: The thermal energy of the gaseous state particles is maximum, then is the thermal energy of liquid particles and the least thermal energy is of solid particles.

$\operatorname{Solid}(\rightarrow)$ Liquid $(\rightarrow)$ Gas

The two factors namely, pressure and temperature are the deciding factors during the inter-conversion of a state of a substance. For example for changing a gas to a liquid, although the pressure is increased to increase the intermolecular interactions the interconversion is not possible unless the thermal energy of the molecule is reduced by lowering the temperature.

For example, water is a liquid at ordinary temperature. When it is heated to 100⁰C it changes into steam (gas). Whereas, when it is cooled below 0⁰C, it changes into ice (solid).

Ice $\underset{(\text { solid })}{\stackrel{\text { Heat }}{\rightleftarrows}} \underset{\text { Cool }}{\rightleftarrows}($ Watuid $) \underset{\text { Cool }}{\stackrel{\text { Heat }}{\rightleftarrows}}$ Steam

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Some Solved Examples

Example 1: For gaseous molecules, which of the following statements is correct:

1)Molecular interactions are stronger and thermal energy is weaker
2) Molecular interactions are weaker and thermal energy is stronger
3)Both of these are equal

4)None of these

Solution

In a gaseous state, all molecules are far away from each other. The molecular interactions between these particles are very low and their thermal energy is very high. Thus, these molecules tend to move in random directions.
Hence, the answer is the option (2).

Example 2: The boiling point of ${CH}_4 $ is much lower than that of HF. This is because:

1)Hydrogen bonding in ${CH}_4$
2) Hydrogen bonding in HF

3)${CH}_4$ is polar

4)None of these

Solution

Because of hydrogen bonding in HF, the HF molecules are strongly bound to each other, and thus HF has a higher boiling point than ${CH}_{n}$.
Hence, the answer is the option (2).

Example 3: The thermal energy of a body is:

1)Inversely proportional to its temperature

2) Directly proportional to its temperature

3)Independent of temperature

4)None of the above

Solution

As we learn

Thermal Energy -

It is the energy of a body arising from the motion of its atoms or molecules.

- wherein

It is directly proportional to its temperature. It is the measure of the average kinetic energy of the particles.

Kinetic energy $alpha^{-1}$

Hence, the answer is the option (2).

Summary

Intermolecular forces and thermal interactions are the driving forces that lead to the physical behavior of substances. Intermolecular forces—van der Waals forces, hydrogen bonds, and ion-dipole interactions—are responsible for the attractions and repulsions between molecules. Such crucial properties, like boiling and melting points, viscosity, and solubility, which impact the interaction and combination of substances, are defined by these forces. Thermal interactions define the role of temperature in the motion of molecules. With an increase in temperature, the kinetic energy increases, and thus the molecules move and collide more. This kinetic energy gained could be adequate to break the intermolecular forces, thereby resulting in phase changes such as melting and boiling. Another implication is thermal expansion: at a higher temperature, the motion of molecules increases, and thus substances expand.