Intermolecular Forces vs Thermal Interactions

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, which are very significant in determining how molecules will interact with one another.

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This Story also Contains

1. Thermal Energy
2. Intermolecular Forces vs Thermal Interactions
3. Some Solved Examples
4. Summary

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.

Solid(→)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

Recommended topic video on ( Intermolecular Forces v/s Thermal interaction)

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 CH₄CH4 is much lower than that of HF. This is because:

1)Hydrogen bonding in CH₄CH4
2) Hydrogen bonding in HF

3)CH₄CH4 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 CH₄ CHn.
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 $alpha^{-1}$ alpha−1

Hence, the answer is the option (2).

Summary

Intermolecular forces—van der Waals forces, hydrogen bonds, and ion-dipole interactions—are responsible for the attractions and repulsions between molecules. Some 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.