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Intermolecular Forces vs Thermal Interactions

Intermolecular Forces vs Thermal Interactions

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:18 PM IST

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.

This Story also Contains
  1. Thermal Energy
  2. Intermolecular Forces vs Thermal Interactions
  3. Some Solved Examples
  4. Summary
Intermolecular Forces vs Thermal Interactions
Intermolecular Forces vs Thermal Interactions

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.

Frequently Asked Questions (FAQs)

1. What are intermolecular forces and how do they differ from intramolecular forces?
Intermolecular forces are attractions between separate molecules, while intramolecular forces hold atoms together within a single molecule. Intermolecular forces are generally weaker and responsible for properties like boiling point and surface tension, whereas intramolecular forces determine molecular structure and stability.
2. How do thermal interactions affect intermolecular forces?
Thermal interactions increase the kinetic energy of molecules, causing them to move faster and vibrate more. This increased motion can overcome intermolecular forces, leading to changes in state (e.g., melting or boiling) as the thermal energy surpasses the strength of the intermolecular attractions.
3. Why do substances with stronger intermolecular forces generally have higher boiling points?
Stronger intermolecular forces require more energy to overcome, meaning more heat is needed to separate the molecules and change the substance from a liquid to a gas. This results in higher boiling points for substances with stronger intermolecular attractions.
4. How does polarity affect the strength of intermolecular forces?
Polar molecules have an uneven distribution of charge, creating stronger intermolecular attractions called dipole-dipole forces. These forces are generally stronger than those between nonpolar molecules, resulting in higher boiling points and other physical properties for polar substances.
5. How do intermolecular forces influence the states of matter?
Intermolecular forces determine how strongly molecules are held together, affecting whether a substance exists as a solid, liquid, or gas at a given temperature. Stronger forces tend to result in higher melting and boiling points, favoring solid and liquid states over gaseous states.
6. How do thermal interactions affect the behavior of gases?
Increased thermal energy causes gas molecules to move faster and collide more frequently with their container walls, increasing pressure. This relationship is described by the ideal gas law and explains why gases expand when heated.
7. How do thermal interactions affect the strength of hydrogen bonds?
Increased thermal energy causes more rapid molecular motion, which can weaken or break hydrogen bonds. However, in liquid water, hydrogen bonds are constantly breaking and reforming, creating a dynamic network that gives water its unique properties.
8. Why does water have a higher boiling point than expected for its molecular mass?
Water's higher-than-expected boiling point is due to hydrogen bonding between water molecules. These strong intermolecular attractions require more energy to overcome, resulting in a higher boiling point compared to similar-sized molecules without hydrogen bonding.
9. What is the difference between evaporation and boiling?
Evaporation occurs at the surface of a liquid at any temperature, while boiling occurs throughout the liquid when the vapor pressure equals the atmospheric pressure. Both processes involve overcoming intermolecular forces, but boiling requires more energy and occurs at a specific temperature.
10. Why do some substances sublimate instead of melting?
Sublimation occurs when the vapor pressure of a solid is greater than the atmospheric pressure at temperatures below its melting point. This happens when intermolecular forces are weak enough that thermal energy can directly overcome them, allowing molecules to transition directly from solid to gas.
11. What role do hydrogen bonds play in intermolecular forces?
Hydrogen bonds are a particularly strong type of intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (usually N, O, or F) and another electronegative atom. They are responsible for many unique properties of water and are crucial in biological systems.
12. How do van der Waals forces compare to other types of intermolecular forces?
Van der Waals forces are the weakest type of intermolecular force, present between all molecules. They include London dispersion forces and dipole-dipole interactions. While individually weak, they can become significant in large molecules or in the absence of stronger forces.
13. What is the relationship between intermolecular forces and viscosity?
Stronger intermolecular forces generally lead to higher viscosity in liquids. This is because the attractive forces between molecules resist flow, making it harder for the liquid to move or change shape.
14. What is the role of London dispersion forces in intermolecular attractions?
London dispersion forces are weak, temporary attractions caused by fluctuations in electron distribution. They are the only intermolecular force present in nonpolar molecules and play a significant role in the behavior of large molecules and noble gases.
15. How do intermolecular forces affect the solubility of substances?
Solubility is influenced by the relative strengths of solute-solute, solvent-solvent, and solute-solvent intermolecular forces. Substances tend to dissolve in solvents with similar intermolecular forces, following the principle "like dissolves like."
16. Why does ice float on water?
Ice floats because its crystal structure, held together by hydrogen bonds, is less dense than liquid water. The open structure of ice, caused by the specific arrangement of water molecules in the solid state, results in a lower density compared to the more compact liquid form.
17. What is the relationship between intermolecular forces and capillary action?
Capillary action occurs when adhesive forces (attraction between unlike molecules) are stronger than cohesive forces (attraction between like molecules). This allows liquids to climb up narrow tubes against gravity, a phenomenon crucial in many biological and technological processes.
18. What is the role of intermolecular forces in crystal formation?
Intermolecular forces guide the arrangement of molecules or ions in a crystal structure. Stronger and more directional forces, like hydrogen bonds, can lead to specific crystal patterns, while weaker forces may result in simpler structures. The balance of these forces determines the crystal's shape and properties.
19. How do intermolecular forces influence the formation of aerosols?
Aerosols form when liquid droplets or solid particles are suspended in a gas. Intermolecular forces between the suspended particles and the surrounding gas molecules affect the stability and behavior of the aerosol. Stronger attractions can lead to coagulation or settling of particles.
20. What is the relationship between intermolecular forces and surface energy?
Surface energy is directly related to the strength of intermolecular forces. Substances with stronger intermolecular forces have higher surface energy because more work is required to increase the surface area by overcoming these attractions between molecules.
21. Why do some substances expand when they freeze while others contract?
The expansion or contraction during freezing depends on how molecules arrange themselves in the solid state. Water expands when it freezes due to its open crystal structure, while most substances contract because their molecules pack more tightly in the solid state.
22. How do intermolecular forces affect the phenomenon of supersaturation?
Supersaturation occurs when a solution contains more dissolved solute than is normally possible under given conditions. Intermolecular forces between solute and solvent molecules allow this metastable state to exist temporarily, but can be easily disrupted, leading to rapid crystallization.
23. How do intermolecular forces influence the formation of clathrates?
Clathrates form when small molecules are trapped within cavities of crystal lattices formed by larger molecules. Intermolecular forces, particularly hydrogen bonding, play a crucial role in stabilizing these structures and determining which guest molecules can be accommodated.
24. What is the relationship between intermolecular forces and osmotic pressure?
Osmotic pressure arises from differences in solute concentration across a semipermeable membrane. Intermolecular forces between solute and solvent molecules affect the tendency of solvent molecules to move across the membrane, influencing the magnitude of osmotic pressure.
25. How do intermolecular forces affect surface tension?
Stronger intermolecular forces result in higher surface tension. This is because molecules at the surface experience a net inward force from other molecules in the liquid, creating a "skin-like" effect that resists penetration or deformation.
26. How does pressure affect the balance between intermolecular forces and thermal interactions?
Increased pressure pushes molecules closer together, enhancing the effect of intermolecular forces. This can lead to changes in state, such as condensation of gases or freezing of liquids, even without a change in temperature.
27. How do intermolecular forces influence the rate of evaporation?
Stronger intermolecular forces result in slower evaporation rates because more energy is required for molecules to overcome these attractions and escape into the gas phase. This is why substances with weaker intermolecular forces, like ethanol, evaporate more quickly than water.
28. What is the difference between adhesion and cohesion?
Adhesion refers to the attraction between unlike molecules (e.g., water and glass), while cohesion is the attraction between like molecules (e.g., water and water). Both are types of intermolecular forces that play important roles in phenomena like surface tension and capillary action.
29. How do thermal interactions affect the diffusion of gases?
Increased thermal energy leads to faster molecular motion, which enhances the rate of diffusion in gases. This is because molecules with higher kinetic energy collide more frequently and spread out more rapidly, increasing the rate at which they mix with other gases.
30. Why do some liquids mix while others form separate layers?
Liquids mix when the intermolecular forces between different types of molecules are similar in strength or stronger than those between like molecules. When these forces are significantly different or weaker, the liquids separate into layers based on density.
31. How do intermolecular forces affect the compressibility of different states of matter?
Gases are highly compressible because their molecules are far apart with weak intermolecular forces. Liquids are slightly compressible due to stronger intermolecular forces and closer packing. Solids are least compressible because their molecules are held tightly by strong intermolecular forces in a fixed arrangement.
32. What is the relationship between intermolecular forces and vapor pressure?
Stronger intermolecular forces result in lower vapor pressure because fewer molecules have enough energy to overcome these attractions and enter the gas phase. This is why substances with weak intermolecular forces, like diethyl ether, have higher vapor pressures than water at the same temperature.
33. How do thermal interactions influence the behavior of polymers?
Increased thermal energy can cause polymer chains to move more freely, overcoming some intermolecular forces. This can lead to changes in physical properties, such as increased flexibility or even melting, depending on the polymer's structure and the strength of its intermolecular forces.
34. Why does the temperature of a substance remain constant during a phase change?
During a phase change, the added thermal energy is used to overcome intermolecular forces rather than increase kinetic energy. This results in a change of state without a change in temperature until the phase transition is complete.
35. How do intermolecular forces affect the critical point of a substance?
The critical point is the temperature and pressure at which the liquid and gas phases of a substance become indistinguishable. Substances with stronger intermolecular forces generally have higher critical temperatures and pressures because more energy is required to overcome these attractions.
36. How do thermal interactions affect the elasticity of materials?
Increased thermal energy causes atoms or molecules to vibrate more, potentially weakening intermolecular forces. This can lead to increased elasticity in some materials as the bonds become more flexible. However, excessive heat can also cause permanent deformation or melting if intermolecular forces are overcome entirely.
37. Why do some gases deviate from ideal gas behavior?
Real gases deviate from ideal gas behavior due to intermolecular forces and the finite size of molecules. At high pressures or low temperatures, these factors become significant, causing gases to behave differently than predicted by the ideal gas law.
38. How do thermal interactions affect the miscibility of liquids?
Increased temperature generally enhances the miscibility of liquids by providing more thermal energy to overcome unfavorable intermolecular interactions. This can lead to the mixing of previously immiscible liquids or an increase in solubility as temperature rises.
39. How do intermolecular forces affect the rate of chemical reactions?
Stronger intermolecular forces can slow down reaction rates by making it harder for reactant molecules to come into contact or by stabilizing reactants. Conversely, weaker forces may increase reaction rates by allowing easier molecular collisions and rearrangements.
40. What is the role of van der Waals forces in gas liquefaction?
Van der Waals forces become increasingly important as gases are cooled and compressed. These weak attractions, particularly London dispersion forces, eventually overcome the thermal motion of gas molecules, leading to condensation and liquefaction.
41. How do thermal interactions influence the behavior of surfactants?
Increased temperature can affect the behavior of surfactants by altering the balance between hydrophilic and hydrophobic interactions. This can lead to changes in micelle formation, surface tension reduction, and overall surfactant effectiveness.
42. Why do some substances have multiple solid phases?
Multiple solid phases, or polymorphs, occur when a substance can form different crystal structures depending on temperature and pressure conditions. These different arrangements result from varying balances between intermolecular forces and thermal interactions.
43. What is the relationship between intermolecular forces and the glass transition temperature?
The glass transition temperature is the point at which a material transitions from a hard, brittle state to a more flexible, rubbery state. It is influenced by the strength of intermolecular forces, with stronger forces generally resulting in higher glass transition temperatures.
44. How do thermal interactions affect the behavior of liquid crystals?
Thermal energy influences the orientation and ordering of molecules in liquid crystals. As temperature increases, the increased molecular motion can disrupt the partial ordering characteristic of liquid crystals, potentially leading to phase transitions or loss of liquid crystalline properties.
45. Why do some gases not follow Charles's Law at very low temperatures?
At very low temperatures, intermolecular forces become more significant relative to the thermal energy of gas molecules. This can cause deviations from Charles's Law as molecules begin to attract each other more strongly, potentially leading to condensation or other non-ideal behaviors.
46. What is the role of thermal interactions in the phenomenon of critical opalescence?
Critical opalescence occurs near a substance's critical point when density fluctuations cause the fluid to become opaque. Thermal interactions at this point are just strong enough to overcome intermolecular forces, resulting in rapid, microscopic phase transitions that scatter light.
47. How do intermolecular forces affect the process of fractional distillation?
Fractional distillation separates liquids based on differences in their boiling points, which are determined by the strength of intermolecular forces. Substances with weaker forces boil at lower temperatures and are collected first, while those with stronger forces require more heat to vaporize.
48. Why do some substances exhibit retrograde solubility?
Retrograde solubility, where solubility decreases with increasing temperature, occurs when the dissolution process is exothermic. In these cases, increased thermal energy shifts the equilibrium towards the undissolved state, as dictated by Le Chatelier's principle.
49. How do thermal interactions affect the behavior of supercritical fluids?
In supercritical fluids, thermal energy overcomes the distinction between liquid and gas phases. This results in unique properties where the fluid can diffuse through solids like a gas but dissolve materials like a liquid, due to the balance between thermal interactions and intermolecular forces.
50. How do intermolecular forces and thermal interactions contribute to the phenomenon of surface melting?
Surface melting occurs when the outermost layers of a solid begin to melt below the bulk melting point. This phenomenon results from the interplay between intermolecular forces, which are weaker at the surface due to fewer neighboring molecules, and thermal interactions that more easily overcome these reduced forces.

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