Intermolecular Forces versus Thermal Energy of three states of matter

In this post, we will first define Thermal energy, and then we will see how competition between molecular interaction energy (Intermolecular Force based energy that keeps molecules together) and thermal energy (that moves the molecules apart) influences the states of matter.

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What is Thermal Energy?

Thermal energy is the energy of a body due to the motion or movement of its atoms or molecules.

It is directly proportional to the temperature of the substance. It is a measure of the average kinetic energy of the particles of the matter and is, therefore, responsible for the movement of particles. This movement of the particles, therefore, is also called thermal motion and keeps the molecules apart.

Intermolecular Forces versus Thermal Energy of three states of matter

According to the particle concept of matter, all matter consists of tiny particles (atoms or molecules) which are constantly moving in all directions. These particles exert attractive forces upon one another called interparticle (or intermolecular) forces. These interparticle forces are quite stronger in solids than those in liquids and are very small (negligible) in gases.

The three states of matter arise due to the competition between molecular interaction energy which keeps molecules together and thermal energy which moves the molecules apart. The competition between molecular interaction energy and thermal energy determines whether a given substance under given conditions is a gas, a liquid, or a solid.

In gases, the molecules have the weakest (almost negligible) molecular interactions (i.e., no attractive forces) but the highest thermal energy. Therefore, the molecules in gases can move at random in all directions with high speeds.

In solids, the molecular interactions are strongest but the thermal energy is weakest and therefore, the molecules in solids have no translatory motion and can only oscillate with respect to their equilibrium positions. Therefore, the solids can have only vibratory motion.

In liquids, the two types of energies are intermediate between solids and gases.

The predominance of molecular interactions results in a change of gases to liquid to a solid-state while the predominance of thermal energy results in a change of solid to liquid to gas as shown below:

Predominance of intermolecular interactions

As Molecular interaction energy rises: change from gas to liquid and then liquid to solid happens.

GAS ———→ LIQUID ———→ SOLID

Predominance of thermal energy

As Thermal energy rises: change from solid to liquid and then liquid to gas happens.

GAS ←——— LIQUID ←——— SOLID

Heating helps solids to melt – why?

In solids, the net forces between the molecules are attractive forces. The molecules in the solid with relatively low thermal motion cannot overcome the attractive forces and cannot move independently. However, on heating, the thermal motion increases, and the solid melts.

Liquid molecules can’t become free like gases – why?

In the liquid state, there is a balance between attractive intermolecular forces and thermal motion. The molecules can have translatory motion but cannot become completely free like gases.

solids and liquids cannot be easily compressed – why?

In solids and liquids, molecules cannot be pushed together by external pressure because the intermolecular forces become repulsive at small distances between the molecules. Therefore, the solids and liquids cannot be easily compressed. Therefore, only small changes in volume occur on their interconversion from solid to liquid and vice-versa.

why liquids do not have any definite shape ?

The breaking and making of bonds between molecules in the liquid give it a non-rigid structure. This explains why liquids do not have any definite shape although they have definite attractive interaction energy between the molecules.

gases are highly compressible – why?

In gases, there are large empty spaces between the molecules and these can be readily compressed when pressure is applied. Hence, the gases are highly compressible, unlike solids and liquids.

gases do not liquify on compression only – why?

In gases, the intermolecular forces are very weak and molecules don’t cling together to make liquid or solid unless thermal energy is reduced by lowering the temperature. Therefore, the gases do not liquify on compression only. However, when the thermal energy of molecules is reduced by lowering the temperature, the gases can be very easily liquified.