[Chemistry Class Notes] Molecular Motion Pdf for Exam

The matter we see around us exists primarily in three states, solid, liquid, and gas. These three states of matter can be easily differentiated based on the degree of motion of constituent particles. In the solid, the particles are restricted to their original positions and cannot move. The only vibration of particles from their mean position is seen. But on the other side, in gasses, the constituent particles can move randomly owing to their high kinetic energy. The gaseous particles don’t have a fixed position. In liquids, the particles exhibit an intermediate state of movement. The particles in liquids don’t have a fixed position but show a higher degree of movement than solids and lesser than that of gasses. To summarize, gasses have the highest degree of molecular motion, followed by liquids and then by solids. This is mainly because gas molecules possess high kinetic energy.

What is Molecular Motion?

Molecular motion is nothing but the movement of constituent particles in a specific direction. The degree of motion of constituent particles is affected by temperature. The temperature is also the measure of the average Kinetic energy of the molecules. The molecular motion is also dependent on the heat as the transfer of heat among molecules increases their kinetic energy.

The mathematical relationship between kinetic energy and temperature is defined as:

E=kT

Types of Molecular Motion:

There are four types of molecular motion:

Translational Motion: 

The movement of molecules may be in a different direction, but the movement axis remains the same. This form of movement is called Translational motion.

Rotational Motion: 

The molecules rotate in and around the axis in this type of motion.

Vibrational Motion: 

This form of movement is prevalent in the solid-state, where the molecules vibrate in their mean position.

Electronic Motion: 

In this type of motion, the movement of electrons takes place from place to place, orbital to orbital. This type of electronic motion causes a change in the color of the substance.

Types of Motion and Intermolecular Forces

Gasses take the shape of the container readily and spread quickly in space. This is because, unlike solids and liquids, molecules in the gaseous state show random movement. Their weak intermolecular forces aid in their free and spontaneous movement. Molecules in a solid-state are capable of only vibrational motion due to their strong intermolecular forces; hence, the solid-state molecules can only vibrate from their mean positions. Thus, solids show the least molecular randomness.

On the other hand, liquids exhibit vibrational motion along with the rotational and translational motion. This is due to their weak intermolecular forces between molecules. Molecular motion in liquids is less than gasses but more than solids. Hence, random motion depends on the intermolecular forces, the kinetic energy of molecules, and heat transfer phenomena.

About Random Molecular Motion in Glasses

These are the properties shown by the gas molecules

• Gasses are constituted of atoms or molecules that are in constant random motion. 

• These gas particles (atoms and molecules) continuously collide with each other and the walls of the container. The collisions that hence occur are elastic, and there is no net loss of energy. 

• Gas particles are microscopic compared to the volume of the container. The relative volume of gas particles is thus negligible.

• There exist no interactive forces, i.e., attraction or repulsion between the molecules of gas.

• The average kinetic energy of the gas molecules is directly proportional to the temperature of the gas. If two gasses are at the same temperature, they possess the same kinetic energy.

Measurement of Molecular Motion

Atoms and molecules require different amounts of energy for the different types of molecular motion. The light that is absorbed during motion can be used for the measurement of molecular motion. The frequencies of absorbed light and compare them with bonding arrangements present in molecules. Various modern techniques like UV spectroscopy, mass spectroscopy, and IR spectroscopy can also quantify molecular motion. These modern methods can tell a lot about the structure of molecules.

Kinetic Energy of Molecules

As the gas particles are in constant motion, they possess kinetic energy. The kinetic energy of every single atom can be calculated by the formula Ek = 1/2 mu2, where ‘m’ is the mass of the atom and u is its speed. The molecules of gas share average kinetic energy but individual molecules have different kinetic energies than the rest due to different speeds. The difference in speeds arises from the collision between themselves that occurs in the gas phase. These collisions are elastic and involve no net loss of energy as a whole, but individual molecular speeds change. One molecule’s energy gets transferred to the other in a collision, but the net energy of the system remains intact.

 

Difference between the Molecular Motion in Solid, Liquid and Gas

Solid: These molecules are crowded tightly collectively in a solid and are unable to flow around freely within the substance. Solids have a definite form that is hard to modify because the molecular motion for the particles in a solid is limited to very slight movements of the atoms around their definite places.

Liquid: In a liquid, molecules are constantly moving. As a result of the pull of gravity, when the container is tilted, particles move to the left and down, and the gaps are filled by many other molecules. The overall flow of liquid out of the vessel is the consequence.

Gas: According to the kinetic theory of gasses, raising the temperature of a gas causes the total kinetic energy of the molecules to increase, leading the molecules to move faster. As a consequence of the enhanced velocity, the gas’s outer pressure rises, as predicted by the ideal gas equation PV=NkT.

 

Properties of Solid

• The intermolecular interactions between nearby molecules are sufficiently powerful to hold them in place.

• Due to the absence of a gap between molecules, solids more like liquids are difficult to compress.

• Crystalline structures are defined as those in which the molecules in a solid acquire a highly organized packing configuration.

Properties of Liquid

• The strong forces between molecules are powerful enough to keep molecules tightly packed together.

• Gasses are less dense and deformable than liquids.

• Liquids have a defined volume that is irrespective of the bottle’s size and shape.

• However, the attraction forces are insufficient to hold neighboring molecules in place, and molecules are free to roam past or glide over one another.

• As a result, liquids can be put into containers and take on the shape of the vessels.

Properties of Gas

• A group of molecules that are far spaced

• The molecules’ kinetic energy is larger than any attraction forces between them.

• A gas can spread to fill its vessel because there is no substantial attraction among molecules.

• When attractive forces get strong enough, gasses begin to behave in non-ideal ways.