States of Matter

Kinetic Molecular Theory of Gases

What is so special about a gas? The structure of all matter is connected to the behavior and structure of the gaseous state. The Kinetic theory of gases studies the molecular or the atomic structure of this state. The theory is based on some assumptions or axioms. Let us find out more!

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Kinetic Theory of Gases

The Kinetic molecular theory provides the microscopic model of gases and gives us a picture of the behavior of gases. We have already had a look at the gas laws which depict the changing behavior of gases with changing physical conditions.

The question for the people studying these laws was to explain the cause of such behavior. As the findings of Kinetic theory of gases related to the behavior of molecules and atoms, this theory tries to answer such questions. It relates the physical changes in a gas with the microscopic behavior of the gases.

Kinetic theory of gases

Postulates of the Theory

Kinetic Molecular theory intensely outlines the molecular behavior of a gas. The theory is based on the following postulates:

Negligible Molecules to Space-Volume Ratio

Gases consist of a large number of atoms or molecules that are generally identical in shape and size and are spaced very far apart. Due to the large space between these molecules, the volume of molecules is negligible as compared to the space that a gas occupies. The space between these molecules is empty which explains the high compressibility of gases.

No Force of Attraction

At normal temperatures and pressures, gaseous molecules generally lack attraction forces. You might have noticed that a gas occupies the whole space, irrespective of the shape of the container. It is when the temperature decreases and the pressure increases, that the force of attraction between the molecules starts to build.

Now how do we say that the molecules in gases have almost nil force of attraction? The property of gases to occupy the available space instantly proves this assumption.

Always in Motion

A large amount of space between the molecules gives them the room to be always in motion. Gases do not have a fixed shape, this shows that the gaseous molecules are always in a random motion. The scattered shape filling behavior of the gaseous element signifies the constantly moving gas molecules.

The molecules of gases move in a straight line, in all directions. And with such random motion, these molecules are bound to collide with one another. The highly mobile molecules in their course of motion collide with the walls of the container, thus exerting pressure on these walls.

Elastic Collision

The collision between the molecules in a gas is perfectly elastic. Collisions are of two types: Elastic and Inelastic. In an elastic collision, the energy stored in the molecules by virtue of their movement is same before and after the collision. If the collisions were inelastic, after some time duration, the molecules would come to a rest. We do not observe this so the collisions must be elastic in nature.

Constantly changing Kinetic Energy

The Kinetic energy stored in the various molecules is never static or constant. It is always changing. The kinetic energy of an object is by virtue of its motion or movement. Since the different molecules or atoms in gases are always in a random motion they move with different speed. Each molecule with a changing speed of its own shows changing levels of kinetic energy.

The changing speed of molecules is also because these particles randomly move in any direction, thence collide. As soon as the collision occurs it is obvious that the speed changes. This results in the changing Kinetic energy of the gaseous element. Now if we presume that the initial speed of the particles is same, yet due to the ever- repeating collision between these molecules the final speed changes constantly. The changed speed in these gaseous molecules results in the changed Kinetic energy level.

Average Kinetic Energy

Like the speed of a moving car with no definite speed, molecules and gas atoms of a gas show a similar behavior in terms of Kinetic energy. The never-ending, movement of molecules and particles results in a changed Kinetic Energy. Since for calculating the kinetic energy, we use the equation: K.E =1/2 mv2, and with the changing speed, the K.E is bound to change.

Kinetic energy like the speed of the car is measured in the form of Average Kinetic energy. The variable speed gives a variable K.E, so for matters of high precision, we use average kinetic energy to keep a track over the changing scenario of the energy system.

Effects of Physical Conditions

According to the Kinetic theory of gases, the kinetic energy of the molecules is directly proportional to temperature. The temperature here is taken as the absolute temperature. At constant volume, the pressure of the gas increases with increasing temperature. This is because with increasing temperature, the kinetic energy of the molecules increases.

The increased kinetic energy of the molecules makes them strike with the walls of the container more often, hence leading to an increased pressure on the walls of the container.

From the postulates of Kinetic Molecular Theory, we now understand the basic nature of molecules in the gaseous state. Kinetic theory supports the various gas laws. The Kinetic theory of gases helps in interpreting the experimental observations of the gas laws.

Solved Examples For You

Q.1 If a gas is expanded at constant temperature:
  • a.The pressure increases.
  • b. The Kinetic energy of molecules remains the same.
  • c. The Kinetic energy of the molecules decreases.
  • d. The number of molecules of the gas increases.

Solution: b) Temperature is the function of Kinetic Energy, so if Temperature remains constant, the K.E of the molecule also remains the same.

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