What is kinetic molecular theory

The significance of the above relationship is that pressure is proportional to the mean-square velocity of molecules in a given container. Therefore, as molecular velocity increases so does the pressure exerted on the container. Pressure and KMT The macroscopic phenomena of pressure can be explained in terms of the kinetic molecular theory of gases.

References Oxtoby, Gillis and Campion. Principles of Modern Chemistry. Chang, Raymond. Physical Chemistry for the Biosciences. California: University Science Books. Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion. These particles move in a straight line until they collide with another particle or the walls of the container.

These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space. There is no force of attraction between gas particles or between the particles and the walls of the container. Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container.

The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else. The assumptions behind the kinetic molecular theory can be illustrated with the apparatus shown in the figure below, which consists of a glass plate surrounded by walls mounted on top of three vibrating motors. A handful of steel ball bearings are placed on top of the glass plate to represent the gas particles.

When the motors are turned on, the glass plate vibrates, which makes the ball bearings move in a constant, random fashion postulate 1. Each ball moves in a straight line until it collides with another ball or with the walls of the container postulate 2. Although collisions are frequent, the average distance between the ball bearings is much larger than the diameter of the balls postulate 3. There is no force of attraction between the individual ball bearings or between the ball bearings and the walls of the container postulate 4.

The collisions that occur in this apparatus are very different from those that occur when a rubber ball is dropped on the floor. Collisions between the rubber ball and the floor are inelastic , as shown in the figure below. A portion of the energy of the ball is lost each time it hits the floor, until it eventually rolls to a stop. In this apparatus, the collisions are perfectly elastic. The balls have just as much energy after a collision as before postulate 5.

Any object in motion has a kinetic energy that is defined as one-half of the product of its mass times its velocity squared. At any time, some of the ball bearings on this apparatus are moving faster than others, but the system can be described by an average kinetic energy.

When we increase the "temperature" of the system by increasing the voltage to the motors, we find that the average kinetic energy of the ball bearings increases postulate 6. The kinetic molecular theory can be used to explain each of the experimentally determined gas laws.

The pressure of a gas results from collisions between the gas particles and the walls of the container. Each time a gas particle hits the wall, it exerts a force on the wall. An increase in the number of gas particles in the container increases the frequency of collisions with the walls and therefore the pressure of the gas.

Amontons' Law P T. The last postulate of the kinetic molecular theory states that the average kinetic energy of a gas particle depends only on the temperature of the gas. Thus, the average kinetic energy of the gas particles increases as the gas becomes warmer. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them close together, and the molecules move further apart, forming liquid water.

This is why liquid water is able to flow: the molecules have greater freedom to move than they had in the solid lattice. If the molecules are heated further, the liquid water will become water vapor, which is a gas. The attractive forces between the particles are very weak given the large distances between them.

Changes in phase : A change in phase may occur when the energy of the particles is changed. The kinetic theory of matter is also illustrated by the process of diffusion. Diffusion is the movement of particles from a high concentration to a low concentration. It can be seen as a spreading-out of particles resulting in their even distribution.

Placing a drop of food coloring in water provides a visual representation of this process — the color slowly spreads out through the water. If matter were not made of particles, then we would simply see a clump of color, since there would be no smaller units that could move about and mix in with the water.

Interactive: Diffusion of a Drop : Click in the model to add a drop of dye. Watch how the molecules move through the water.