Kinetic Molecular Theory of Matter

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Chapter 10: Physical Characteristics of Gases

Section 10-1
The Kinetic-Molecular Theory of Matter

Prerequisites

The kinetic-molecular theory is based on the idea that particles of matter are always in motion. The theory can be used to explain the properties of solids, liquids, and gases in terms of the energy of particles and the forces that act between them. ideal gas: an imaginary gas that perfectly fits all the assumption of the kinetic-molecular theory.

Kinetic-Molecular Theory of Gases
(based on the following 5 assumptions)

Most of the volume occupied by a gas is empty space
accounts for the lower density of gases compared with that of liquids or solids and explains the fact that gases are easily compressed. No kinetic energy is lost when particles collide with each other or the walls of the container (elastic collision). KE is transferred between two particles during collisions; however, the total KE of the two particles remains the same as long as temperature is constant. Gas particles are in constant or random motion and therefore possess KE, which is energy of motion. KE of the particles overcomes the attractive forces between them, except near the temperature at which the gas condenses and becomes a liquid. Gases do not attract or repel each other.

When gas particles collide, they do not stick together but immediately bounce apart. All gas particles have the same kinetic energy at a given temperature. All gases at the same temperature have the same average kinetic energy. Therefore, at the same temperature, lighter gas particles, such as hydrogen molecules, have higher average speeds than do heavier gas particles, such as oxygen molecules.

Properties of Gases

Expansion
Since gases do not have a definite shape or volume, they completely fill any container in which they are enclosed, and they take its shape. The kinetic-molecular theory explains this because gas particles move rapidly in all directions (Assumption 3) without significant attraction or repulsion between them (Assumption 4).

Fluidity
The ability to flow causes gases to behave similarly to liquids. Because liquids and gases flow, they are both referred to as fluids. Because the attractive forces between gas particles are insignificant (Assumption 4), gas particles glide easily past one another. Low Density

The density of the substance as a gas is about 1/1000 the density of the same substance in the liquid or solid state. This is because the particles are so much farther apart in the gaseous state (Assumption 1). Compressibility

The volume of a gas can be greatly decreased if it is compressed. During compression, the gas particles, which are initially very far apart (Assumption 1), are crowded closer together. Diffusion
Gases spread out and mix with one another, even without being stirred. The random and continuous motion of gas molecules (Assumption 3) carries them throughout the available space. Such spontaneous mixing of the particles of two substances caused by their random motion is called diffusion. The rate of diffusion of one gas through another depends on three properties of the gas particles: their speeds, their diameters, and the attractive forces between them. Hydrogen gas diffuses rapidly into other gases at the same temperature because its molecules are lighter and move faster than the molecules of the other gases. Effusion

Effusion is the process by which gas particles under pressure pass through a tiny opening. The rates of effusion of different gases are directly proportion to the velocities of other particles. Thus, molecules of low mass effuse faster than molecules of high mass.

Deviations of Real Gases from Ideal Behavior

When their particles are far enough apart and have enough kinetic energy, most gases behave ideally; however, all real gases deviate to some degree from ideal-gas behavior. Particles of real gases occupy space and exert attractive forces on each other, and at very high...
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