5 Postulates of the Kinetic Molecular Theory (KMT)

Postulate 4 - Describe the way gas particles stick to each other or push away from each other.

The particles (molecules or atoms) are assumed not to attract or to repel each other.

What kind of gases are NOT defined as behaving according to the postulates of the Kinetic Molecular Theory of Gases?

Real gases

What is a postulate?

Sometimes these postulates are called assumptions, but it's a bit more than just an assumption. The Oxford Languages definition states that a postulate is "a thing suggested or assumed as true as the basis for reasoning, (or) discussion."

Are there any ideal gases?

The answer is both yes and no. In reality, NO GAS behaves ideally ALL THE TIME. But real gases behave ALMOST IDEALLY if conditions are not extreme. Non-polar gases are the most ideal and the more polar a gas molecule is the less ideally it behaves. So, we can think of ALL kinds of gas molecules as behaving ideally unless the conditions are extreme.

Postulate 5 - How is the kinetic energy of a group of gas particles related to temperature?

All gases have the same average kinetic energy at a given temperature. The average kinetic energy of gas particles (molecules or atoms) is directly proportional to the Kelvin temperature of the gas (Charles' Law)

Non-ideal behavior contradictory to postulate 2 (volume of a gas particle)

All matter takes up space. The space occupied by gas gas molecules becomes significant when gasses are highly compressed or super cooled.

Which gas laws require that these postulates be assumed?

All the gas laws used in an introductory high school chemistry class, including: a) Charles' Law, b) Boyle's, Law, c) Gay-Lussac's Law, d) Avogadro's Law, e) the combined gas law, and f) the Ideal (or Perfect) Gas Law.

What are ideal gases?

An ideal gas does not actually exist. It is a hypothetical construct used by chemists and students because it simplifies the math and works well so long as conditions are not extreme. The concept of an ideal gas assumes things like intermolecular forces do not exist to complicate Charles' Law, Boyle's Law, the Combined Gas law, and the Ideal Gas Law. Ideal gases are essentially point masses moving in constant, random, straight-line motion. Ideal gas behavior behavior is described by the assumptions listed in the Kinetic-Molecular Theory of Gases.

Why does EXTREMELY HIGH PRESSURE and EXTREMELY LOW TEMPERATURE result in NON-IDEAL gas behavior?

Because EXTREMELY HIGH PRESSURE and EXTREMELY LOW TEMPERATURE causes gas structural particles to move very close together. The first postulate (or assumption) states that gases consist of PARTICLES that are treated as if they have NO VOLUME. The closer gas molecules are to each other, the less ideally they behave. This is because the volume of the molecules starts to significantly affect the actual volume of the gas. So, since extremely low temperature and extremely high pressure cause the structural particles to be much closer together on average, the gases behave less ideally.

Postulate 2 - Describe the volume of a gas particle.

Gases consist of tiny particles (atoms or molecules); SO SMALL and the distances between the particles are SO LARGE that they can be treated as if they have NO VOLUME.

What kind of gasses are defined as behaving according to the postulates of the Kinetic Molecular Theory of Gases?

Ideal gases

Non-ideal behavior contradictory to postulate 3 (gas particle collisions)

Molecules are attracted to and repulsed from other gas molecules and surfaces resulting in collisions that are not perfectly elastic. Inelastic collisions produce heat..

Non-ideal behavior contradictory to postulate 4 (gas particle attract or repel)

Molecules are attracted to and repulsed from other gas molecules and surfaces. For example, the partially negative ends of polar molecules are attracted to the partially positive ends of polar molecules (dipole-dipole forces). Temporary shifts in electron density also results in attraction and repulsion (London forces).

Are temperature and heat the same thing?

No. Temperature is also different from heat, although the two concepts are linked. Temperature is a measure of the internal energy of a system, while heat 1) is a measure of how energy is transferred from one system (or body) to another, or, 2) how temperatures in one system are raised or lowered by interaction with another. You don't need to know this until later but temperature and heat are related by the equation heat = mass x heat capacity x change in temperature.

Postulate 3 - Describe the way gas particles bounce.

Particles (molecules or atoms) bounce off each other or bounce off a surface with perfectly elastic collisions. This also means that kinetic energy is conserved.

Postulate 1 - Describe the motion of gas particles.

Particles (molecules) of a gas are in constant random motion and moving in straight lines, colliding with each other or with a surface. Collisions with a surface such as the walls of a container cause the pressure exerted by the gas.

Non-ideal behavior contradictory to postulate 1 (particle motion)

Particles (molecules) of a gas can move in the same direction instead of random directions. If gas molecules are primarily moving in the same direction, the pressure is reduced in all but the direction the molecules are moving (this is why the Bernoulli effect works). Particles can be made to curve. This can happen, for example, when molecules are moving in an electromagnetic field. .

Non-ideal behavior contradictory to postulate 5 (kinetic energy and temperature)

The effect of all the exceptions to postulates 1 through 4 can result in a less-than-perfect proportional relationship between average kinetic energy of gas particles and Kelvin temperature.

What is temperature?

Various definitions of temperature can be found but all result in the same thing. The teacher who developed this Quizlet prefers, "A relative comparison of the average random momentum of structural particles in a substance or mixture." So, it is also a comparison of kinetic energy if the particles are moving randomly.

When do real gases typically follow ideal gas behavior?

When conditions are not extreme. This is especially true when the PRESSURE is NOT EXTREMELY HIGH and when the TEMPERATURE is not EXTREMELY LOW.