# Kinetic Molecular Theory

• The Kinetic Molecular Theory attempts to represent the properties of gases by modeling the gas particles themselves at the microscopic level.

## Kinetic Molecular Theory:

1. Gases are composed of very small particles, either molecules or atoms.

2. The gas particles are tiny in comparison to the distances between them, so we assume

that the volume of the gas particles themselves is negligible.

1. These gas particles are in constant motion, moving in straight lines in a random fashion

and colliding with each other and the inside walls of the container. The collisions with

the inside container walls comprise the pressure of the gas.

1. The gas particles are assumed to neither attract nor repel each other. They may collide

with each other, but if they do, the collisions are assumed to be elastic. No kinetic energy

is lost, only transferred from one gas molecule to another.

1. The average kinetic energy of the gas is proportional to the Kelvin temperature.

• A gas that obeys these five postulates is an ideal gas.

• The average velocity of the gas particles is called the root mean square speed and is given the symbol urms.

# Gas Law Relationships

## Pressure

• pressure, we may be referring to the pressure of a gas inside a container or to atmospheric pressure, the pressure due to the weight of the atmosphere above us.

• Atmospheric pressure is measured using a barometer

• To measure the gas pressure inside a container, a manometer  is used.

## Volume–Pressure Relationship: Boyle’s Law

• Boyle’s law describes the relationship between the volume and the pressure of a gas when the temperature and amount are constant.

## Volume–Temperature Relationship: Charles’s Law

• Charles’s law describes the volume and temperature relationship of a gas when the pressure and amount are constant.

• In any gas law calculation, you must express the temperature in kelvin.

## Pressure-Temperature Relationship: Gay-Lussac’s Law

1. Gay-Lussac’s law describes the relationship between the pressure of a gas and its Kelvin temperature if the volume and amount are held constant.

## Combined Gas Law

• If we keep the number of moles of gas constant—that is, no gas can get in or out—then we can combine these three gas laws into one, the combined gas law,

• a container is kept at constant pressure and temperature, and you increase the number of gas particles in that container, the volume will have to increase in order to keep the pressure constant. This means that there is a direct relationship between the volume and the number of moles of gas (n).

## Dalton’s Law of Partial Pressures

• Dalton’s law says that in a mixture of gases (A + B + C...) the total pressure is simple the sum of the partial pressures

## Graham’s Law of Diffusion and Effusion

• Graham’s law defines the relationship of the speed of gas diffusion (mixing of gases due to their kinetic energy) or effusion (movement of a gas through a tiny opening) and the gases’ molecular mass.

## Common Mistakes to Avoid

1. In any of the gas laws, be sure to express the temperature in kelvin. Failure to do so is a quite common mistake.

2. Be sure, especially in stoichiometry problems involving gases, that you are calculating the volume, pressure, etc. of the correct gas. You can avoid this mistake by clearly labeling your quantities