## Tuesday, September 8, 2020

### Kinetic Theory and Gasses

General Chemistry Index

Where are we going with this? Getting to the current models of atomic theory didn't happen over night. This page will support the ability to use the kinetic molecular theory with the combined and ideal gas laws to explain changes in volume, pressure, moles and temperature of a gas and apply the ideal gas equation (PV = nRT) to calculate the change in one variable when another variable is changed and the others are held constant.

Kinetic Theory and Gasses
Yes, there will be math. Eventually.

If all matter is made of particles that speed up and take up more room as they get hotter (gain kinetic energy / gain molecular velocity), then it follows that the effect will be interesting to look at in gases, which can change both shape and volume.

To do that, we need to be grounded in a few concepts first:

With those concepts understood, we are ready to start talking about what happens when

Changing Temperature
When the temperature of a sample of gas in a container goes up…

By definition, this means that the molecules have higher kinetic energy. Therefore:
• if the volume of the container cannot change, the molecules (moving faster) will hit the container more often and with more kinetic energy and raise the pressure.
• if the volume of the container CAN change, the molecules (moving faster) will take up more space (spread out) and increase the volume.
Raising the temperature of a gas will increase its pressure if the volume of the gas and number of molecules are constant.

Opposite: Lowering the temperature of a gas will decrease its pressure if the volume of the gas and number of molecules are constant.

Raising the temperature of a gas will increase its volume if the pressure of the gas and number of molecules are constant.

Opposite: Lowering the temperature of a gas will decrease its volume if the pressure of the gas and number of molecules are constant.

The math… Boyle's Law, Combined Gas Law

Changing Volume
When the size of the container decreases…

If the volume changes, the molecules will strike the walls of the container more often (because they keep moving, but have less space to move in). Therefore:
• if the volume of the container decreases and the temperature stays the same, the molecules will strike the walls of the container more often and, thus, increase pressure.
• if the volume of the container decreases and the pressure stays the same, the temperature will decrease.

Reducing the volume of a gas increases its pressure if the temperature of the gas and the number of particles are constant.

Opposite: Increasing the volume of a gas decreases its pressure if the temperature of the gas and the number of particles are constant.

Changing the Number of Molecules
When the number of molecules goes up…
• more molecules in the container will result in more frequent collisions with the container.

• increasing the number of molecules will increase the pressure if the temperature and volume are kept constant.

• increasing the number of molecules will increase the volume if the pressure and temperature are kept constant.
The opposites are also true.

If the volume cannot change, such as in a basketball or volleyball that "isn't flat" something else must happen. The most obvious example is adding molecules of air to a balloon. Blowing into a balloon obviously changes its volume. It is easy to see that one affect adding molecules can have is to increase volume. The opposite is also true.

If the volume cannot change, such as in a basketball or volleyball that "isn't flat" something else must happen. Pumping more air into a basketball or volleyball does not change the size (unless it was flat to start with). It makes the ball harder. Why? The more molecules inside the set, fixed volume, the more collisions with the inside wall of the ball, and therefore, the higher the pressure. So a second affect adding molecules can have is to increase pressure.

Understanding the affect adding molecules has on temperature is not as intuitive. To examine this, we have to imagine a situation where molecules are added but that neither the volume, nor pressure changes. In this case we have more molecules in the same space, which means more collisions with the unmovable wall. For there to be more collisions but to NOT have a pressure change must mean that the molecules are moving slower. Therefore, if molecules are added and if pressure and volume do not change THEN temperature must go down (it must be lowered on purpose).

The math… Universal Gas Law

Changing the Pressure
When the pressure goes up…

An example of changing pressure could be rising from one altitude to a higher one. In such a case, the pressure would go down. Or going down deeper in water would cause the pressure to go up.

Reducing the pressure of a gas increases its volume if the temperature of the gas and the number of particles are constant.

Opposite: Increasing the pressure of a gas decreases its volume if the temperature of the gas and the number of particles are constant.