Tuesday, September 8, 2020

Gas Laws and Pressure

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.

Gas Laws and Pressure
Yes, there will be math. Eventually. 

Pressure is the continuous physical force exerted on or against an object by something in contact with it. The force exerted per unit area.

Pressure is the result of a force distributed over an area. There are several ways to measure pressure.

Pressure is measured in pascals, Pa - sometimes expressed as newtons per square meter, N/m2. These mean exactly the same thing. Pascals is the SI unit for pressure.

Be careful if you are given pressures in kPa (kilopascals). For example, 150 kPa is 150,000 Pa. You must make that conversion before you use the ideal gas equation.

However, the unit, atmospheres (atm) is often used. The atm unit for pressure is very handy for introductory exercises.

Should you want to convert from other pressure measurements:

1 atmosphere = 101,325 Pa

1 bar = 100 kPa = 100,000 Pa

Another way to measure pressure in in millimeters of mercury, which is a measure used in weather and comes from the use of mercury barometers.

Within the framework of the kinetic theory of matter, pressure can be understood as the sum of all the forces of all the molecules of a gas colliding with the sides of the container. Each molecule is moving quickly and sometimes the collide with the container. When the molecules of the gas strike the molecules of the container, kinetic energy is transferred, and the effect is noted as pressure. The more often and more energetically molecules strike the container, the higher the pressure will be.

How Does Pressure Affect Gases?
When the pressure goes up…

Changing pressure is actually not something that can be done without changing one or more of the other quantities. To change the pressure requires an increase or decrease in volume, temperature, or number of molecules.

Thus, pressure changes can be thought of as responding to changes in the other values. If a pressure change is measured, it will be because some other change was also present. 

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