Gas Laws For Real Gases

Ideal Gases do not exist. But they are a useful idea. They are like real gases except: Molecules are assumed to be just points in space with no length, width, or volume. Molecules have no attraction with one another. They do collide and bounce off of one another. Ideal gas molecules do have mass and speed.

Real gas molecules are made up of atoms with protons, neutrons and electrons, so they do take up space and they do have electrostatic attractions for one another. So real gases deviate from the ideal gas equation, PV = nRT, at some pressures and temperatures.

The compressibility of a gas is defined as for 1 mole of an ideal gas at all P, V, and T However, the P, V, and T can be measured for real gases and the compressibility plotted. This is done for several gases in the next two slides.

Slide 10

slide 11

Using the van der Waals Equation. Let a = 0 and n = 1 (no attractive forces and 1 mole of gas) P(V-b) = RT so V= has V = b at T=0 instead of V = for an ideal gas: V = 0 at T = 0 b is the volume of 1 mole of gas molecules.

Using the van der Waals Equation. Let b = 0 and n = 1 (no molecular size and 1 mole of gas) (P + )V = RT so P= and P < Or pressure is reduced from an ideal gas when the molecules are attracted to one another because they are slowed down slightly as they pass each other and do not collide with the walls of the container as hard or as often. If they slow down enough they condense to a liquid and exert no gas pressure at all.

Notice in General: Every gas acts like an ideal gas when the pressure is low. See the graph. Every real gas deviates from ideal when the pressure is high enough to push the molecules close together. When molecules are close together the gas is more like a liquid than a gas.

Every gas acts most like an ideal gas when the temperature is high. Notice in General: Every gas acts most like an ideal gas when the temperature is high. See the graph. The molecules move so quickly that the attractions between molecules have hardly any effect. If the temperature is low enough every real gas will condense to a liquid, in which case it will not act like a gas at all.