What is an Ideal Gas? A gas that behaves according to the Kinetic Molecular Theory. It obeys all the postulates of KMT.
When is a gas ideal? Never!!!! Ideal gases dont exist. However, real gases act like ideal gases at conditions of low to moderate pressure and moderate to high temperatures.
Huh? Real gases follow the Kinetic Molecular Theory (act like ideal gases) until……. The pressure gets very high OR The temperature gets very low
What causes real gases to stop following the KMT? Reality…. KMT says particles have no volume and no attractive or repulsive forces In reality we know they do Experimental data shows they do
The Experimental Evidence that KMT is false 1. At very low temperatures, the observed volume of a gas is smaller than the KMT (all of our gas laws) would predict.
The Experimental Evidence that KMT is false For example, Charles Law might predict a volume of gas to be 1.0 L at 1K, but when we do the experiment in the lab we see the volume is really 0.75 L. We think this is because when particles get very close (like at low T) they are close enough to attract each other and make the volume even smaller.
The Experimental Evidence that KMT is false 2. At very high pressures, the observed volume of a gas is larger than the KMT (all of our gas laws) would predict.
The Experimental Evidence that KMT is false For example, Boyles Law might predict a volume of gas to be 0.25 L at a pressure of 3 atm, but when we do the experiment in the lab, the volume is actually 0.28 L. We think this is because the particles of gas themselves contribute to the observed volume.
So why do we use the KM Theory if it is not true? Because it is a model that helps us understand and predict what gases will do. Under most conditions, real gases act like ideal gases and thus follow the K-M Theory. Only under conditions of very low T and very high P does the KM Theory start to break down.
Are you ever going to tell us what the Ideal Gas Law is?
The Ideal Gas Law PV = nRT P = pressure (in atmospheres) V = volume (in Liters) N = number of moles R = Universal Gas Law Constant (.0821 L atm/mol K) T = Temperature (in Kelvins) Kelvins = o C + 273
Example What is the volume of 2.3 moles of hydrogen gas at a pressure of 1.2 atm and temperature of 20 o C? Ans: V = nRT/P V = (2.3 mol)(.0821 L atm/mol K)(293K) 1.2 atm = 46 L