Chapter 18 Ideal vs. Real Gases

Variables Affecting Gases 4 Variables: 1.Pressure 2.Volume 3.Temperature 4.Number of particles By changing any one of these variables we change another.

Letting air out of a ballon

Heating the air in an empty can.

Ideal Gases vs. Real Gases In a “Real Gas” the molecules of a gas take up space (have volume) and they attract each other (have Van der Waals forces). In an “Ideal Gas” we disregard the different particle sizes and attractive forces.

Ideal Gases vs. Real Gases An ideal gas is an “imaginary gas” made up of particles with negligible particle volume and negligible attractive forces.

This chapter will require you to do many calculations, all of which will be wrong. Helium gas vs. Water Vapor Do they have the same size particles? Do they have the same attractive force? Helium is more ideal (However no gas is truly ideal).

Ideal Gases vs. Real Gases We will assume helium gas and water vapor have no particle volume or attractive force. Since this is only an estimation of the true nature of these gases our calculations involving them will only be an estimation of the true value.

Why then use ideal gases if there are no such things and our answers are not completely accurate?

Real Gases: The van der Waals equation a corrects for the effect of molecular attractions (van der Waals forces), and b corrects for the molecular volume

Ideal Gases vs. Real Gases Calculations for ideal gases are only estimations but they are good estimations and are much easier to do.

(This chapter will require you to do many calculations, all of which will be wrong – really will be estimations). –Real gases {He, H 2 O(g)} require different formulas because they have different sizes and different attractive forces. –Ideal gas particles have “no volume” (volume of particles doesn’t matter compared to empty space in between the particles since gas particles are so far apart. In a solid the particles size matters because the particles are packed together next to each other. –Ideal gas particles have “no attractive forces”. That’s why they are gases. –Both the above assumptions are close to the truth and so your “ideal gas” calculations (estimations) will be close to being correct.