Lecture 19: Real Gases and Condensation Improvement of Ideal Gas Law: Van der Waals Equation of State Critical Point: The Onset of Condensation Intermolecular Forces

Collision Rate and Mean Free Path Imagine following a particular molecule moving through a gas for one second: hit miss d crel x 1s = c (m) How many collisions will our molecule make per second?

PRS Question: Air has a density of about 1 g/L at room temperature and pressure Solid nitrogen has a density of about 1 g/cm3 Estimate the volume of one nitrogen molecule for the purpose of estimating the collision rate in air. 1. 14 L/mol 2. 14 mol/cm3 3. 14 cm3/mol 4. 28 L/mol 28 mol/cm3 28 cm3/mol

d ~ [(28 cm3/mol)/(6x1023/mol)]1/3 = 3.6x10-8 cm = 3.6x10-10 m l = (V/n)/(pd2) = [(28 L/mol)(10-3 m3/L)/(6x1023/mol)]/[3.14 x (3.6x10-10 m)2] = 1.1x10-7 m = 110 nm At 500 m/s, there are about 2x10-5 s between collisions. What is good about this as an estimate? What is bad about it?

Attenuation data for the scattering of a thermal beam (1100 K) of CsCl by Ar atoms and by the polar CH2F2 molecules in a 44 mm cell. The log of the transmission decreases linearly with the pressure of the target gas. [Adapted from H. Schumacher, R. B. Bernstein, and E.W. Rothe, J. Chem. Phys. 33, 584 (1960).]

The Ideal Gas Law is Remarkably Good at High T When does it break down?

In which is the mean free path of molecular motion largest? liquid water at 1 atm and 298K CO2 at its critical point CO2 at 1atm pressure and 298K CO2 at 1 atm pressure and 596K

Predict which a cation of sodium is most strongly attracted to: Br- anion HBr molecule Kr atom H2O molecule