Presentation on theme: "Chem 300 - Ch 16/#3 Today’s To Do List l More on IM Forces l A bit on Potential Energy l Lennard-Jones & Other Potentials l What is a London Dispersion."— Presentation transcript:
Chem 300 - Ch 16/#3 Today’s To Do List l More on IM Forces l A bit on Potential Energy l Lennard-Jones & Other Potentials l What is a London Dispersion force?? l The return of Van der Waals
Compare with “Real” Equations of State l Attractive and repulsive (excluded volume) forces: l Attractive forces are influential at far distances and are (-) l Excluded volume effects are short-range and are (+)
To Evaluate B 2V l Find & for selected gas l Substitute & into Lennard-Jones equation l Substitute L-J into expression for B l Integrate l With B 2V gas pressure can be calculated from virial eq.
(3) London Dispersion Force l Exists even without perm dipole l QM effect l Electronic Wave function of one molecule is momentarily distorted by passing molecule l Creates an instant induced dipole l Feeds back to other molecule
Other Potential Functions (1) l Hard-sphere Potential Mimic molecules as hard spheres of diameter For r u(r) = 0 H-SP assumes no attractive force Pretty good at high T l Substitution gives: B 2V = (2 2 N)/3 (T-indep)
Other Potential Functions (2) l Square-Well Potential For r< u(r) = for < r< u(r) = - for r > u(r) = 0 S-WP assumes crude attractive potential Gives T-dependent B 2V Reduces to HSP when = 1 or =0
Van der Waals Again l P = [RT/(V m -b)] - a/V 2 m l Compare with Z = 1 + B 2V /V m +...
Next Time l Start Chapter 19: 1st Law P-V work State Functions 1st Law Adiabatic Processes