Download Coulomb's Law and Electrostatics: Intermolecular Interactions and Electric Fields and more Study notes Physical Chemistry in PDF only on Docsity! 1 Lecture 35 Chapt 20 Coulomb’s Law (electrostatics) Announce: • Give exam distribution Outline: Coulomb’s Law Electric Field Electrostatic Potential Intermolecular Interactions Okay, for this week we are changing our focus to talk about various kinds of intermolecular interactions. Let’s look at the strongest interactions between molecules – electrostatics. (nuclear forces are stronger, but that’s about it.) Electrostatics includes charge-charge interactions, charge-dipole, dipole-dipole, dipole-quadruple, etc. (often described as monopole/multipole interactions) Coulomb’s law Let’s start with Coulomb’s law which everyone remembers from physics ( ) r qqru BA 04 1 πε = u is the interaction energy (units J = V⋅C) (note W = J/s = V⋅C/s = V⋅A) ε0 is the permittivity of vacuum (units F/m = C/V⋅m = C2/J⋅m) So, we can calculate the electrostatic interaction between two charged particles. In practice this is a difficult calculation. Why? Because the interaction falls off very slowly with distance ~ r−1 Text does an example of NaCl crystal. If you just look at one line of charges that surround a central Na+: +−+−+−= L 6 2 5 2 4 2 3 2 2 2 1 2 4 1 2 0 a eu πε 2 we get something that converges VERY slowly. In this case there is a regular arrangement of charges, so we can find a series approximation that helps us out [defines the Madelung constant]. e.g. a eNU A 2 04 1747.1 πε −= This is great for cyrstalline solids, but for more general charge distributions (like dissolved salts, amorphous materials, or proteins you have to try to just add them all up. Fortunately, in most media charges are shielded by the surroundings. This is especially important in water. We can write Coulombs law the way it is really defined ( ) r qqru BA πε4 1= where ε = ε0D where D is the dielectric constant D ≡ 1 for vacuum D = 1.00059 ≈ 1 for air D ≈ 2 for hydrocarbons D ≈ 1.4 often used for proteins D = 33 MeOH D = 78.54 for water Book mentions Bjerrum length, which is a handy tool. Remember our golden tool for connecting microscopic properties to macroscopic behavior? Partition function. Energy always appears as E/kT or E/RT depending on units. The Bjerrum length is just the distance two charged bodies are apart that corresponds to U = RT. Book points out that Bjerrum length is 80 times shorter (i.e. a factor of D, 560 Å versus 7.13 Å) for NaCl dissolved in water than for two bare Na+ Cl− particles. Their interaction is severely weakened by the presence of the water. If we had many charges, we would just add up the u for each of them, until we had accounted for everybody. However, there are easier ways, which we will work up to now.
