Pre-exam Intermolecular Forces Confusion
December 13, 2011 12:05 AM   Subscribe

Intermolecular forces questions: Why do H-S-S-H molecules have dipole-dipole attractions to eachother? Why does ion-dipole attraction make for a stronger solute-solvent interaction than hydrogen bonding?

My chemistry program keeps spitting out the answer that H-S-S-H molecules have dipole-dipole attractions. I'm struggling a little with determining whether molecules are polar here... are we supposed to be able to figure out that the H-S-S-H molecular geometry creates some sort of weakly polar attraction due to its symmetry? I drew the Lewis structure, and I guess I'm confused as to how you would get H-S-S-H being bent (if it is bent... best guess) when there are those two Sulfurs next to eachother as opposed to just one Sulfur, which would definitely be bent with the two lone pairs pushing it down. Can someone explain the Lewis structure and how the symmetry of it relates to polarity of the H-S-S-H molecule? A drawing of H-S-S-H would be appreciated. Google yielded nothing.

Also, I thought less than 0.4 debye electronegativity difference meant that the dipole wasn't significant, but I guess it's different in cases of intermolecular attraction. Could you clarify how EM difference plays into the polarity of a molecule?

Finally, why does ion-dipole attraction make for a stronger solute-solvent interaction than hydrogen bonding? I thought hydrogen bonding was the strongest IMF. Are IMF significantly different in solutions?
posted by sunnychef88 to Science & Nature (8 answers total)
Addendum: For the solvent-solute attraction, the program said KCl in water has a stronger interaction than methanol (CH3OH) in water, which has the OH group on the end which would form the hydrogen bonds with water in a solution.
posted by sunnychef88 at 12:08 AM on December 13, 2011

I'm going to guess that it's because it's not a linear molecule, like hydrogen peroxide.
posted by oonh at 12:26 AM on December 13, 2011

The simple answer to your second question is that stronger charges bond more tightly. Hydrogen bonding involves two partial charges being attracted to each other, say H and O. It's really a type of dipole-dipole bonding, but it's considered a separate type of bonding because the dipoles are particularly strong due to oxygen's electronegativity.

The difference is that in ion-dipole interactions, the ion has a full charge. This full charge attracts the partial charge from the solvent dipole and forms a stronger bond than the two partial charges of hydrogen bonding.
posted by TungstenChef at 12:28 AM on December 13, 2011 [1 favorite]

also: wikipedia page for hydrogen disulfide
posted by oonh at 12:30 AM on December 13, 2011

I'm looking for an explanation for how to derive the molecular geometry of hydrogen disulfide from the Lewis Structure. We didn't discuss skew and torsion of molecules - just linear, bent, trigonal pyramidal, and tetrahedral. Perhaps with two central molecules with two lone pairs it's like two bent molecules put together? Trying to recognize any factors contributing to a dipole moment to identify whether dipole-dipole forces or at work, or just London dispersion forces.
posted by sunnychef88 at 12:38 AM on December 13, 2011

In H-S-S-H, each S has 2 bonds and 2 lone pairs. That means each S atom has a tetrahedral electron geometry, and a bent molecular geometry. Two bent geometries put together form a W more or less.
posted by TungstenChef at 1:45 AM on December 13, 2011

In this case, just use VSEPR theory to find the geometry of each sulfur separately (bent), then imagine linking two bent things together. There's a little more to it than that (keeping things from bumping into each other by rotating them), but that should be sufficient. If you can get your hands on a model kit for 5 minutes, I highly recommend it.

General rule of thumb, though: if you have a molecule more complicated than just one single central atom, and there are polar bonds, the odds of the dipoles cancelling is very very small--it will almost certainly be polar.

When people say H-bonds are the strongest intermolecular force, they're discounting ions, because ionic compounds don't have molecules. That said, ion-dipole has to be stronger, because (as mentioned above) there's full-blown positive and negative charges involved instead of just partial charges.
posted by Dr.Enormous at 2:44 AM on December 13, 2011 [2 favorites]

Dr. Enormous is right; Lewis Structure theory can't tell you that HSSH is bent. You need VESPR theory for that. Lewis structure does tell you the order atoms, but it doesn't have enough detail to derive the bond angles that sulphur imposes. Here's a nice discussion of Lewis bond theory from the Chem Ed. Journal. The rules for di-sulphides like HSSH are in "multi-player molecules", case 4.
posted by bonehead at 6:27 AM on December 13, 2011

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