Chemistry - hydrates and other such magical beings

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Chemistry - hydrates and other such magical beings

Postby ST47 » Tue Feb 19, 2008 1:35 am UTC

I'm in high school chemistry right now, and we're up to electron configurations/atomic theory right now. While preparing for the science league test, the whole thing about 4s23d4 becoming 4s13d5 came up, and the AP chem teacher said this is why some transition metals form 1+ ions and others don't, but that we can't really predict anything beyond that. While thinking about that, I thought of something else: Hydrates. Based on the chemical formula for an anhydrate, can we determine the formula of the possible hydrates? (Wikipedia said some confusing stuff about organic chem, so just to be clear, I'm talking about inorganic salts)

Also, Wikipedia mentioned fractional coefficients for the H2O. Would AB·5/2H2O mean 5 water molecules 'trapped' in 2 AB molecules, which are working together to hold the water, or is the 5/2 an average, where some are dihydrate and others are trihydrate?

Am I completely missing the point of this? Can it be ANYTHING?

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Re: Chemistry - hydrates and other such magical beings

Postby ducksan » Tue Feb 19, 2008 2:06 am UTC

For the transition metal electron configuration: 3d and 4s, as you've probably seen, are very similar in energy. They can fill differently and somewhat out of order. Determining what oxidation states the transition metals can take just from their positions on the periodic table can be hard; using the electron configurations can make it easier. Indeed, your example could be a 1+ ion almost exclusively because the half-full 3d is stable. Depends on the metal, though. Again, you've probably touched on this before.

Hydrates: These are confusing. So many things form hydrates that we don't even think about; this is almost exclusively Lewis acid-base theory if I've done my research correctly. When dissolved in water, ions are hydrated. They act as Lewis acids. Aluminum chloride is both Lewis and Bronsted acidic; the 3+ cation is hydrated by six waters, and this complex ion (hexaaquaaluminum(III), if my nomenclature is correct) can spit out a proton to form something with an overall 2+ charge.

For the weird formula bit: Not sure if one water can hydrate two molecules. Could be an average. Oxygen does have two lone pairs there, but two coordinate covalent bonds (one atom contributes both electrons; cf. ammonia acting as a base)...not sure. It probably makes two hydrogen bonds in water, but those tend to be weak. The bonds between waters of hydration and the hydrated molecules, in terms of bond strength, (kJ/mol) no idea where they rank. H-bonding in water/proteins/etc. is only about 5-maybe 10% as strong as an actual bond between the two atoms. (O-H in water = about 20x O - - - H in H-bond)
You'll probably never know what hydrates will be formed as solid guessing, that is. Some coordination chemistry (complex ions, etc.), knowledge of what hydrated ions are formed when the salt is dissolved in water, stuff like that MIGHT help. I've done a very small amount with hydrates, but they seem easy enough to understand...

The organic chem probably talked about anhydrides, etc. If two molecules react and spit out a water molecule, an anhydride might be the product. Two acetic acid molecules, e.g., can react under certain conditions to "dimerize" into an anhydride. Their carboxylic -OH groups react, leaving a single O atom joining the remainder of the acetic acid molecules and a water molecule produced from the extra H and OH. The liquid anhydride will readily take in water to revert to acetic acid, and thus even smells like it (vinegar) due to trace moisture in the air.

I never got to take AP Chem, but I took an advanced course designed to be as close as possible. Didn't learn nearly as much as AP, I would guess. Though, I'm in second semester freshmen chem at Tufts currently, and it's mostly review.

Hope this post made sense. I'm not always good at that. =/
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Re: Chemistry - hydrates and other such magical beings

Postby ST47 » Tue Feb 19, 2008 11:40 am UTC

Ah, I see. So, essentially, no reasonable way to predict these things. The bit about organic hydrates is interesting, I think we touched on that last year in bio without actually calling them hydrates. Thanks!

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Re: Chemistry - hydrates and other such magical beings

Postby Boxcar Aldous Huxley » Wed Feb 20, 2008 2:53 am UTC

For transition metals, there's the "18-electron rule," which is like the 8-electron rule, except not nearly as good a predictor ( It states that the amount of electrons in the atoms's atomic orbitals plus the amount of electrons in its ligands will be 18. For example, in Fe(NO3)2*6H2O, Fe2- has 6 electrons, and each water ligand contributes 2 electrons (if I remember the counting rules rightly). This makes 18 electrons (the nitrates are not coordinated).
I think this could be used as a rough predictor of how many waters will be hydrated, although the rule is often violated (usually the amount is 16-19 electrons).
I think this just about exhausts everything I learned from inorganic chem...
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Re: Chemistry - hydrates and other such magical beings

Postby BlackSails » Wed Feb 20, 2008 1:00 pm UTC

I believe the number of water molecules in the hydrate is the number of water molecules that crystallizes out along with the compound.

In solution-phase, it is possible to calculate solvation shells, which is the number of solvent molecules that coordinate to each molecule of solute, as well as futher shells, which is solvent coordinated to the solvation shells.

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