[kaliaden]

Every version of the solubility rules says that all nitrate salts are soluble in water but why?

[Schrödinger]

What is the type of bonding present in nitrate salts?

[kaliaden]

What is the type of bonding present in nitrate salts?

Ionic bonding, right? And covalent bonding inside the nitrate ion itself.

[Schrödinger]

Well there you have it then, aren't ionic compounds soluble in water, an extremely polar solvent?

[kaliaden]

But barium sulfate is also an ionic compound, yet it isn't soluble...

[Schrödinger]

Well ofcourse you need to see solubility and Ksp products as well. Probably this is favourable too


edit : I initially thought you were trying to ask why alkyl nitrates are insoluble, and yet the text says all nitrate salts are soluble. So I replied by saying what bonding is present in 'salts'  Just got your question. Let's start over, in that case

[kaliaden]

Well ofcourse you need to see solubility and Ksp products as well. Probably this is favourable too


edit : I initially thought you were trying to ask why alkyl nitrates are insoluble, and yet the text says all nitrate salts are soluble. So I replied by saying what bonding is present in 'salts'  Just got your question. Let's start over, in that case

ok.. so why are all inorganic nitrate salts soluble. Does it have to do with some property of the nitrate ion itself?
Also.. although silver salts are mostly insoluble (and ionic in nature), silver nitrate is soluble.. what makes the nitrate ion so special that it can make the Ag⁺ ion soluble??
and thanks for your help btw.. 

[Babcock_Hall]

I don't recall seeing this question addressed in my textbooks, but I have not looked carefully yet.  It is interesting that barium phosphate and barium sulfate are insoluble, but those two anions are tetrahedral and can have more than one negative charge, whereas nitrate is trigonal and has only one negative charge.

[kaliaden]

I don't think it can be explained in terms of hydration enthalpy either, since hydration enthalpy is directly proportional to charge density.

However the nitrate ion is large and the charge is more diffused due to delocalization (therefore lower charge density) whereas the chloride ion is smaller..

that should mean that silver nitrate is less soluble than silver chloride which doesn't make sense..

[Babcock_Hall]

My recollection is that the bond in silver chloride has some covalent character.

[Zyklonb]

I know this is an old thread,and I see it hasn't been answered yet. Unfortunately, I don't know the answer either, but am interested if there is an recent information on the subject.
I know there are many organic nitrates which are insoluble (ie. urea nitrate), which is also ionic, so what's the differance?

[Corribus]

Solubility is complicated, and as soon as you start to generalize, you'll almost just as soon run into exceptions that defy your explanations.

That said, if I was forced to try to explain it quickly and simply I'd point to entropy. When an ion dissolves in water, water molecules will tend to orient themselves around the ion. This is entropically unfavorable. An ion that is more highly charged, or has more concentrated charge, will exacerbate this effect. Thus we might predict that ions that have smaller, more diffuse charge density will also tend to be more soluble. Nitrates have a sweet spot for solubility because they are both low-charge (only -1) and that charge is delocalized over a large volume (4 atomic centers). This will minimize the unfavorable entropy change due to water molecule reorientation during dissolution of the anion.

We can turn to some standard entropies to see if this bears out. If the Gibbs energy for dissolution of a salt into ions is given by ΔH - TΔS, then a larger (more positive) ΔS will give rise to more driving force for solution. Since the ions are the products, a larger standard entropy will result in a lower ΔG.

Standard entropies of nitrate, chloride, bromide, hydroxide, and sulfate offer some instructive examples. The values for these anions are (in J/K mol) 116.5, 56.5, 82.4, and -10.3, 20.1. (I found these values through a Google search, then neglected to copy the link. Sorry!).

You can see that of these ions, nitrate has by far the highest standard entropy, in agreement with my argument above. Chloride, the salts of which are decently soluble, has a more modest value. You might say that sulfate is also a large polyatomic ion - why isn't it quite so soluble as nitrate, usually? Well, sulfate has a formal charge of -2. And this is reflected in its far lower standard entropy value. Hydroxide also offers a nice comparison. It, too, has a formal charge of -1, but it is quite small compared to nitrate, and actually has a negative standard entropy.

So, my guess is that the very large positive standard entropy of nitrate has quite a bit to do with the fact that nitrate salts are always more soluble than similar salts featuring the same cations but different anions. Even silver nitrate is fairly soluble! (For the same reason, ammonium salts tend to be highly soluble: large, diffuse cation.) Of course, solubility depends on more than just the thermodynamics of the anion - there is the cation and the starting salt to worry about, so as I said, it is hard, and probably unwise, to generalize too much.

(The fact that nitrate is both lowly charged and large/diffuse not only plays a role in favorable water orientation/entropy; it also means that the bonds in the solid salt that hold the cations and anions together are fairly weak, which favors thermodynamics of dissolution from this angle as well. I.e., this is a lattice energy concept - salts with weaker inter-ion bonds will tend to dissolve more easily.)

Anyway, that's my 5-minute hand-waving argument.

[Zyklonb]

OK, thanks. That clears up a lot!