Relationship Between Solubility and Reactivity

pzona

I just want to make sure I have this right. I know the general solubility rules (i.e. nitrates are highly soluble, hydroxides aren't, etc.), but I'm not completely sure how to compare the solubilities of two salts of the same anion. I'm pretty sure that the salt with the more reactive ion attached to it will be more soluble, since higher reactivity means lower energy required to break the ionic bond between the two ions. For example, if the two salts are KOH and Ca(OH)2, KOH should be more soluble because K is more reactive than Ca. Would this be a fair generalization of the degree of solubility?

chemisttree

I don't think it would be a useful system for ranking solubilities. I haven't really thought about it but there are likely many exceptions to such a rule. Kind of reminds me of the hard and soft acids/bases arguments I've heard.

pzona

I looked into this a little more and found a better rule. Solubility largely depends on the relative sizes of the atoms and the magnitude of their charges. But if there are two compounds, one with similar sizes but small charges, LiF for example, and one with slightly dissimilar ionic sizes but larger charges, such as CaO, what determines solubility? Is charge more important, since it is muliplicative? I assume that in this case, LiF would have the weaker bond because of the smaller charges, but I'm not sure. Any thoughts?

chemisttree

As a first order estimation, you would look at the likelyhood of the solvent forming some type of association with the solute. In water that is mainly hydrogen bonding. Ions tend to form more stable associations with water (not necessarily by hydrogen bonding) than do intact molecules. Using your LiF vs CaO (really CaOH₂ in water) as an example, you would examine the likelyhood that LiF would dissociate vs CaOH₂. The reactivity of lithium vs calcium might suggest which would be more likely to dissociate but I leave that for you to determine. Comparing Group I vs. Group II ions in this way might not be as helpful as learning Group trends by themselves.

pzona

Sorry, that was a bad example, I forgot about oxides forming hydroxides in solution. Obviously LiF will be more soluble in this case because of the lithium ion and the hydrogen bonding between F and H2O.

What if the two ions to be compared were MgCl2 and NaCl? Would NaCl be more soluble because Na and Cl are both singly charged, or is there another reason as well?

chemisttree

MgCl2 is soluble at a rate of 56g per 100 mL water. NaCl is soluble at a rate of 36g per 100 mL water. Which is more soluble? The MgCl2 solution will be 0.59M and the NaCl will be roughly 0.62M. How might atomic mass skew these relationships?