general chemistry - water solubility vs. change and atomic radius

[Monkeymaniac]

How does the ion charge or atomic radius affect the water solubility?

While reading atomic theory section in BR, the following statement regarding water solubility of Alkaline earth metal cations came across.

Their cation form is not as soluble in water as are the alkali metals, primarily due to their +2 charge and smaller radius.

Ok, so I understand that AEM's ion form has +2 charge and they have smaller radius than ions of AM due to larger effective charges, but how do these two quantities affect water solubility?

In biology, we learned that molecules with charges are hydrophilic (water-loving), so I don't understand how an ion having a greater charge would negatively affect its solubility in water. Any ideas?

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[Evergrey]

That's pretty confusing. I just finished reading the BR text info about amino acids and they explicitly state that charged amino acids are always more soluble. I see no reason why that logic doesn't extend to AEM cations... Although, if the molecule is very small, it will be difficult for water to solvate it. Maybe that's what their referring to, since the +2 charge would reduce radius significantly? Not sure...

 

[Monkeymaniac]

For those of you who have BR books, refer to general chemistry part I page 107 for the sentence. Maybe having more contexts would help you understnad it.

 

[Isoprop]

I don't have the TBR books and I never read them. But I believe it has to do with electrostatic attraction between cations and anions. Ions with stronger charges and smaller sizes tend to be in their lowest energy states bonded to other ions. That's because the electrostatic forces are stronger when the distance is smaller and the charges are bigger.

F = kQq/r^2 (for alkali earth metals q is larger and r is smaller so F is bigger).

For larger ions with smaller charges, there is a smaller attractive force between the ions. Their lowest energy states tends to be when dissolved because the increase in entropy from solvation is greater than the weaker energy it takes to make the ions bonded.

 

[Monkeymaniac]

Ok. But, how would that describe more hydrophilic molcules tending to have greater charges then? Besides, ion forms of AM or AEM atoms are not bonded to another ions. They exist in neutral state and then become ionized when they get mixed with water only. There are only the cations and water molecules in the solutino, no third set of ions bonded to the cations.

 

[Isoprop]

Ok. But, how would that describe more hydrophilic molcules tending to have greater charges then?

You are confusing the concepts between solubility of charged organic ions and the solubility of ionic compounds. Solubility of organic molecules are best predicted by polarity, which is why we look at number of charges. Solubility of ionic compounds are governed mostly by electrostatic forces.

Besides, ion forms of AM or AEM atoms are not bonded to another ions. They exist in neutral state and then become ionized when they get mixed with water only. There are only the cations and water molecules in the solutino, no third set of ions bonded to the cations.

No, ionic compounds of solids can be thought of as arrangement of charged atoms held together by electrostatic attraction. Na of NaCl is definitely not neutral. An ionic bond is essentially electrostatic attraction between a positive charge and a negative point charge. When put in water, the compound will solvate if the interactions between the ions and water molecules is stronger than the interaction between the ions themselves.

 

[Monkeymaniac]

No, ionic compounds of solids can be thought of as arrangement of charged atoms held together by electrostatic attraction. Na of NaCl is definitely not neutral. An ionic bond is essentially electrostatic attraction between a positive charge and a negative point charge. When put in water, the compound will solvate if the interactions between the ions and water molecules is stronger than the interaction between the ions themselves.

Where did you get the NaCl from? The passage was taking about pure M(s) reacting with H2O(l). So we're talking about Na or Li in its neutral forms, not in its cation forms attached some halogen counterparts.

 

[Isoprop]

Where did you get the NaCl from? The passage was taking about pure M(s) reacting with H2O(l). So we're talking about Na or Li in its neutral forms, not in its cation forms attached some halogen counterparts.

Even if you are using pure elemental metal, we need to evaluate the electrostatics of the cation and anion and compare it to the entropy gain from solvation.

Example:
Na (s) + H20 --> NaOH + H2 (not balanced)

Is Na+ so strongly ionically bonded to OH- that its energy state is lower than the entropy increase from Na+ and OH- being dissolved in water? That's the question we need to ask.

When we compare NaOH to Mg(OH)2, we see that the latter is less likely to be dissolved because of the higher electrostatic forces according to the equation F = kqq/r^2.