Ionic Bond vs. Ion-Dipole Force? Which is stronger...

[ilovemcat]

From my understanding, when a salt dissolves into water it's because the forces between water and the ion (ex. Na+ and Water or Cl- and Water) are much stronger (and therefore more favored) as opposed to the ionic bonds within an Ionic Solid.

What I'm having trouble understanding is this: Salts have a very high boiling point due to the strong ionic force. However, from experience, I know the bonds between water and an ion (in this case, Na+ or Cl-) requires a much lower amount of heat input than an Ionic Solid would.

But isn't that contradicting the whole logic behind solubility (Bonds being reformed to form higher bonds)? Wouldn't you expect the water to boil at a much higher temperature than an Ionic Solid alone?

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[ilovemcat]

Since nobody has answered, I'm just going to make the assumption that since dissolving the salt into water would compose a fraction of ion-dipole AND hydrogen-bonds is the reason for the much lower boiling point.

I think I figured it out. Please correct me if I'm wrong.

A salt doesn't completely dissolve into a pure solvent (ex. water). Instead it forms SOME ion-dipole bonds but it's MOSTLY composed of hydrogen bonds (which are much weaker than ionic forces). The rest of the undissolved salt (ionic forces) remain intact. This would explain why the boiling point doesn't change much. So by the time all of the water vaporizes, some of the salt added to the solvent will remain at the bottom (the undissolved portions of the salt).

I have a different question though. How does ion-dipole forces rank in comparison to the other intermolecular forces?

 

[aidama]

Since nobody has answered, I'm just going to make the assumption that since dissolving the salt into water would compose a fraction of ion-dipole AND hydrogen-bonds is the reason for the much lower boiling point.

I think I figured it out. Please correct me if I'm wrong.

A salt doesn't completely dissolve into a pure solvent (ex. water). Instead it forms SOME ion-dipole bonds but it's MOSTLY composed of hydrogen bonds (which are much weaker than ionic forces). The rest of the undissolved salt (ionic forces) remain intact. This would explain why the boiling point doesn't change much. So by the time all of the water vaporizes, some of the salt added to the solvent will remain at the bottom (the undissolved portions of the salt).

I have a different question though. How does ion-dipole forces rank in comparison to the other intermolecular forces?

What I'm having trouble understanding is this: Salts have a very high boiling point due to the strong ionic force. However, from experience, I know the bonds between water and an ion (in this case, Na+ or Cl-) requires a much lower amount of heat input than an Ionic Solid would.

But isn't that contradicting the whole logic behind solubility (Bonds being reformed to form higher bonds)? Wouldn't you expect the water to boil at a much higher temperature than an Ionic Solid alone?

Covalent > Ionic > Ion-Dipole > All Other Dipole including H-Bonding > The Ever Vague London Dispersion

- Covalent is strong because it's actually a bond between molecules.
- Ionic bonds are electrostatic attractions (+/+, -/-, +/-)
- Ion-Dipole is weaker than ion-ion because it's between a dipole (like the negative dipole in water) and an ion

 

[Isoprop]

First of all, ionic bonds are usually "stronger" than covalent bonds. The lattice energy of NaCl is higher than even a triple bond (the absolute value of lattice E I should say). All bonds are stronger than intermolecular forces (ion-dipole, H-bonds, etc.).

Second, the biggest reason why it is "easy" to break ionic bonds and replace them with ion-dipole bonds when you dissolve in water is because of the LARGE increase entropy when you dissolve a crystalline solid into an aqueous solution. This entropy increase overcomes the strong ionic bonds of salts.

But, ionic bonds are still stronger than H-bonds. This is why it takes a lot of heat to melt NaCl than water.

 

[ru4real1666]

Isoprop is completely correct. Ionic> all other bonds, but obviously the solvent, entropy/free energy, etc need to be taken into account.