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aggle-rithm
Ardent Formulist
Evilgiraffe
Scatterer of X-rays
The "what happens" of dissolution is fairly simple. Ions at the surface of a crystal are less strongly bound than those in the bulk crystal (they have fewer neighbours).
Thermal vibration of the surface ions means that, at any given time, a few of them have broken away from their neighbours and have moved away from the surface, leaving a hole.
Consider the crystal in a vacuum (or air, same diff to a first approximation)
Let's say a sodium atom breaks free of the surface. It's positively charged, the hole it left behind is lined with chloride ions and so is negative. Electrostatics dictates that the most likely outcome is that the sodium ion drops back into its hole as though nothing happened.
Now consider the crystal surrounded by water. When the sodium atom breaks free, leaving a hole. The positively charged sodium ion will quickly be surrounded by the partially negative oxygen ends of water molecules. The sodium ion is now solvated, it doesn't fit back into the hole it left behind and floats off into solution.
Repeat ad nauseam to dissolve the entire crystal.
A similar argument work if a chloride ion breaks free of the surface, but this time it is the partially positive hydrogen atoms of water that coordinate to the ion to form the solvation sphere.
The "why it happens" of solvation depends pretty much on the second law of thermodynamics.
Thermal vibration of the surface ions means that, at any given time, a few of them have broken away from their neighbours and have moved away from the surface, leaving a hole.
Consider the crystal in a vacuum (or air, same diff to a first approximation)
Let's say a sodium atom breaks free of the surface. It's positively charged, the hole it left behind is lined with chloride ions and so is negative. Electrostatics dictates that the most likely outcome is that the sodium ion drops back into its hole as though nothing happened.
Now consider the crystal surrounded by water. When the sodium atom breaks free, leaving a hole. The positively charged sodium ion will quickly be surrounded by the partially negative oxygen ends of water molecules. The sodium ion is now solvated, it doesn't fit back into the hole it left behind and floats off into solution.
Repeat ad nauseam to dissolve the entire crystal.
A similar argument work if a chloride ion breaks free of the surface, but this time it is the partially positive hydrogen atoms of water that coordinate to the ion to form the solvation sphere.
The "why it happens" of solvation depends pretty much on the second law of thermodynamics.
This Guy
Master Poster
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Wait! There really is a second law of thermodynamics? I thought that was just something christians made up to disprove evolution!
Madalch
The Jester
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An excellent description, but note that the water will be attracted to the sodium ions while they are still part of the crystal; the cations won't "break free", they will be pulled off by the water molecules.
sol invictus
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Yeah, there really is.... and all it says about evolution is that it would have been impossible if the sun didn't exist (and the earth had no molten core).
Shocking, huh?
Both effects contribute. The salt would evaporate (very slowly) even in a vacuum. When the water is hotter, the salt dissolves more quickly, because both the water and salt molecules are vibrating faster and are thus more likely to pop an atom off the crystal.
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Evilgiraffe
Scatterer of X-rays
Six of one, half a dozen of the other.
If there isn't sufficient thermal energy to break the Na-Cl bonds, all you get is a water molecule adsorbed onto the surface of a salt crystal.
ETA.
Unless you're suggesting that adsorption of water onto a salt crystal weakens the surrounding bonds, making it more likely that the atom will leave the surface and solvate. But I think that's getting a little outside the scope of the OP
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shadron
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Six of one, half a dozen of the other.
If there isn't sufficient thermal energy to break the Na-Cl bonds, all you get is a water molecule adsorbed onto the surface of a salt crystal.
ETA.
Unless you're suggesting that adsorption of water onto a salt crystal weakens the surrounding bonds, making it more likely that the atom will leave the surface and solvate. But I think that's getting a little outside the scope of the OP
On the other hand, the asymmetry of the charges on a water molecule are also essential to dissolution. Note that table salt won't dissolve nearly as much in a symmetrical solvent (like a hydro-carbon) or alcohol.
Evilgiraffe
Scatterer of X-rays
Absolutely... At the risk of sounding like a homeopath, Like dissolves Like.
Ionic compounds will dissolve better in a polar solvent such as water, because the water molecules can orient themselves to reduce the strong electric field densities associated with "naked" ions.
Ionic compounds don't dissolve in non-polar compounds because this mechanism will not work. There's nothing to counteract the electric field of the ion, hence the ions prefer to stay bound together as a crystal.
Non-polar molecules dissolve in non-polar solvents because there is an entropic drive for them to do so. Essentially there are infinitely more ways for the molecules to be organised if they are dissolved than if they remain crystalline.
Ionic compounds will dissolve better in a polar solvent such as water, because the water molecules can orient themselves to reduce the strong electric field densities associated with "naked" ions.
Ionic compounds don't dissolve in non-polar compounds because this mechanism will not work. There's nothing to counteract the electric field of the ion, hence the ions prefer to stay bound together as a crystal.
Non-polar molecules dissolve in non-polar solvents because there is an entropic drive for them to do so. Essentially there are infinitely more ways for the molecules to be organised if they are dissolved than if they remain crystalline.
SezMe
post-pre-born
Does the Na or Cl ion stay attached to the water molecule it "leaves" with.
Madalch
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Only briefly- the water molecules are only weakly attached, and are constantly popping off and on for most cations. The exceptions would be Co3+ and Cr3+ ions, which are extremely slow to replace their water molecules (and other ligands).
Evilgiraffe
Scatterer of X-rays
Any one water molecule is only in the hydration sphere temporarily, but the overall level of hydration is constant for a given ion at a given temperature/pressure.
Also, the number of water molecules in an ion's hydration sphere depends on its charge density. Small, highly charged ions like Ca++ will keep hold of many more water molecules than big, "floppy" ions like Cs+.
In fact, ions like Ca++ are capable of holding onto more than one layer of water molecules, the outer ones being easily exchanged and the inner shell being held much more tightly.
Dammit, Madalch beat me to it... Must type faster.
Also, the number of water molecules in an ion's hydration sphere depends on its charge density. Small, highly charged ions like Ca++ will keep hold of many more water molecules than big, "floppy" ions like Cs+.
In fact, ions like Ca++ are capable of holding onto more than one layer of water molecules, the outer ones being easily exchanged and the inner shell being held much more tightly.
Dammit, Madalch beat me to it... Must type faster.
Loss Leader
I would save the receptionist., Moderator
Excelent explanation for the layman. Thanks.
The "why it happens" of solvation depends pretty much on the second law of thermodynamics.[/QUOTE]
A robot ion may not harm an ion or, through inaction, allow an ion to come to harm.
Evilgiraffe
Scatterer of X-rays
Cheers
I've been lurking for a while and it's not that often that a question comes up here that is pure chemistry. Well, not any chemistry that I know about at any rate.
patchbunny
Graduate Poster
To clear up my own ignorance about chemistry (let's not discuss my grades in that course, ok?), given how wonderfully Na behaves when you drop it in water, why does the sodium not now interact with all that water? Is it because it is an ion?
If you read the rest of this thread, let me ask you - why do you think it doesn't?
Madalch
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Yes- it's because it's an ion. Sodium is much more stable as the ion than as the metal, so the metal will react with very many things in order to form the ion. Once it's the ion, it doesn't do very much at all.
Evilgiraffe
Scatterer of X-rays
Indeed, Na+ that occurs in NaCl has a stable electronic configuration.
Sodium as an element has one too many electrons to be stable, as a lone atom, and is desperate to get rid of it. Drop it it water and it can dump an electron onto a passing water molecule giving you Na+ and H2O-.
H2O- isn't particularly happy like this and will grab an H+ ion from any passing water molecule giving you H3O and OH-
OH- is a reasonably stable ion in solution but H3O is unstable and spits out an H2 molecule leaving behind another relatively stable OH-.
All of these reactions are fast and exothermic (releasing heat) meaning that you get a high concentration of hot H2 molecules when you drop sodium into water. Fuel + Heat + Oxygen = pyrotechnic fun!
Drop Sodium into acid (with a ready supply of H+ hanging around) and you can miss out a couple of steps for even more pyrotechnic fun
Some of the details of reacting species may be inaccurate, but the general idea is there. I also apologize for anthropomorphising atoms, but I think it's just easier to get a handle on this way
Sodium as an element has one too many electrons to be stable, as a lone atom, and is desperate to get rid of it. Drop it it water and it can dump an electron onto a passing water molecule giving you Na+ and H2O-.
H2O- isn't particularly happy like this and will grab an H+ ion from any passing water molecule giving you H3O and OH-
OH- is a reasonably stable ion in solution but H3O is unstable and spits out an H2 molecule leaving behind another relatively stable OH-.
All of these reactions are fast and exothermic (releasing heat) meaning that you get a high concentration of hot H2 molecules when you drop sodium into water. Fuel + Heat + Oxygen = pyrotechnic fun!
Drop Sodium into acid (with a ready supply of H+ hanging around) and you can miss out a couple of steps for even more pyrotechnic fun
Some of the details of reacting species may be inaccurate, but the general idea is there. I also apologize for anthropomorphising atoms, but I think it's just easier to get a handle on this way