Chemistry Question

[peter eldergill]

I was reading an article in Astronomy magazine about chemistry and my mind began to wander.

Putting salt in water dissolves it into ions

What happens if you put salt into an electric field (say two charged plates)?

Pete

[swampyankee]

No clue, although I think solid salt is an insulator. You could, however do the experiment. There is unlikely to be an explosion.

[PraedSt]

I was reading an article in Astronomy magazine about chemistry and my mind began to wander.

Putting salt in water dissolves it into ions

What happens if you put salt into an electric field (say two charged plates)?

Pete

I don't think pure solid NaCl can conduct. It has to be molten or doped with impurities for conduction to happen.

[Swift]

Actually, you will get some conduction, at least if your contacts are good enough (you will probably need to polish the crystal faces or use a conductive paint for your electrodes) and if your voltage is high enough. Sodium chloride crystals can be thought of as a closed packed array of chloride ions, into which sodium ions are located in the spaces in the array. If you put enough voltage on them, you can move the sodium ions. You can also create defects called F-centers. But given a band gap of about 9 eV (compare to 1 or 2 eV for a typical semiconductor), you would need a lot of voltage.

Compared to molten or aqueous sodium chloride, solid NaCl is a very poor conductor.

[El Jefe]

What happens if you put salt into an electric field (say two charged plates)?

Pete

It serves as a good insulator between the two charged plates. Nothing much happens unless you heat it to the melting point.

[peter eldergill]

I was thinking more along the lines of the idea that the water molecules break the ionic bond of the NaCl (at least that's what I think happens), so wouldn't an electric field do the same thing?

Pete

[El Jefe]

I was thinking more along the lines of the idea that the water molecules break the ionic bond of the NaCl (at least that's what I think happens), so wouldn't an electric field do the same thing?

Pete

Well, every insulator has a dielectric strength, the maximum field intensity it can withstand before it breaks down and becomes a conductor. For crystals of sodium chloride, that field strength is 150 kV / mm (ref. CRC handbook). Generally only of academic interest unless you're designing an experiment or dealing with transmission-level voltages.

[PraedSt]

Actually, you will get some conduction, at least if your contacts are good enough (you will probably need to polish the crystal faces or use a conductive paint for your electrodes) and if your voltage is high enough.

Biscuits. I should have remembered this. Thanks Swift.

[PraedSt]

I was thinking more along the lines of the idea that the water molecules break the ionic bond of the NaCl (at least that's what I think happens), so wouldn't an electric field do the same thing?

In case you're wondering why an easier process (dissolving) achieves the same result as a difficult process (melting and/or shocking), it's because the very act of dissolving a solute into a solvent is exothermic. It releases energy which balances (or at least nets) the energy required to separate the solute ions. At least that is what I was taught, if I remember right.
However, it seems all very quantum to me- you use the energy to break solute bonds before you've acquired it. Very strange.

(Chemists please correct)

[BioSci]

it's because the very act of dissolving a solute into a solvent is exothermic.

(Chemists please correct)

Correction: dissolution of some salts are exothermic, some are endothermic. The 'why' is a bit more complicated.

[PraedSt]

Thanks BioSci. Well that just makes everything weirder. If all three processes are exothermic, where does the energy come from? The environment? And what makes it occur spontaneously? Do you have to account for entropy as well?

ETA: That should read "If all three processes are endothermic" of course. Brain freeze.

[clop]

Correction: dissolution of some salts are exothermic, some are endothermic. The 'why' is a bit more complicated.

Perhaps I'm not the only one to wonder - why are the dissolution of some salts exothermic and some endothermic? I'm assuming dissolution implies water? Is it something to do with the dipole moment?

clop

[Strange]

Thanks BioSci. Well that just makes everything weirder. If all three processes are exothermic, where does the energy come from? The environment? And what makes it occur spontaneously? Do you have to account for entropy as well?

Partly entropy. It also depends on the relative bond energies. It all comes down to which reaction is most thermodynamically favourable - which can involve either taking energy from or releasing energy to the environment. You might want to look up "Gibbs free energy" as a starting point. (I studied this stuff for years and was never completely comfortable with it - I learnt enough to solve the course/exam problems. Luckily I never had to apply it!)

[kzb]

If you put NaCl between two charged plates nothing will happen at low energies because the ions are in deep energy wells in the crystal lattice. They will stay put.

If you ramp up the current enough, eventually the salt will melt and conduct electricity. Or at least it will appear to. In reality, the sodium ions will pick up an electron and become metallic sodium, and the chloride ions will lose an electron and end up as chlorine gas. So at this point it has become potentially very messy if you do not take precautions.

[kzb]

For the majority of cases, a substance dissolving in water is an endothermic process, that is it requires energy to do it. That is why most substances are more soluble in hot water than cold water. The reason that it can be a spontaneous process is because the entropy term in the "free energy" equation is bigger than the heat-energy term.

[peter eldergill]

Thanks everyone for your answers

I thought the electric field would act in the same way as the water molecules. This is clearly not the case. I didn't take into the account of the lattice structure as well. I suppose this is why I teach math and not chemistry

Pete

[chornedsnorkack]

Put clean water into a modest electric field. There are very few free ions in clean water (and even less in most nonpolar solvents) so the few ions will start slowly drifting and cause a weak current.

Put ice into the same weak electric field. The ions are fixed in crystal, so in a weak field they cannot even drift, and there is no current at all.

If you put water or ice into a strong field, though then the field can create ions.

If a needle or wire is charged in a gas, then the strong field near the tip creates ions, which can drift away causing a corona. Same applies to liquids. But in nonconducting solids, charges cannot move in a weak field. So, if a needle were injected in an insulator with sufficient field to create ions nearby, would the current stop as the vicinity of the needle gets charged?

[PraedSt]

Partly entropy. It also depends on the relative bond energies. It all comes down to which reaction is most thermodynamically favourable - which can involve either taking energy from or releasing energy to the environment. You might want to look up "Gibbs free energy" as a starting point. (I studied this stuff for years and was never completely comfortable with it - I learnt enough to solve the course/exam problems. Luckily I never had to apply it!)

For the majority of cases, a substance dissolving in water is an endothermic process, that is it requires energy to do it. That is why most substances are more soluble in hot water than cold water. The reason that it can be a spontaneous process is because the entropy term in the "free energy" equation is bigger than the heat-energy term.

Thanks for the pointers. I see I have some reading to do.