dawsonsuen - 22-11-2015 at 09:17
I know this is a very stupid question.. but why is it not possible to make hydrochloric acid by dissolving a chloride salt and distilling off the
liquid? Is it because for the hydrochloric acid to be distilled as an azeotrope, the H+ ions in solution has to be in large excess, (i.e from sulfuric
acid or a bisulfate) in order to shift the equilibrium in favour of the product?
Sorry for being naive. 
[Edited on 22-11-2015 by dawsonsuen]
Hawkguy - 22-11-2015 at 11:02
Well if you dissolve a Chloride salt, you won't really have what you want, disassociated H+ and Cl-, which is Hydrochloric Acid. Instead you'll have a
metallic cation and a Chloride anion. Despite disassociating in water, the ions are still 'together', if you will.
Instead you'll have what I think is Na+ and Cl-. You need a stronger acid with more disassociation than Hydrochloric Acid (ie: Sulfuric Acid) to react
with the Sodium, and giving the Hydrogen cation to the Chloride. Now, you'll have Sodium Bisulfate, and Hydrochloric Acid. The Hydrochloric Acid can
be distilled. Sodium Bisulfate is used by Nurd Rage in his Hydrochloric Acid video because it decomposes to Sulfuric Acid and Sodium Sulfate when
heated, letting the above reaction occur.
[This is Chem 11 explanation, anyone can correct me if I'm wrong.]
[Edited on 22-11-2015 by Hawkguy]
aga - 22-11-2015 at 12:16
It is not posible to make HCl by simply dissolving NaCl in water (for example).
However, depending on what reaction you wanted the HCl for, the dissolved NaCl would also give Cl- ions in solution.
So, if your reaction were to require Cl- ions, and not be affected by the Na+ ions, then table salt in water could easily work just as well as actual
HCl.
e.g. dissolve some Gold
http://www.sciencemadness.org/talk/viewthread.php?tid=63606#...
[Edited on 22-11-2015 by aga]
Upsilon - 22-11-2015 at 14:14
Water has an extremely low disassociation constant. To make HCl, you need free H+ ions in solution. The vast majority of regular water is made up of
bound H2O molecules. A very very very small fraction of these molecules dissaciociate into H+ and OH-. So, in theory a very small amount of
HCl is formed, however it instantaneously would react with the OH- ions reverting back to NaCl and H2O.
dawsonsuen - 22-11-2015 at 15:00
Thank you very much for your responses!
ave369 - 22-11-2015 at 22:06
Well, actually you can, if the chloride salt is from a very weak base, such as FeCl3. When you try to crystallize FeCl3 in an evaporating bowl, you
can smell a quite strong smell of hydrogen chloride.
Magpie - 23-11-2015 at 16:29
I want to add to ave369's post:
FeCl3 is the salt of a weak base, Fe(OH)3, and a strong acid, HCl. When it is dissolved in water the following reaction takes place:
Fe+++ + 3HOH --> Fe(OH)3 + 3H+ (approximately)
Fe(OH)3 forms because it is a weak base, ie, poorly dissociated.
This provides the surplus of H+ ions. Together with the Cl- already in solution from the dissolution of FeCl3 there is enough HCl to evolve from the
solution. The pH will also be quite acidic.