[lemonoman]

This is absolutely the most appalling series of posts I've read.  I'm not trying to be a jerk or anything, but it's obvious that's something's wrong.

Let's get this straight.

HCl is a strong acid.

HCl dissociates in water.

HCl does NOT dissociate 100% in water.  It dissociates a very very high amount, maybe 99.7% - but NOT 100%.  In school and such you are taught to use 100% as an approximation, because the 0.3% (or whatever is actually is) that doesn't dissociate is negligible.

The reaction is reversible.  The WHOLE time you have an HCl-in-water mixture, I don't care what concentration it is...HCl molecules will be dissociating into H+ (as H3O+) and Cl- ... just as H+ and Cl- will be simultaneously recombining somewhere else.  The EQUILIBRIUM that is set up comes to rest (as in, no change occurs...but the equilibrium exists) at a point where most (i.e. 99.7%) of the HCl molecules you put in are in dissociated form.

Any more questions that need to be resolved?

[mike]

So are any of you contributors going to have a go at actually answering the original question?

(I think I made an attempt somewhere up there )

[lemonoman]

When dissolved in water, HCl is in equilibrium with dissociated ions due to the molecular forces of water.

H₂O + HCl (aq) <-> H₃O⁺ + Cl^-

Toluene does not react with HCl in this way, and so very few ions are formed to conduct.

There isn't actually a strictly distinguished difference between ionic and covalent bonds.  The hydrogen in HCl has enough control over its electron to be considered covalent, but if they were to split it's probable that the Chlorine would strip the Hydrogen of its electron. 

I think it just dissociates "better" in water than in toluene (from what the original posters says).

Both of these answers are correct.  Bakegaku and mike deserve countless Scooby Snacks for their contributions to answering this question and attempting to resolve the million questions which followed.

[Borek]

In water K_a for HCl is listed as 10⁴-10⁷ depending on source. Assuming 10⁴ 0.1M solution contains about 10^-6M undissociated HCl.

list of dissociation constants

[Donaldson Tan]

The simplest way to look at it is:

1. HCl dissociates in water to produce ions
2. This ionisation of HCl does not occur in the toulene solvent.

There is no ions in toulene solvent, hence zero conductivity is measured.

Water had a high dielectric constant, unlike toulene.

[AWK]

Gaseous HCl has polar covalent bond which dissociates easily in solvents with high dielectric constants. It also dissociates in benzene or toluene but in minute ratio.
Probably dissociation can be writen as
2HCl + CH3ph = HCl2^- + CH3phH⁺

[jdurg]

And to answer the whole "fuming" debate, "fuming HCl" is designated as such because the amount of hydrogen chloride gas dissolved in the water is GREATER than the amount that can remain dissolved in the water.  As a result, the excess HCl fumes off.  If you take some fuming HCl and let it sit out in the atmosphere for a while, eventually it will stop fuming as the undissolved HCl exits the solution.  It's like carbonated beverages.  Sodas bubble and emit gas because there is more CO2 inside the soda than can dissolve in the water.  To reach an equillibrium, the excess CO2 "fumes" out until the soda goes flat.

Also, to confirm what was mentioned above, HCl does NOT 100% dissociate in water.  That approximation is just given to prevent confusion amongst those just beginning to learn chemistry and because the tiny bit that doesn't dissociate plays no factor in the calculations and labs peformed my beginning level chemists.

Just to throw a wrench into things, just because a substance is a metal doesn't mean that it forms ionic bonds.  Beryllium is a metal, but it typically will form covalent bonds with non-metals.

[rctrackstar2007]

This is absolutely the most appalling series of posts I've read.  I'm not trying to be a jerk or anything, but it's obvious that's something's wrong.

Let's get this straight.

HCl is a strong acid.

HCl dissociates in water.

HCl does NOT dissociate 100% in water.  It dissociates a very very high amount, maybe 99.7% - but NOT 100%.  In school and such you are taught to use 100% as an approximation, because the 0.3% (or whatever is actually is) that doesn't dissociate is negligible.

The reaction is reversible.  The WHOLE time you have an HCl-in-water mixture, I don't care what concentration it is...HCl molecules will be dissociating into H+ (as H3O+) and Cl- ... just as H+ and Cl- will be simultaneously recombining somewhere else.  The EQUILIBRIUM that is set up comes to rest (as in, no change occurs...but the equilibrium exists) at a point where most (i.e. 99.7%) of the HCl molecules you put in are in dissociated form.

Any more questions that need to be resolved?

that would make a lot of sense why i don't see any equilibrium, thank you 

[tamim83]

And to answer the whole "fuming" debate, "fuming HCl" is designated as such because the amount of hydrogen chloride gas dissolved in the water is GREATER than the amount that can remain dissolved in the water.  As a result, the excess HCl fumes off.  If you take some fuming HCl and let it sit out in the atmosphere for a while, eventually it will stop fuming as the undissolved HCl exits the solution.

This is absolutly correct.  It has more to do with the solubility of HCl in the water.  Fuming HCl is probably made at lower temperatures to dissolve the HCl gas (since the solubility of a gas decreases with increasing temperature).  When the solution warms up, some gas comes out of solution, hence the "fuming" part.  There is probably some Ksp value for this process. 

Also, to confirm what was mentioned above, HCl does NOT 100% dissociate in water.  That approximation is just given to prevent confusion amongst those just beginning to learn chemistry and because the tiny bit that doesn't dissociate plays no factor in the calculations and labs peformed my beginning level chemists.

I believe this.  Even ef somthing has a super high value for K, it still has a "K" value.  Which means there could be the presence of a slight reverse reaction.  It may not be significant to care about but it is still there. 

Honestly, this is the craziest thread I have seen.  But it is such an interesting topic, makes me think a lot harder. 

[jdurg]

Exactly.  It's just like knowing that based on the laws of quantum mechanics, there is a finite probability that if I run full speed into a wall that I will pass right through it unharmed.  While that probability is not zero, it is so amazingly close to zero that I'm not willining to take any chances.   

[Borek]

I remember our quantum chemistry lecturer (or someone from his team) stating that part of students passes QC exam by tunneling effect

[tamim83]

Hmm, you are so right.  That must be where I got that from, my quantum theory obsession  .

[deyicheng]

wait, if HCl is a strong acid then why does it not dissociate 100%? 

and because i am confused about this leads to my incorrect statements in the other posts 

it is really an strong acid ,and it dissociate 100%,but HCl has a high vilatility,so we can only get a 35%HCL solution.

[Dan]

Why bonds in gaseous HCl are covalent? Since HCl in toluene gives a conductivity of zero and HCl in water is 100.

I think about it like this:

Why would HCl dissociate? Energy gain.

Splitting HCl into ions requires energy to break the bond, so where does the energy gain come from? Solvation! This is the key.
The energy gain is from the electrostatic interactions of the ion with the solvent. These interations are high and favourable if the solvent is polar.

So, in water, HCl gains much more solvation energy than the energy cost of breaking the HCl bond => it dissociates.
Toluene is not polar, and hence does not solvate ions well. The energy gain from solvation is much smaller than the energy required to break the HCl bond => no dissociation

[Mitch]

it is really an strong acid ,and it dissociate 100%,but HCl has a high vilatility,so we can only get a 35%HCL solution.

Volatillity is not related to solubillity. Why a certain substance is only so soluble in a give solvent is an involved question. But, if like dissolves like you can start from there.

Physical Chemists will kill all of you except Mike, bakegaku, Borek and some others, it is of course an equilibrium, an equilibrium whose dissociation direction is much faster and favored over the recombination direction. The fact it is in equilibrium is proven by the fact it has a Ka: http://www.chemicalforums.com/index.php?page=pkavalues