"Sandy" wrote in message
...


In article , Sandy
says...
I'm wondering why the salt. It surely does nothing but perhaps cause
problems later on.

See your friendly local chemistry professor :-).

Do you not know what it does?
I've got a fairly reasonable handle on chemistry, and I can't see what
the salt is there for. The acetic acid will dissolve rust, and slowly
dissolve iron. No need for the salt unless you know of a good reason.
Please expound.



Sandy - I provided that rather long post that you responded to yesterday. I
deleted it because of its size, and most of it is unrelated to your
question. Also, you should realize (if you didn't) that I was passing on an
article written by someone and didn't add any of my own thoughts, since I
have forgotten so much chemistry I didn't want to dust off the brain cells.
However, after reading your correct responses to his article, I had some
thoughts that might possibly help, although I don't have a proven answer to
your question.

Let me say that I agree that the driving force in the reaction to dissolve
the iron oxide is the hydronium ion, and formation of water on the right
side of the equilibrium. I think the answer to why you have to add salt to
the mix is that the salt increases the ionic strength of the solution. This
improves the conductivity of the solution, and might have the effect of
improving the rate of reaction by allowing an easier transfer of charges
among the reacting ions. I know this is a bit vague, but I don't know if
anybody has a really good answer on this.

On the other hand, I recall that the equilibrium constant is really based on
the activity of the ions in solution, and not their concentration. At
higher ionic strengths (due to salt addition), I believe the activity, and
therefore, the equilibrium constant, is lower than it would be without the
salt. It seems to me this would tend to lower the dissociation of the
acetic acid, and bind up even more H+ than without the salt. I think if
this is correct, that it isn't the prevailing factor as I believe the salt
does increase the reaction rate.

I think the case of cleaning your copper pots is similar to the rust issue.
In that case, if you pour vinegar on the copper pot with the oxides on it,
you won't see much if anything happen. When you sprinkle salt on the
surface wetted with vinegar, you quickly see the oxides disappear from the
spots where the salt is dissolving. It is very clear that salt does speed
the rate of reaction in the case of copper oxides, and I have to think it
does the same with iron oxides.

If you don't believe salt does anything, it is simple enough to test for
yourself on two equal rusty spots. As to "why" it works, I have to think it
is because of improved migration of electrons through solution due to the
higher ionic strength of the solution. You might find, for example, that it
takes two weeks to do the job with acetic acid alone, and one day with the
salt added. Whatever problems you say the salt may cause later on may be
outweighted by the time factor. Of course it is also possible (probable?)
that the acid alone will not only take longer, but might in fact not remove
as much oxide in the end.

I had one other thought, improbable as it may be: I'd say the Fe3+ ion is
relatively large compared to the Cl- ion. On the other hand, the acetate
ion is of course a molecule and not a single ion. Maybe there is also a bit
of steric hindrance going on as well. With no Cl- present, all the Fe3+ has
to bond with the acetate molecule (3 of them). If there is some difficulty
fitting 3 of these molecules on one Fe ion, the reaction could be inhibited.
If free Cl- is present, it could more quickly and easily neutralize the Fe
ion and move the molecule away from the reacting area.

regards,
dwhite