Re: boiling vinegar

[Brian Whatcott <betwys@SBCGLOBAL.NET>]

I have noticed that there are several competing analytical equations
dealing with the parameters affecting change of state temperatures
and pressures.
This suggests that there are complexities in the effects which
"stick" molecules/atoms/ions together well enough to withstand
thermal  "jiggling".
The frequent effect of mixing liquids is to elevate their joint boiling point.
As an example, mixes of water and nitric acid show the same order of
 BP elevation as the acetic acid  and water mix shows BP depression.

I notice too, that a phase diagram for water shows numerous forms;
ten or more, existing mostly at high pressures.

For want of a rigorous theory, one could easily visualize added ionic bonds
sticking water together into long chains or space filling structures on
a small scale,so as to elevate BP, and as easily, conjecture additions which
shield adjacent water structures so as to make them more mobile at
lower temperature.
Besides the ionic bond, chemists refer to valency bonds, Van der Waals,
and hydrogen bonds - perhaps others too.

Brian Whatcott      Altus OK

At 12:26 AM 4/13/2003 -0400,  John Denker, you wrote:
> Hi Folks --
>
> Consider the following table:
>
>                                 Pure    Aqueous Azeotrope
>                                 -----   -----------------
> Substance       Formula         Boiling         Boiling
>                                 Boint   %age    Point
> Formic acid     H-COOH          100.7   77.5    107.1
> Ethanoic acid   H-(CH2)-COOH    118.1    3.0*    76.6*
> Propanoic acid  H-(CH2)2-COOH   141.6   17.7     99.9
> Butanoic acid   H-(CH2)3-COOH   164.5   18.4     99.4
>
>    Ethanoic acid is also known as acetic acid.
>    Dilute aqueous solutions are called vinegar.
>
>    Butanoic acid is also known as butyric acid.
>
>    All data from CRC Handbook.
>
> This leads to a whole package of riddles.
>
> First, suppose I had showed you this table with
> the two starred items not filled in.  Would you
> have been able to predict anything close to the
> observed answer?
>
> Note that adding some water to formic acid raises
> the boiling point above the BP of either constituent.
>
> For the propanoic and butanoic acids, the BP of
> the azeotrope is within a fraction of a degree of
> the dominant constituent (water).
>
> So why is ethanoic acid so different?  The azeotrope
> contains only a small percentage of acid.  Adding
> a tiny amount of a high-boiling substance lowers
> the BP more than 23 degrees.
>
> To put it in simplest terms, what is going on here?
>
> I have no clue.  I've rummaged around.  I've found
> a number of "just-so stories" that allegedly explain
> the boiling of mixtures, but for every correct
> prediction they make multiple incorrect predictions,
> so they can't be taken seriously as hypotheses.
>
> I even entertained the hypothesis was that it might
> be a misprint ... 96.6 would have been more plausible
> than 76.6 ... so I did the experiment.  It's a rather
> simple experiment:  pot of vinegar plus thermometer
> plus small amount of patience.  It ain't a misprint.


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