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Author: Subject: Usable heat of dilution data
chornedsnorkack
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[*] posted on 23-5-2023 at 23:41
Usable heat of dilution data


What could be practicably usable sources for heats of dilution of hygroscopic substances (like strong acids) at various concentrations?

There is the slogan "Always add acid into water - not water to acid!" Why?

I get it that the heat of dilution can prompt water to boil. So if you are diluting the acid in an open beaker, then the spray of hot acid can of course get out of the beaker.

But how about diluting acids in a flask topped by a Claisen adapter? Dripping something in, and even if the contents of the flask boil, the flask simply contains the spray because there is no straight way out.

Now, I found an example calculation of heat of dilution for 98% sulphuric acid. This shows heating by about 165 degrees around the concentration of 83%.
185 degree sulphuric acid can certainly cause water to boil by contact.
But how could you look up the peak temperature of dilution depending on the acid concentration? E. g. starting from 60% sulphuric acid at 20 C? Or 38% hydrochloric? 90% nitric?
Also note that while 100% sulphuric acid does not fume, some others do. Like hydrochloric, nitric, oleum...

100% perchloric acid is said to consistently detonate at 90 C at the latest. Am I correct in guessing that 100% perchloric acid will be detonated by its own heat of dilution?
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Lionel Spanner
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[*] posted on 24-5-2023 at 14:40


There is no simple way to calculate the heat change in a given situation; it depends on the respective volumes of the two reagents, the nature of the chemistry between them, and the container-specific nature of heat losses (mostly due absorption by the container's walls, or convection.)

Believe me, if you put enough water into concentrated sulphuric acid, the water will boil nearly instantly, even if the acid is at room temperature.




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chornedsnorkack
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[*] posted on 24-5-2023 at 22:50


Quote: Originally posted by Lionel Spanner  
There is no simple way to calculate the heat change in a given situation; it depends on the respective volumes of the two reagents, the nature of the chemistry between them, and the container-specific nature of heat losses (mostly due absorption by the container's walls, or convection.)

Believe me, if you put enough water into concentrated sulphuric acid, the water will boil nearly instantly, even if the acid is at room temperature.

Container-specific nature of heat losses just takes a bit off the maximum. So this leaves the issue of calculating the maximum assuming no losses (good heat insulation).

No matter whether you put a small amount of water into a large amount of 100% sulphuric acid, a large amount of water into a small amount of concentrated sulphuric acid, a small amount of concentrated sulphuric acid into a large amount of water or a large amount of concentrated sulphuric acid into a small amount of water, at some time during mixing, you are going to have all the concentrations in between - 10%, 50%, 90%...

You can tabulate the temperatures reached in a well insulated vessel for each concentration. Someone must have done it.
And from what I understand, the heats of dilution are one-dimensional, not two-dimensional. If, for example, you produce 10 kcal in diluting 100% sulphuric acid to 98% and 80 kcal in diluting it to 10%, then you already know that if you have the same 98% sulphuric acid already cool, you produce just 70 kcal cooling it to 10%. Correct?
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Lionel Spanner
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[*] posted on 28-5-2023 at 07:55


To be absolutely honest, I think this whole exercise is like trying to make a four-dimensional fractal from a piece of string.



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BromicAcid
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[*] posted on 29-5-2023 at 05:40


I was going to comment on this some time back. I had some vague memory of heat of dilution really coming down to heat of formation of H3O+.

I thought that you could really just determine the mols of H3O+ formed for a strong acid, look at the heat capacities of both liquids, and determine final temperatures from there. Again, this was from some memory - possibly from college.

I found this reference going over the heats of dilution of various acids.
https://diverdi.colostate.edu/all_courses/CRC%20reference%20...

At just a cursory glance I noted that the heats of dilution of HCl, HBr, and HI are all very similar which would make sense if just the H3O+ formation were the driving force behind the exothermicity and that the anion was just along for the ride. The table though is a little weak since I wanted to compare sulfuric acid to its brethren.

Anyway, just some vague remembrances.




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[*] posted on 29-5-2023 at 17:01


Too many variables to evaluate.

There is
initial amounts and temperatures
reaction rates
speed of mixing of the products into the bulk acid
heat conduction through the bulk acid

Some of these will be affected by container geometry, viscosity, rate and manner of water addition (pour versus dropwise addition) and many other factors.

The rule of adding acid to water is a good one. With experience you can bend or ignore the rule in certain circumstances.
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