How to turn a weak acid into a strong acid

  • #26
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My experience is with large lead acid storage batteries kept at float voltage, therefore in a fully charged state. These batteries have see through cases allowing for views of any boiling or extreme chemical reactions. In maintaining proper electrolyte level no violent reactions were ever observed. Certainly not a lab setting, this is why your statement baffled me.
I doubt lead-acid batteries contain highly concentrated sulfuric acid anyway. The reason you should always add acid to water is to reinforce a safety mindset so when you are working with 18 M sulfuric acid, you don't do the opposite and risk getting yourself hurt. But if you add water to 0.1 M acetic acid ... yeah, nothing will happen.

First you need to understand why water to acid is bad. Dilution of sulphuric acid is a strongly exothermic reaction. Adding a small amount of water to a large amount of acid would generate heat around the diminishing water droplets and could be enough to boil them. The steam could blow acid around.
I don't *know* why the reverse process is safer. It also generates heat, but the nucleus is a diminishing drop of concentrated acid, which if it boils (BP much higher than water), maybe likely to blow water and diluted acid around. Still, I'd pour cautiously, pausing frequently, looking, listening and feeling (indirectly!) for signs of heating. (Not to mention goggles and other PPE.)

Spills - you want to get rid of the acid, so you can hardly do that by adding more concentrated acid. You have to add water. Undesirable and potentially harmful, but necessary. And you add massive amounts of water very quickly, hopefully overwhelming the heating effect with a small quantity of acid.

If there were a large spill, you would need to be very careful. Perhaps use a large hosepipe from a good distance?? (I'really don't know. Just thinking what I might try if I absolutely had to.)

Batteries - Again, you're stuck with the necessity of adding water, so must just try to do it carefully. I think the sulphuric acid gets up to about 4M (but not sure). I can only assume that this is low enough to allow topping up with water, without boiling. One could make a point of adding the water when the battery is at a low state of charge, when the acid is less concentrated.
Adding water to sulfuric acid and adding sulfuric acid to water are equally exothermic. However, adding acid to water allows the heat to disperse throughout the solution so nothing exciting happens. This won't cause acid to spray around.
 
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  • #27
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The heat capacity shouldn't be that different between the two liquids, but splashes from a container with water are safer than splashes from a container of concentrated acid.
 
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  • #28
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For properties of hydrogen cloride, such as heat of solution/dilution, see:
https://www.jsia.gr.jp/data/handling_02e.pdf
If you add 1 g of water to 1 l of hydrogen chloride, would hydrogen chloride be spilt from the bottle, or would air be sucked in?
 
  • #29
Ygggdrasil
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The heat capacity shouldn't be that different between the two liquids, but splashes from a container with water are safer than splashes from a container of concentrated acid.
The specific heat capacity is not likely to be different between the two liquids, but often one would be diluting a small quantity of concentrated acid in a larger volume of water, so heat capacity would be another reason to add acid to water (in addition to the point you bring up about the concentration of acid in splashes). For example, a drop of water in 1 mL of concentrated acid would cause a larger temperature change than a drop of concentrated acid in 10 mL of water.
 
  • #30
Merlin3189
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For properties of hydrogen cloride, such as heat of solution/dilution, see:
https://www.jsia.gr.jp/data/handling_02e.pdf
If you add 1 g of water to 1 l of hydrogen chloride, would hydrogen chloride be spilt from the bottle, or would air be sucked in?
I've not done that. But when dissolving hydrogen chloride in water, we used an inverted funnel to avoid the water being sucked back into the HCl generator.
So I'd expect air to be sucked in.
HCl_funnel.png


You can see water being sucked in when some is added to a flask containing some HCl. This is the HCl fountain experiment
 
  • #31
The basic confusion here is between acid strength and acid concentration. A strong acid has a weak bond between hydrogen and the atom to which it is attached and a weak acid has a strong bond. The dividing or reference bond is the strength of the hydrogen-oxygen bond in water. Molarity is a measure of concentration not strength and is defined as the number of moles (one gram molecular weight) dissolved in one liter of solution.
In terms of acid strength it would be necessary to change the structure of the acid molecule in such a way as to weaken the hydrogen bond to the atom to which it is attached. One example would be to replace the hydrogens attached to carbon in acetic acid with a more electronegative element such as chlorine. The resulting trichloroacetic acid is a strong acid.
Looking at the concentration aspect, glacial acetic acid is a weak acid but is pure acetic acid so its concentration is 100% and its molarity is 17.4 meaning that one liter of glacial acetic acid contains 17.4 moles of acetic acid.
Fuming sulfuric acid is not pure sulfuric acid but is sulfuric acid containing dissolved sulfur trioxide.
When looking at the issue of adding acid to water, as stated, when a small amount of water is added to concentrated sulfuric acid the highly exothermic dissolution causes the small amount of water (boiling point 100 celsius) to boil splattering the sulfuric acid. When adding the sulfuric acid (boiling point 377 celsius) to water the sulfuric acid does not boil and the heat generated is readily absorbed by the large amount of water.
 
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  • #32
Tom.G
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The easy thing to remember is that acids are generally denser than water. HCl acid has a specific gravity of 1.18; water is, of course, 1.0.

If you have acid in the bottom of a container and add water, the water floats on the acid. You then have a large exothermic reaction at the interface surface which causes both to boil... and spatter acid far and wide.

Conversly, when adding acid to water, the acid migrates downward through the water, distributing the heat and avoiding the boiling.

That's why the recommendation is to SLOWLY add acid to water.

Hope this helps.

Cheers,
Tom
 
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  • #33
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The easy thing to remember is that acids are generally denser than water. HCl acid has a specific gravity of 1.18; water is, of course, 1.0.

If you have acid in the bottom of a container and add water, the water floats on the acid. You then have a large exothermic reaction at the interface surface which causes both to boil... and spatter acid far and wide.

Conversly, when adding acid to water, the acid migrates downward through the water, distributing the heat and avoiding the boiling.

That's why the recommendation is to SLOWLY add acid to water.
That seems to be the better insight!
Sulphuric acid being high boiling is not applicable to acids in general. At azeotrope (98 %), sulphuric acid does boil at 337 (not 377) Celsius, but sulphur trioxide itself releases even more heat on dilution (duh! all the same heat sulphuric acid would release plus the reaction heat to sulphuric acid) and boils at +45 Celsius. Suitably concentrated HCl of course boils at room temperature. As does HF.

But the abovesaid substances still form aqueous solutions that are denser than water, sink to bottom and confine bubbles and spray.
Of acids, the ones that are less dense than water include carboxylic acids starting with propanoic acids. But simple carboxylic acids don´ t have large heats of dilution.
A strong acid might be organic sulphonic acids. But sulphonic acid groups also add density: C2H5CO2H density is 0,99, but C2H5SO3H density is 1,35. Higher aliphatic sulphonic acids and sulphuric acid esters would have to be lighter than water (hard to find actual data), and still strong acids (due to nature of group), with possibly large heat of transferring the proton to water - but the organic tail would participate in diluting the heat of dilution!
 
  • #34
That seems to be the better insight!
Sulphuric acid being high boiling is not applicable to acids in general. At azeotrope (98 %), sulphuric acid does boil at 337 (not 377) Celsius, but sulphur trioxide itself releases even more heat on dilution (duh! all the same heat sulphuric acid would release plus the reaction heat to sulphuric acid) and boils at +45 Celsius. Suitably concentrated HCl of course boils at room temperature. As does HF.

But the abovesaid substances still form aqueous solutions that are denser than water, sink to bottom and confine bubbles and spray.
Of acids, the ones that are less dense than water include carboxylic acids starting with propanoic acids. But simple carboxylic acids don´ t have large heats of dilution.
A strong acid might be organic sulphonic acids. But sulphonic acid groups also add density: C2H5CO2H density is 0,99, but C2H5SO3H density is 1,35. Higher aliphatic sulphonic acids and sulphuric acid esters would have to be lighter than water (hard to find actual data), and still strong acids (due to nature of group), with possibly large heat of transferring the proton to water - but the organic tail would participate in diluting the heat of dilution!
The functional factors are boiling points and heat capacity. Water has a very high heat capacity so adding a small amount of acid to a large amount of water results in a small temperature rise. Density is not a factor as any acid with a high heat of dilution has a very high solubility in water and would dissolve far faster than any possible segregation due to density.
Sulfuric acid does not boil at 45 celsius. Adding sulfuric acid to water RAISES the boiling point of the mixture. If I said 377 celsius, that was a typo I did not catch. Hydrochloric acid DOES NOT boil at room temperature. Otherwise, all those lab reagent bottles would have to be pressure containers and muriatic acid would not be sold in one gallon plastic containers at retail outlets. Sulfur trioxide in fuming sulfuric acid would immediately react with any water to form more sulfuric acid: SO3 + H2O --> H2SO4. This multi-step reaction is used in the industrial production of sulfuric acid.
In any event, the question was about the strength of acids not the hazards of their dilution.
 
  • #35
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Sulfuric acid does not boil at 45 celsius. Adding sulfuric acid to water RAISES the boiling point of the mixture. If I said 377 celsius, that was a typo I did not catch.
45 Celsius is the boiling point of pure (and non-polymerized) sulphur trioxide.
Hydrochloric acid DOES NOT boil at room temperature. Otherwise, all those lab reagent bottles would have to be pressure containers and muriatic acid would not be sold in one gallon plastic containers at retail outlets.
I specified sufficiently concentrated.
Boiling point of concentrated hydrochloric acid drops rapidly with concentration:
20 % - 108 C
30 % - 90 C
32 % - 84 C
34 % - 71 C
36 % - 61 C
38 % - 48 C
42 % - 20 C
45 % - 0 C
48 % - -18 C
The hydrochloric acid sold by retail seems to go to 32 %, which boils at 90 %.
Scroll up at the post where hydrogen chloride generator is used to feed a funnel... there filled with NaCl/H2SO4 mixture, and heated.
Well, you can use the same arrangement to generate gaseous hydrogen chloride by heating hydrochloric acid above 20 %! Say, 32 % acid would give off mostly HCl and little water Thus you can distil one portion of 32 % acid down to 30 % and absorb the fumes into the other portion of 32 % acid which you are cooling. So you can get from two portions of 32 % acid a portion of 30 % and a portion of 34 % acid. Or if the portions are different size and you are using enough cooling of the second portion, as much as 48 % acid.
 
  • #36
45 Celsius is the boiling point of pure (and non-polymerized) sulphur trioxide.

I specified sufficiently concentrated.
Boiling point of concentrated hydrochloric acid drops rapidly with concentration:
20 % - 108 C
30 % - 90 C
32 % - 84 C
34 % - 71 C
36 % - 61 C
38 % - 48 C
42 % - 20 C
45 % - 0 C
48 % - -18 C
The hydrochloric acid sold by retail seems to go to 32 %, which boils at 90 %.
Scroll up at the post where hydrogen chloride generator is used to feed a funnel... there filled with NaCl/H2SO4 mixture, and heated.
Well, you can use the same arrangement to generate gaseous hydrogen chloride by heating hydrochloric acid above 20 %! Say, 32 % acid would give off mostly HCl and little water Thus you can distil one portion of 32 % acid down to 30 % and absorb the fumes into the other portion of 32 % acid which you are cooling. So you can get from two portions of 32 % acid a portion of 30 % and a portion of 34 % acid. Or if the portions are different size and you are using enough cooling of the second portion, as much as 48 % acid.
Please cite your source as this data does not make sense.
 

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