Sciencemadness Discussion Board - Unexpected observation

Unexpected observation

wells_c - 2-12-2024 at 01:13

Hi! First post, though I am often directed here anytime I'm looking for information about practical chemistry.

I'm the quintessential amateur scientist; jack of all trades, master of none. I started out in photography a few years before digital began it's takeover, and have recently made the natural and obvious transition to welding (ahem...)

Which helps explain how I came to have some solid NaOH, and a solution of NaOH, on my workbench. I recently bought a new welding machine, and thought I'd test some settings that my old machine didn't have, specifically AC frequency. Anyway I had a few samples of aluminum cut and polished, and was using the NaOH as an etch so I could see penetration. This doesn't explain why I also had a small amount of sulfuric acid out, but there it was.

Safety warning--do this safely, if at all. if you don't know whether you're being safe, you're not. Also I don't need to be informed that I was being unsafe. I was aware at the time.

Energetic reactions are fun if amounts are kept small. So I sent a couple of drops of the sulfuric acid into the saturated hydroxide solution. As expected, each drop made a CHOOOP sound as it went in, and a little cloud of vapor came up. Coooool. I recalled doing similar with individual prills of solid NaOH into hydrochloric acid, with a similar result. the NaOH I had this time was composed of smaller spheres, pretty tiny. I gathered about 10-15 of them carefully into a spoon, leaned back and carefully tipped them into the acid, and...nothing.

Wha??

I looked, and there they were, floating on the surface. As I watched, several slowly began to sink. After a minute, I picked up the jar and carefully swirled the acid around. The prills swirled around in the acid and slowly sank again. They did absolutely nothing. I checked an hour later, and again four hours after that, and they were more or less intact. By the next morning, they had disappeared.

I was surprised to say the least, and thought if anyone would be interested and/or would have an explanation for this un-phenomenon, it would be here. So there you are, I look forward to hearing theories and discussion about what's not going on, and why. A little more background: the NaOH was "100% lye" from a hardware store, and the H2SO4 was from a plumbing supply place. It was fairly strong, it turned a tongue depressor black pretty quickly. However I doubt it was pure enough that there was no water present.

Thoughts?

W

teodor - 2-12-2024 at 05:03

You mean those spheres were liquid spheres after you drop concentrated H2SO4 into concentrated NaOH?
So, you probably get a saturated Na2SO4 solution inside saturated NaOH which doesn't mix because of some surface differences of those two liquids, most probably connected with the properties of their water shells/molecular polarisation. It would be interesting if such a shell can preserve some H2SO4 inside the tiny drop, but for checking this you need to suck a bit from inside with pasteur or micropippete and check it's acidity.

[Edited on 2-12-2024 by teodor]

Sir_Gawain - 2-12-2024 at 07:08

My guess is not enough water present to dissolve the formed sodium sulfate. So it coats the beads, preventing further reaction. Sort of like how nitric acid over a certain concentration (~90%) can’t dissolve copper. Dilute some sulfuric acid and try again.

wells_c - 2-12-2024 at 07:47

No, sorry if I wasn't clear...

Two separate experiments:

First, sulfuric acid was droppered into a conc. aq. solution of sodium hydroxide. Each drop reacted immediately and strongly, as I'd expect. They made a SHOOOP sound and let up some vapor.

Second, SOLID sodium hydroxide (in the form of tiny spheres) was added to sulfuric acid. There was no apparent change to the individual spheres (prills?) of hydroxide, even after an hour or more. No discernable action or change whatsoever.

I knew better than to add aqueous hydroxide solution to sulfuric acid; even a drop of water might pop and splatter. I was expecting the little prills to pop immediately upon hitting the surface of the acid. The fact that they did not react for at least an hour or more was amazing to me. By adding the solid, I was combining the chemicals in an unorthodox way that no sensible procedure would ever call for. Therefore it may be a relatively understudied little corner of chemistry.

I suspect it may have something to do with the reaction conditions and my guess would be that water (or it's ions) is needed in more abundance than what was present in my sulfuric acid. I am in no way a chemist, so this is literally an uneducated guess. The only reaction I've heard that's similar is nitric acid with copper metal, stronger acid reacts less strongly. This is due to the formation of a passivation layer, and somehow I don't think that's what happened here. I am not about to combine greater quantities, nor would I add sulfuric acid to solid sodium hydroxide. I feel like it would reach some sort of tipping point and react all at once. There's just so much potential energy sitting there in intimate contact. It was just so surprising I thought i'd share.

wells_c - 2-12-2024 at 07:56

Quote: Originally posted by Sir_Gawain

My guess is not enough water present to dissolve the formed sodium sulfate. So it coats the beads, preventing further reaction. Sort of like how nitric acid over a certain concentration (~90%) can’t dissolve copper. Dilute some sulfuric acid and try again.

Ah, same time

Great minds... I wrote the clarification to teodor's comment, I wasn't saying "no" to yours. It's possible, though if so it was not visually apparent. The sulfuric I was using was "drain cleaner grade" so I'd be shocked if there was no water present. I'm tempted to do it again, take some photos, and then try dilution and report back. I will stick to using minute quantities...

teodor - 2-12-2024 at 08:00

Ah, sorry, I really misunderstood your experiments.
But the results are really interesting.
They proof that for acid-base reaction the base should be soluble in the acid.
The same happens with diluted acids and water insoluble carbonates: they just don't react. So, in the later case there is no "passivation" layer, so I think you are right, the solubility of NaOH in conc. H2SO4 is close to zero.

[Edited on 2-12-2024 by teodor]

Texium - 2-12-2024 at 08:06

I don’t think sodium sulfate formation even needs to be invoked. In order for the ions to be exchanged, they need to be mobile. Without water present, the sodium and hydroxide ions are locked up in a solid matrix and can’t exchange with the acid’s ions.

I wouldn’t recommend diluting the solution. It would probably react quite quickly as the sodium hydroxide starts to dissolve. I’d recommend decanting as much sulfuric acid away from the sodium hydroxide prills as you can and then carefully diluting the residue with water.

If you left the mixture sitting out, I’d expect to see the prills gradually dissolve as the solution absorbs moisture from the air (both components are very hygroscopic). As more water is absorbed, tiny amounts of NaOH are dissolved and immediately react, until none is left. I highly doubt there would be a violent reaction unless the mixture was added to water all at once.

Sulaiman - 2-12-2024 at 08:37

Definitely unexpected by me - I'd expect an immediate violent reaction,
but the explanations seem logical.
Interesting experiment and observation, I'll try it sometime....

teodor - 2-12-2024 at 08:53

Quote: Originally posted by Sir_Gawain

If I am not mistaken sulfates have much greater solubility in H2SO4 than in water (BaSO4 is a prominent example). This is a common property of salts with the same anion as solvent (at least for di-basic acids...).

Another thing, NaOH and H2SO4 are a base and an acid in H2O solvent system only.

(There is no such thing as a "base" or "acid" without relation to some solvent. NaOH is a base because it contains OH- anion, the result of the solvent dissociation: 2H2O <-> H3O+ + OH-. For H2SO4 the base anions are HSO4- and SO4(2-). They can react with acids which are stronger than solvent (H2SO4) (probably) vigorously)

[Edited on 2-12-2024 by teodor]

wells_c - 2-12-2024 at 09:18

Ok, interesting new info. See pics attached.

First I replicated the experiment. You can see in the last 2 pics the intact spheres of NaOH.

Next, on a whim, I grabbed a tungsten welding electrode (I don't have a glass rod, and I knew tungsten wouldn't react with sulfuric) and started crushing the beads. Hard to tell at first, but I eventually heard little !ZIP! sounds when I would get a good shot square onto one of the spheres. Once in a while I'd see a tiny bubble form. However, most of the powdered remains hadn't reacted, as I could further grind it against the glass and get more sizzle sounds.

I then made up a dilution with 25% water, 75% acid. I started with the water in the "beaker" and added the acid a small portion at a time. The first addition steamed a bit but the rest went in without event. I then added the solid NaOH to the dilution, and still no reaction. However when I swirled it around I heard a few sizzles here and there. When I crushed the beads, the ZIP sounds were more pronounced and easier to make happen. Also the resulting powder showed more activity after being crushed--a few tiny bubbles, and some turbulence generated around the area, with more zip sounds.

I think there are a few things going on, maybe a combination of passivation and the lack of mobility of the ions given the relative lack of water. From my understanding, sulfuric acid holds onto water molecules pretty tightly, that's what makes it such a good dehydrating agent. So even with some present, it's not freed up enough to liberate the hydroxide and sodium ions from the solid so they can react.

To defuse my little chembombs, I dumped each beaker into a large container of water all at once. This could then be safely put down the drain. All told I probably used 20-25 ml of acid. Before I dumped them, and as I was writing this, I heard a few more little !ZIPS! come from one of the beakers

Good times

teodor - 2-12-2024 at 10:10

Quote: Originally posted by wells_c

From my understanding, sulfuric acid holds onto water molecules pretty tightly, that's what makes it such a good dehydrating agent. So even with some present, it's not freed up enough to liberate the hydroxide and sodium ions from the solid so they can react.

It doesn't "hold" the water molecules, it changes them to something else which is not water anymore. Normally water is a mixture of H2O molecules and H3O+ and OH- ions which can form H2O again by combination. In concentrated H2SO4 there are no water molecules neither those ions. If you drop water into concentrated H2SO4 it completely converts it to H3O+ and HSO4- ions. That probably means that there is no way how OH- from hydroxide can combine with H3O+ ions in the sulfuric acid if there are huge forces working for their separation even for water molecules.
It's quite close to your description except there is no water as well as "normal" base ions.

[Edited on 2-12-2024 by teodor]

[Edited on 2-12-2024 by teodor]

wells_c - 2-12-2024 at 14:01

That makes sense, and I'm glad I joined and posted. Thanks for the insight! My chemistry, like the other sciences I interact with, lacks rigor. But I will always absorb well-explained concepts, that help me refine my understanding. One of my hindsight regrets that I would go back and explain to my (much) younger self before college, is the difference between physics and engineering. I chose a small school that had a good physics department but no engineering. The first year and a half was good as a physics major, but then everything went theoretical and I lost interest. Looking back, I really should have been testing steel beams to failure, creating shaped explosives, shortening laser pulses, and helping the high voltage lab build a new tesla coil. But there has never been a better time for seeking out the things that interest me now. Sometimes I think back (not going to get too specific

) and imagine how hard it would have been to learn about something in furtherance of a hobby, and connect with others doing the same stuff, pre-internet. I mean, what? go to the library? Join a group? How would I know there were groups to join? No job in engineering would let anyone skip around disciplines like I have, to whatever interests me at any given time.

Anyway, enough mumbling into my coffee. And I should put this NaOH away, looks too much like sugar