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Hydrochloric acid is more complicated than you think

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Re: Hydrochloric acid is more complicated than you think

#51

Post by toddm »

aabajian wrote: post #2

I remember a bit of this in undergrad gen chem. If pH stands for -log([H+]), how can pure water (H2O) have a pH of 7? In theory, there shouldn't be any hydrogen ions in pure water. Turns out there are two simplifications being made: i. H+ in aqueous solution is synonymous with hydronium ion (H3O+). ii. Water is in an equilibrium autoionization reaction with itself and the hydronium and hydroxide ions: 2H2O H3O+ + OH-…

Ph.D. in chemistry here.

The stoichiometry which factors into the definition of pH is

    H_{2}O  H^{+} + OH^{-}
which actually includes the activity of proton - that's H^{+} multiplied by a coefficient that reflects the ionic strength of the solution and in non-zero outside of really pure 18 mega-ohm RO water - but that's beyond the scope of this. I will note that your balanced equation is in fact valid, as it includes bulk aqueous water.

(1) H^{+}(aq) is not synonymous with H_{3}O^{+}; the former is aqueous solvated proton, the latter is hydronium ion;

(2) Moving on to the so-called "autoionization" reaction, what you mean to say is that water is self-ionizable or, more appropriately, amphoteric, i.e., is can act as an acid (in this sense, a proton donor) or a base (the hydroxide ion);

(3) The pH of 7 is an ideal and is dependent on temperature, ionic strength, and a few other things (including pressure) that do actually move us off the pH point of 7 for a so-called "neutral" solution; and finally

(4) The units are moles per liter; and hydronium vs. proton does make a difference, as proton is one hydrogen and hydronium is 3 hydrogens.

Re: Hydrochloric acid is more complicated than you think

#52

Post by caf »

aj7 wrote: post #16

Many people have never seen stainless steel rust. I have. At a slightly leaky Swagelok compression fitting on the HCl-containing gas mixture bottle for a XeCl excimer laser. The leaking HCl combines with the room humidity to make an acid that will rust stainless. And it’s brown oxide rust, induced by the stripping of protective oxides by the HCl.

At a house that fronts onto the ocean, even marine-grade stainless steel fittings will get a patina of rust after a few years exposed.

Re: Hydrochloric acid is more complicated than you think

#53

Post by selimthegrim »

jamal-kumar wrote: post #50
adaml_623 wrote: post #42

Earlier quoted context omitted.

Since we are on a tangent anyway I've just realised that the current British PM just this week appeared to be using the non-standard definition when talking with a "homeless" man at a soup kitchen over Xmas. I think I'm this case the PM was corrected fairly quickly but the assumption that homeless means having fewer homes explains a lot of government policy

No it's worse, he asked the dude if he's in business hahaha Imagine if the prime minister was LARPing a role in the service industry asked you that when you're living in a cardboard box

If you read the whole conversation it's not as bad as it sounds

Re: Hydrochloric acid is more complicated than you think

#54

Post by rcthompson »

toddm wrote: post #51
aabajian wrote: post #2

I remember a bit of this in undergrad gen chem. If pH stands for -log([H+]), how can pure water (H2O) have a pH of 7? In theory, there shouldn't be any hydrogen ions in pure water. Turns out there are two simplifications being made: i. H+ in aqueous solution is synonymous with hydronium ion (H3O+). ii. Water is in an equilibrium autoionization reaction with itself and the hydronium and hydroxide ions: 2H2O H3O+ + OH-…

Ph.D. in chemistry here. The stoichiometry which factors into the definition of pH is H_{2}O H^{+} + OH^{-} which actually includes the activity of proton - that's H^{+} multiplied by a coefficient that reflects the ionic strength of the solution and in non-zero outside of really pure 18 mega-ohm RO water - but that's beyond the scope of this. I will note that your balanced equation is in fact valid, as it includes b…

I don't think the GP is saying that H+ and H3O+ are literally synonymous, but rather that "H+" in the context of an aqueous solution is actually shorthand for a hydronium ion and not a solvated proton (with an H2O implied on the other side of the equation for balance). My recollection from high school and undergrad chemistry is that an unassociated proton is very highly energetically unfavorable in solution relative to a hydronium ion, so any "H+" in solution would exist almost exclusively as the latter. Is that accurate, or is it just a simplification they make for the sake of not overcomplicating things in intro chem classes?

Re: Hydrochloric acid is more complicated than you think

#55

Post by selectodude »

bee_rider wrote: post #48
zdragnar wrote: post #38

Earlier quoted context omitted.

Linguistically wrong, factually correct. There are many grades of stainless steel, and they will in fact develop rust over time. 430 is a mild stainless steel that has decent corrosion resistance, but you're not going to want to test it too hard. 304 is more common, albeit more expensive, and will hold up pretty well under most normal conditions. Any expected prolonged exposure to salt requires 316 or better, though.

What do the numbers mean?(slightly confused because your description of 316 makes it sound generally more badass than 304, but I can’t fit that in sequence with 430 being the worst).

https://en.wikipedia.org/wiki/SAE_steel_grades#Stainless_ste...

Re: Hydrochloric acid is more complicated than you think

#56

Post by midoridensha »

colechristensen wrote: post #26
ace2358 wrote: post #23

Earlier quoted context omitted.

Remember, it’s stain LESS not stain FEEE. It merely stains less than regular steel. Not that it is unable to stain at all. Haha I remember when I first realised that.

Literally, the suffix -less means a complete absence of the thing. It is not a synonym for fewer . “Stainless except for odd situations you wouldn’t normally find messing with the oxide protective layer” isn’t a very good marketing name. Even though it is a lie, stainless does mean the absolute.

It may have meant the absolute, because the term was likely invented by a marketer, but in reality "stainless" steel has never been rustproof. It really depends on the alloy, plus the manufacturing quality. Lower grades with less chromium rust pretty easily, just not nearly as much as regular carbon steel.

Another fun fact about "stainless steel": it's usually said to be non-magnetic: you can't stick a magnet to it. But again this isn't completely true: the lower grades are indeed magnetic, but the higher chromium-content ones are not.

Why bother with a low-chromium-content SS? Probably either cost, or performance reasons. The high-chromium alloys are more brittle.

Re: Hydrochloric acid is more complicated than you think

#57

Post by midoridensha »

bell-cot wrote: post #20
323 wrote: post #5

One thing I always wonder is what exactly does HCl dissolve in the stomach, and how does it not destroy the nutritional value of the food?

> ...and how does it not destroy the nutritional value of the food? Acids destroying whatever they touch is mostly a "kiddie" version of reality. 99.9% of acids found outside of chemistry labs and industrial settings are no more dangerous or destructive ( as acids ) than soda pop, lime juice, or human vomit (all of which are acids). The reason is that some far stronger and/or more dangerous acids occasionally show up…

So what kind of acid do xenomorphs use as blood, which can quickly eat through multiple levels of deck plating in the Nostromo?

Re: Hydrochloric acid is more complicated than you think

#58

Post by hilbert42 »

mtlmtlmtlmtl wrote: post #13
aabajian wrote: post #2

I remember a bit of this in undergrad gen chem. If pH stands for -log([H+]), how can pure water (H2O) have a pH of 7? In theory, there shouldn't be any hydrogen ions in pure water. Turns out there are two simplifications being made: i. H+ in aqueous solution is synonymous with hydronium ion (H3O+). ii. Water is in an equilibrium autoionization reaction with itself and the hydronium and hydroxide ions: 2H2O H3O+ + OH-…

Interestingly this was all in the curriculum for my high school chemistry class.

Yes, it was. And it'd be nice if they left it alone (so I didn't have to relearn stuff a new every decade or so). ;-)

Chemistry's a quixotic master.

Re: Hydrochloric acid is more complicated than you think

#59

Post by zimpenfish »

selimthegrim wrote: post #53
jamal-kumar wrote: post #50

Earlier quoted context omitted.

No it's worse, he asked the dude if he's in business hahaha Imagine if the prime minister was LARPing a role in the service industry asked you that when you're living in a cardboard box

If you read the whole conversation it's not as bad as it sounds

I dunno, I've heard/read it and it is pretty bad - "What are you doing this weekend?" is about as tone-deaf as you can get towards a homeless person.

Re: Hydrochloric acid is more complicated than you think

#60

Post by zimpenfish »

mabbo wrote: post #31
ace2358 wrote: post #23

Earlier quoted context omitted.

Remember, it’s stain LESS not stain FEEE. It merely stains less than regular steel. Not that it is unable to stain at all. Haha I remember when I first realised that.

Well, linguistically speaking, adding the suffix "-less" to a word implies "without", in English. You can be penniless, having not one penny. You can be homeless- not just fewer homes but by golly none at all. You could be endlessly fearless and remain motionless when facing things that aren't harmless. Doubtlessly, I've been tasteless in this reply.

> Well, linguistically speaking, adding the suffix "-less" to a word implies "without", in English.

It's also frequently an exaggeration and I doubt most people who heard "I'm penniless" or "It's endless" would think "they have zero money" or "this will continue until I die (and beyond)".

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