XPost: rec.arts.sf.written
On 13/06/2024 01:39, John W Kennedy wrote:
On 6/7/24 9:09 PM, Peter Fairbrother wrote:
On 07/06/2024 22:55, Your Name wrote:
On 2024-06-07 21:30:55 +0000, Lynn McGuire said:
xkcd: Unsolved Chemistry Problems
https://xkcd.com/2943/
Sounds like a good start to me. I fight with ionic chemistry all
the time in my job.
Explained at:
https://www.explainxkcd.com/wiki/index.php/2943:_Unsolved_Chemistry_Problems
Lynn
Solved the last one of those:
"The letters pH stand for potential of hydrogen, since pH is
effectively a measure of the concentration of hydrogen ions
(that is, protons) in a substance."
:-)
SPOILER ALERT
s'aktually an abbreviation of an abbreviation. It should be p[H+]
The "p" is used to denote the decimal negative logarithm of the
following quantity; you can have p(tons of concrete) or p(the number
of atoms in the universe), though I don't know why you might want to.
The square brackets nowadays mean the concentration of whatever is
inside them in moles per liter, ie [H+] is the concentration of
hydrogen ions in a sample.
And so p[H+] or more commonly pH is the negative logarithm of the
concentration in moles per liter of +ve hydrogen ions in a sample.
The "p" does not stand for anything, contrary to many contrary
opinions. Sorenson had the concept of negative logs, but the p in his
symbol - a p with an individualised H subscript - referred to test
tube p, with the unique H subscript meaning the negative log of the
concentration of H ions.
pH was easier to print than (a range of) individualised subscripts, so
quite quickly the p came to mean the negative log of the concentration
of whatever.
:)
He says that he mistrusts H+ ions (which are naked protons). When I was
in high school, I’m pretty sure I was taught that it was actually
hydronium ions (H₃O+) that were counted.
Yep, you probably were taught that. But..
Another chemistry problem, but this one is mostly solved. I don't trust
H+ naked protons either, but chemists often make simplifications as the
real explanations are often loooong.
Teachers sometimes refer to it as "lying to children", but strictly
amongst themselves. Indeed I will have to leave some things out here:
like the role of the water molecules pulling on the A moiety of the A-H
acid during dissociation, the historical background (textbook authors
knew the protons were hydrated, then poly-hydrated, but until recently
didn't know how), or the occasional naked proton; otherwise it will be
too long.
When an acid - call it A-H - dissociates in water to form A- and H+
ions, both the A- and H+ ions are conjugated with water molecules to
form complex ions.
The mechanism of dissociation requires water (or other) molecules
(plural) to be present. The water/other molecules grab onto the H atom
in the acid and rip it off, leaving the A part and an electron behind.
In some cases these other molecules can be the acid molecules
themselves, but otherwise generally no water means no dissociation.
Also, generally speaking, there are no naked protons. In the case of the conjugated H+ ions in water or aqueous acid solution the hydrated
protons can be H3O+ hydronium (we Brits spell it "hydroxonium") cations,
but several other cations will be produced as well.
In most cases the most common hydrated proton cation is not hydroxonium
but the Stoyanov cation, H+(H2O)2(H2O)4, a hexahydrate.
Other cations will include the hydronium monohydrate, one or perhaps
more dihydrates, several hydrated symmetrical dihydrates, a
tetrahydrate, and so on. The H3O+ hydronium monohydrate does exist, but
is comparatively rare.
When we want to measure the strength of an acid (more correctly the
acidity of an acid) what is important is the total number of hydrated
protons, of whatever type: sometimes we use the symbol H+(aq) for this,
but often we are key-lazy or simplifying and just use H+.
tl:dr
Sometimes we are even lazier and use the term hydronium ions; but that
term is ambiguous as it can be used to mean strictly H3O+ ions or total hydrated protons; so the usage is discouraged - and ambiguity in
chemical naming is BAD, whereas simplification or common names are
acceptable where widespread and there is no ambiguity.
If we say H+ ions we know that they aren't really H+ ions, but we know
what we do mean; whereas if we use "hydronium ions" we may be unsure.
just had knee surgery with a general anaesthetic yesterday, so I'm a bit
out of it,
Peter Fairbrother
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