jimmyboy
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Acid Strengths - Sulfuric VS Hydrochloric
just a random thought - i find that hydrochloric acid has a lower Ka (Acid Ionization Constant) than sulfuric - i always thought you could make HCL
with sulfuric and a chlorine salt and not the other way around making sulfuric the stronger acid - am i missing something? maybe it has to do with the
heavier mass/boiling point of sulfuric -- or that it is a water scavenger?
pKa Measurements are at 25ºC in water
# - 25.00: Fluoroantimonic acid
# - 15.00: Magic acid
# - 10.00: Fluorosulfuric acid
# - 10.00: Perchloric acid
# - 10.00: Hydroiodic acid
# - 9.00: Hydrobromic acid
# - 8.00: Hydrochloric acid
# - 3.00, 1.99: Sulfuric acid
# - 2.00: Nitric acid
# - 1.76: Hydronium ion
# 3.15: Hydrofluoric acid
# 3.60: Carbonic acid
# 3.75: Formic acid
# 4.04: Ascorbic acid (Vitamin C)
# 4.19: Succinic acid
# 4.20: Benzoic acid
# 4.63: Aniline*
# 4.74: Acetic acid
[Edited on 11-9-2006 by jimmyboy]
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12AX7
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Equilibrium owns all. You can easily make HCl (or I would assume even HClO4 as well) by distilling from H3PO4, which simply does not ever evaporate.
The only thing that *can* happen is the volatile (HCl in this case) is removed.
Sulfuric is on the edge of the glassy minerals (boron, silicon, phosphorous oxides) so shares some characteristics (high boiling point, weak acidity),
but also shares characteristics with the strong, volatile mineral acids (high acidity, low boiling point).
Hmm, come to think of it, I should melt some boric acid and toss on salt. The reaction should be similar to that with sulfuric acid, namely bubbling
and release of gas (although with anhydrous B2O3, the salt's anhydride would be given off in exchange, i.e., chlorine gas, hmm).
Tim
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unionised
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I don't know where that table came from but you cannot measure the strength of strong acids in water because they completely dissociate.
The equilibrium for the reaction between HCl and H2SO4 vs their sodium salts may well favour H2SO4 rather than HCl but, because the HCl can be
continuously removed as the gas, the reaction goes to completion (or very near it).
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Nick F
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"you cannot measure the strength of strong acids in water because they completely dissociate."
Some dissociate more completely than others - it's an equilibrium, you'll never get complete dissociation. Although how they actually
measure it, I don't know. Maybe it'll tell me in my lecture notes at home...
"although with anhydrous B2O3, the salt's anhydride would be given off in exchange, i.e., chlorine gas, hmm"
Nah, B2O3 isn't going to oxidise chloride to chlorine.
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12AX7
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It shouldn't, but where's hydrogen going to come from?
Only option is ala 3B2O3 + 6NaCl --> Na3BO3 + uh... BCl3? Hmm
I don't see where it's coming from either, oxygen is going to like boron more than that...
Only other option is it doesn't react, which suggests NaCl would float on B2O3 with no reaction. Hmm, another possibility I suppose...
Tim
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jimmyboy
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its from Wiki
http://en.wikipedia.org/wiki/Acid_dissociation_constant
i was thinking maybe it is that sulfuric is a stronger oxidizer than hydrochloric but that can't be either - nitric is a stronger oxidizer than
sulfuric and you get nitric from sulfuric with nitrate salts - this table makes no sense to me.. i feel that if sulfuric is used to make another acid
then it would be the stronger acid - can nitric displace chlorine salts? i am starting to think this boils down to atomic mass - if that is the case
then nitric would make hydrochloric from NaCl since it is heavier - not a gas
actually perchloric is made with sulfuric and its mass is slightly higher - so this isnt mass at work either..
[Edited on 12-9-2006 by jimmyboy]
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woelen
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If you would have read 12AX7's post, then you would have understood. It all is a matter of equilibrium: proton-exchange between H2SO4 and the other
ion, e.g. Cl(-) + H2SO4 <---> HCl + HSO4(-).
In the case of HCl (and also HNO3 at higher temperature), the product is gaseous, and hence it can escape as gas bubbles. So, one of the reactants at
the right side of the equilibrium is taken away from the system, causing the reaction to proceed to the right.
If HCl were highly soluble in H2SO4 or were not volatile at all, then this method would not be suitable for making HCl. In that case, the HCl would
not be taken away from the system and only a small amount were formed and the system then would reach equilibrium.
As you can see here, acid strength is not the only factor, which determines which acid is formed.
With HNO3 you cannot make HCl from a chloride salt. This is because HCl and HNO3 mix very well in normal concentrated HNO3, and no HCl will escape.
There also is a practical problem. HCl is slowly oxidized by HNO3, forming Cl2 and ONCl, among other products.
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not_important
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Echoing woelen and 12AX7, it's just not strength but the reaction conditions. The commericial production of phosphorus works by reacting 'P2O5' with
carbon, the products of which leave as gases, and makes the 'P2O5' by reacting rock phosphate - essentially a rather impure calcium phosphate - with
the weak acid SiO2 in the formof sand. It works because it's run at bright red heat, the SiO2 is not volatile but the P2O5 is.
In a similar fashion sodium has been made by reducing sodium fluoride with iron, the iron fluorides are still solid at the boiling point of sodium.
Thus sodium distills off even though it is a stronger reducing agent than metallic iron at STP.
Boron oxide will function as a strong acid at high temperatures. If you exclude oxygen the it will mix with halides to varying degrees, depending on
solubilities. It will displace SO3 from sulfates, and nitrogen oxides from nitrates.
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mick
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The stength of the acid is usually measured in water. The strong acids give a low Ph value which I think means they pick up the OH leaving the H+.
When the strong acids are concentrate they will pull the water and then separate the H and O. They are oxidising then.
mick
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woelen
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Mick, what you write is not correct. Strong acids give off H(+) ions and do not absorb OH(-) ions. There are compounds, which absorb H2O and then
split off H(+). These compounds usually are called the acid anhydrides (examples are: SO3, N2O5, P4O10).
The statement about separation of H and O does not make sense to me at all. Any acid can be oxidizing (e.g. when a metal dissolves and H2 is
liberated), and some acids are more oxidizing than others, because they can oxidize in another way than the acid-specific H(+) ---> H2 conversion.
Examples of such acids are HNO3, HClO4, H2Cr2O7, and to a lesser extent, H2SO4.
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jimmyboy
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the temperature of the reactions aside - i guess sulfuric acid would be considered the most "reactive" acid - since i dont have a better word for it -
since it is used to make all the others - but isn't the strongest hydrogen donor
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12AX7
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Yabbut- you can distill sulfuric from phosphoric, and by outright reactivity, something like nitric acid dissolves things faster I would say.
Tim
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jimmyboy
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every acid has its strength of course -- hydrofluoric dissolves most minerals/metals very quickly -- aqua regia would totally dissolve organic
material i think.. if something is made of teflon i guess your SOL 
[Edited on 13-9-2006 by jimmyboy]
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12AX7
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Reminds me, is teflon affected at all by magic acid?
Could be you get the reaction (-CF2-CF2-CF2-) + 4H = 2(-CF2H) + CF2H2 (and further reaction on the ends if the hydrogen makes it more reactive.. )
Hmm fluorine is very electronegative, -CF3 should pull in protons better than anything..?! Ah, but it's pulling electrons from the carbon, which
would make them less available for the carbon; something like (-CF3) + 2H = (-CF2H) + HF seems more likely.
Tim
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not_important
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| Quote: | Originally posted by 12AX7
Reminds me, is teflon affected at all by magic acid?
...
Tim |
Magic acid is typically stored in teflon bottles.
And this patent says that teflon, amoung other things, can be used as a solid suport for magic acids http://www.freepatentsonline.com/4116880.html
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12AX7
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Ah. Figured they must've performed all those experiments in something 
Is it fluorine then, possibly hot, that burns teflon?
Tim
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unionised
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"you cannot measure the strength of strong acids in water because they completely dissociate."
Some dissociate more completely than others - it's an equilibrium, you'll never get complete dissociation. Although how they actually measure it, I
don't know. Maybe it'll tell me in my lecture notes at home..."
Strange as it may seem, I'm familiar with the concept of equilibrium.
OK, that table gives the dissociation of fluoroantimonic acid as about 10^-25
so, for each undissociated molecule there must be about 10^25 molecules of the ionised stuf. That's roughly 10 moles of the stuff ie several Kg in
weight.
How, exactly, do you think anyone hunts for a single molecule in among that?
At the other end of the scale it's estimated that about 1 in 10^50 molecules of methane is actually CH3- and H+
Trust me, they really don't do this sort of thing in aqueous solutions. Apart from anything else, in order for the activities to be even remotely
comparable with the concentrations, the solutions need to be dilute. Essentially the definition of a strong acid is that, in dilute solution, it is
almost completely dissociated.
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Nerro
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Perhaps a measurement in a different solvent, like MeOH might tell you more about the acidity of some strong acids. I don't really know if the
dissociation in waterlike solvents is equal to the dissociation in water though.
#261501 +(11351)- [X]
the \"bishop\" came to our church today
he was a fucken impostor
never once moved diagonally
courtesy of bash
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Nick F
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Hey unionised, I wasn't trying to insult you, I wasn't trying to imply that you didn't know what an equilibrium was!
I was just mentioning that they don't completely dissociate for people who might not realise that. I certainly remember being told at school
"strong acids dissociate completely," and thought it was worth while pointing out that that isn't true.
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jimmyboy
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the dissolving teflon got me searching -- found this..
Commercially Teflon is etched by "free" electrons liberated by sodium in liquid ammonia. While this is more convenient than the elevated temperatures
required by alkali metals alone, the process could probably be improved further by using primary or secondary aliphatic (not aromatic) amines that are
liquid at room temperature or slightly higher allowing for the very exothermic reaction.
i guess you can birch reduce teflon lol - pretty nasty
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12AX7
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Ah, I see.
Then my confidence is once again restored in the fucking nearly indestructible material known as polytetrafluoroethylene 
Tim
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