SaccharinSlayer157
Harmless
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But WHY is concentrated sulfuric acid such a strong dehydrating agent?
I've had this question for a long time, especially after being asked to explain the "carbon snake" demo by a friend and finding myself unable to do so
without resorting to "Sulfuric acid really likes water for some reason", so I finally decided to try and find and answer. After poking around on the
web all I found was this thread, and there seemed to be some disagreement and confusion. Everyone knows conc. H2SO4 will pull water out of just about anything, but I
want to know why.
I get that it has something to do with its ability to readily protonate water to form bisulfate and hydronium, and that a solution of acid and water
is usually more thermodynamically stable, but it's not like there's any sort of real "reaction" going on with these explanations as they're all
reversable.
What exactly is going on when concentrated sulfuric acid irreversibly "dehydrates" sucrose leaving carbon? Is it more accurate to look at it as a
decomposition reaction to H2O and CO2 with H2SO4 as the catalyst? Any insight would be greatly appreciated! 
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Keras
National Hazard
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If H₂SO₄ is a strong acid, that’s because it is quite willing to lose at least one proton to become HSO₄⁻. This ionised form is much more
stable, as you can infer from the fact that diluting concentrated sulphuric acid creates a lot of heat. Because the ionised form is so much stable,
the ∆G of the reaction consisting of concentrated sulphuric acid pulling out water from whatever organic material comes into contact with it is
almost always super negative, meaning the reaction is highly favoured. You can also envision it like this: creating concentrated sulphuric acid means
evaporating almost all water from regular sulphuric acid by heating it up to 330 °C. In doing so, you transform part of the heat into potential
chemical energy, which the substance releases each time it has the opportunity to do so by reacting with something else.
Concentrated sulphuric acid is so eager to lose a proton that it does protonate itself to appreciable levels: 2 H₂SO₄ ⇋ H₃SO₄⁺ +
HSO₄⁻. That’s why it is somewhat conductive.
Hydrogen bonding also explains part of this behaviour, and applies to other very hygroscopic substances like 100% pure ethanol.
As to the reaction with glucose/sugar, if you look at glucose you’ll see a lot of —OH groups. Those get protonated by the sulphuric acid,
resulting in H₂O⁺ groups which detach, and the water molecule thus created is immediately sucked by a nearby sulphuric acid, transformed into
H₃O⁺ which can then carry on protonating another —OH terminal group, and so on. At the end, glucose has lost about all its —OH group, and only
the carbon skeleton remains.
[Edited on 18-4-2023 by Keras]
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unionised
International Hazard
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In principle, all reactions are reversible, but some go a lot nearer to completion than others.
One reaction you could consider is this
H2SO4 + H2O --> H3O+ + HSO4-
That "uses up" the water and thus encourages water to enter from elsewhere.
However, it is realistic to say that the acid catalyses the decomposition of the sugar.
Something like this.
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Ma...
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Fulmen
International Hazard
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Reversible reactions are still reactions. When sulfuric acid reacts with water it liberates a lot of energy, if you want to reverse that you will also
need to return that energy. So it won't happen spontaneously.
Calling the acid a catalyst isn't correct. Yes it's still "sulfuric acid", but you can't just decant the acid and redo the reaction.
We're not banging rocks together here. We know how to put a man back together.
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