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I learned that all strong acids have a 100% degree of ionization and that there are no degrees of strength in acids; however, I also read that fluoroantimonic acid is 20 quintillion times stronger than 100% sulfuric acid !!

Can you please explain the chemistry behind this?

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  • $\begingroup$ There are degrees of strength in acids, else there would not be strong acids and weak acids. There would just be acids. $\endgroup$
    – Ben Norris
    Commented Oct 8, 2013 at 22:06
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    $\begingroup$ I think the way I phrased that is confusing. I meant that there are degrees of weakness but no degrees of strength because all strong acids 100% ionise. That it just what I learnt in high school so it must be wrong :) $\endgroup$
    – please delete me
    Commented Oct 9, 2013 at 15:04
  • $\begingroup$ The acid strength dichotomy of strong/weak is relative to $\ce{H3O+}$, the conjugate acid of water (see Brian's answer). We can make the divide relative to other conjugate acids, and then some of our "strong" acids may become weak acids. $\endgroup$
    – Ben Norris
    Commented Oct 9, 2013 at 19:12
  • $\begingroup$ Dissociation as in $\ce{HB + H2O<=>H3+O + B-}$ is not ionisation. $\endgroup$
    – Martin - マーチン ♦
    Commented Apr 29, 2014 at 16:24

2 Answers 2

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For many common purposes we say there's weak acids and strong acids, and define strong acids to be ones that dissociate completely in water. This is because if the acid dissociates completely in water, then every proton dissociated will just protonate water, and turn it into $\ce{H_3O^+}$ (hydronium ion). The hydronium ion is the strongest acid that can exist in water as far as I understand, and hence why strong acids are simply strong acids in some contexts.

What you're describing is a super acid, that is not used in aqueous solution Take HCl for example. This is a classic strong acid, that will protonate a wide range of structures. But if we take something that is a VERY, VERY poor base (something that really does not want a proton), such as an alkane, HCl can't protonate it. The proton will rather stay with chlorine than protonate the alkane. That is one of the things you can do with the acid you mentioned.

The strength of an acid depends on many different factors, but it all comes down to how strongly the $\ce{H^+}$ is bound to the acid structure. If it is very loosely bound, the structure is high in energy, and the proton would like to find a more stable structure to partake in. In this case protonating an alkane to form a carbocation and molecular hydrogen is low enough in energy, that the acid can protonate it.

But remember that protonation is an equilibrium. If we achieve a vastly lower energy state by having the acid protonate another structure, then the protonation will be complete. If it is the same energy, half of the acid will have protonated (assuming equal concentrations of acid and base).

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No water! $\ce{SbF5}$ is a tremendously strong Lewis acid: high oxidation state, fluorine electron withdrawal, and terrible mismatch of pi-orbital size inhibiting electron back donation. Solvo-cation, solvo-anion. $\ce{F^-}$ is a strong Lewis base. $\ce{SbF6^-}$, given its large size, symmetry, charge delocalization, and lack of polarizabilty (those hard fluorines) is an exceptionally weak base and weak nucleophile. The proton in $\ce{HSbF6}$ is naked and exceptionally acidic.

The system is anhydrous, so there is no leveling effect making hydronium the most acidic solvo-cation.

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  • $\begingroup$ But It is said that it is 10^16 times more powerful than our good old friend H2SO4, You can't do much research on it.. It's definitely most corrosive thing ever $\endgroup$
    – Connor Verlekar
    Commented Dec 30, 2015 at 17:42

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