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I understand that $\ce{H2SO4}$ is a very good dehydrating agent because it is a strong acid, so that it quickly donates $\ce{H+}$ to hydroxide ions in water to form $\ce{HSO4-}$ and $\ce{H3O+}$ .But we know that there are a lot of other acids stronger than $\ce{H2SO4}$ like $\ce{HClO4}$ which are not used as dehydrating agents. They can also readily give $\ce{H+}$ to hydroxide ions in water and act as dehydrating agents but we don't use them for this purpose. Why is that?

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As pointed out by Maurice and myself, the argument provided in the answer by iad22agp is rather incorrect. Sulfuric acid is a dehydrating agent just because it is available in concentrated form whereas rest of the common acids like HCl(aq) are not. No, it is not a concentration effect. First of all, it is not possible to have HCl(aq) more concentrated than 38% wt/wt under normal conditions. This is already a saturated solution of the gas. Secondly, liquified HCl cannot be a good dehydrating because as everyone knows HCl g fumes heavily in the presence of moisture. It cannot fulfill the criterion of a dehydrating or dessicant. The same arguments apply to conc. nitric acid as well.

The two main reasons which make conc. sulfuric acid a good dehydrating agent are (a) it has a very low vapor pressure and (b) secondly the hydration of sulfuric acid is quite exothermic as compared to other common acids. Maybe a quantum mechanical study reveal "why" this is so. Acid strength alone cannot be used to predict the dehydrating capability. One has to think about the vapor pressure as well.

General chemistry book also preach that sugars are dehydrated by conc. sulfuric acid. It is actually a wrong notion to say that sulfuric acid extracts water out of sugar. There is no water molecule in sugar. This is a chemical reaction between sugar and conc. sulfuric which generates water and carbon. The true demo for sulfuric acid dehydration is the dehydration of hydrated copper sulfate. Adding a few drops on the blue crystals makes them white (anhydrous) copper sulfate almost immediately.

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  • $\begingroup$ Just a note in favour of textbooks. Once one know what it is spoken about, it is more than acceptable to say that reaction between sulphuric acid and, eg, a sugar, is a dehydratation one. Of course, it has nothing to do with a real extraction or with drying. Indeed water isn't there, but it is a dehydratation reaction. Beside this, plus 1 for fixing the thread. $\endgroup$ – Alchimista Jan 11 at 19:12
  • $\begingroup$ @m.farooq (b) secondly the hydration of sulfuric acid is quite exothermic as compared to other common acids. Is available comparison of with concentrated HNO3 and HClO4 ? Is possible this exothermic nature is just more commonly known for H2SO4, as it is more common acid form? $\endgroup$ – Poutnik Jan 12 at 6:43
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    $\begingroup$ The properties of sulfuric acid in reactions have very little to do with its properties as a dehydrating agent. In a closed dessicator, for example, the acid will be separate from the substance to be dried. All that matters here is that any water vapour in the vessel will be rapidly absorbed by the acid as that is highly favoured by the thermodynamics of water mixing with the acid. And the low vapour pressure of the acid won't contaminate the substance. $\endgroup$ – matt_black Jan 12 at 14:09
  • $\begingroup$ @Poutnik, I don't have the enthalpies but if you recall even the hydration of sulfuric trioxide is very exothermic but when HCl gas dissolves in water, it is just warm. The same must be true for nitrogen dioxide. There is indeed something special about sulfuric acid and water combination. $\endgroup$ – M. Farooq Jan 12 at 14:48
  • $\begingroup$ @M.Farooq Hm, I guess hydration comparison of SO3, NO2, HCl is not much fair and relevant. NO2 is not HNO3 anhydrid and HCl is out of game. Add there P4O10, used to prepare N2O5 from HNO3 or in other extreme dehydrations. Acids of P(V) are not particularly strong dehydration agens. $\endgroup$ – Poutnik Jan 12 at 15:10
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Nitric and perchloric acids are sold at 70% aqueous, so most of their dehydrating ability has already been consumed, while sulfuric acid is sold as 98%. At 98% concentration, nitric and perchloric acids are likely to be very good dehydrating agents, but these are also very strong oxidizers, less stable, etc.

Likewise, commercial hydrochloric acid is sold as a 38% solution in water, so it already has had its dehydrating ability consumed. However, I imagine that liquid hydrogen chloride would be a very good dehydrating agent.

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    $\begingroup$ H2so4 also has strong oxidising properties. If the oxidising nature of nitric acid is the reason we do not use it as dehydrating agent then h2so4 should also not be used. $\endgroup$ – Arishta Dec 6 '16 at 17:33
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    $\begingroup$ Oxidizing properties of H2SO4 may be called strong, but they are nowhere near those of HNO3 and HClO4. $\endgroup$ – Ivan Neretin Dec 6 '16 at 17:45
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    $\begingroup$ I dried $\ce{NiCl2 . 6 H2O}$ in a dry $\ce{HCl}$ gas stream in an inorganic lab course to give anhydrous $\ce{NiCl2}$. So yes, $\ce{HCl}$ even in gaseous form is a good dehydrating agent. $\endgroup$ – Jan Dec 6 '16 at 23:40
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    $\begingroup$ iad22agp's argument is that the dehydrating power is due to the minimum amount of water in the commercial solution of the acid. I am not sure about the value of this argument. For example acetic acid is available with a concentration equal to 100%. It is not a strong dehydrating agent. $\endgroup$ – Maurice Jan 10 at 19:21
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    $\begingroup$ This is a completely wrong argument. I agree with Maurice. $\endgroup$ – M. Farooq Jan 10 at 23:01

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