Chemistry - Why is H2SO4 a good dehydrating agent while other strong acids are not?

Solution 1:

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.

Solution 2:

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.