Sulphuric acid (H2SO4) has a dehydrating property, but what is the cause of this property? Can anyone explain? Is it about its structure?

Thank you!

I guess, it is because of possible H bonding that occur with O atoms of SO42- and H atoms. In other words, hydration emergy of sulphate group released is high and association with water is highly favourable.

H-bonding occurs with both sulphuric acid and sulphate anion, though.

Well, H₂SO₄ is an acid. An acid is defined, somehow, as a substance that easily gives away H⁺-ions. It will, as a consequence, react easily with the watermolecule, an ampholyte, in this example acting as a base.

In reaction with water, or H₂O, H₂SO₄ wil first of all give one H⁺-ion to water, leaving a HSO₄^--molecule behind:

 

[math]H_2SO_4 + H_2O \leftrightarrow HSO_4^-+H_3O^+[/math]

 

The HSO₄^- can react again with water, leaving another H₃O⁺-molecule and an SO₄^2--molecule behind:

 

[math]HSO_4^-+H_2O \leftrightarrow SO4^{2-}+H_3O^+[/math]

 

So, according to me, both equations in one would give:

 

[math]H_2SO_4 + 2H_2O \leftrightarrow SO_4^{2-} + 2H_3O^+[/math]

 

 

Now about this 'dehydration' property: as far as I'm aware of, (2)H₃O⁺ is not water, dehydrating the mixture, as per consequence. (What I'm trying to say is that the reaction transfers water to non-water, which is, in my opinion, dehydration)

But I could be wrong... Can anyone more scientifical tell me whether I'm correct or not?

Edited March 18, 201411 yr by Function

I guess, it is because of possible H bonding that occur with O atoms of SO42- and H atoms. In other words, hydration emergy of sulphate group released is high and association with water is highly favourable.

Once there's enough water around to hydrate any sulphate ions you have a material that's mainly water and isn't really hygroscopic any more. So hydration energy can't be much of a factor.

 

The reaction of water with sulphuric acid to give H3O+ and HSO4- is very energetically favoured so any water that gets into the acid is effectively trapped as involatile ions.

Once there's enough water around to hydrate any sulphate ions you have a material that's mainly water and isn't really hygroscopic any more. So hydration energy can't be much of a factor.

 

The reaction of water with sulphuric acid to give H3O+ and HSO4- is very energetically favoured so any water that gets into the acid is effectively trapped as involatile ions.

So, 'enough water' due to hydration would be so less that hydration can be neglected as the main cause of hygroscopic nature? Though, sulphate can move in the material and create residual force for more h-bonding at new sites. Nevertheless, it is still H-bondind in the case when H3O+ exists.

Edited March 19, 201411 yr by rktpro

You can't get hydration unless there is water.

Initially, practically all the water that is added to the acid reacts.

 

So you don't get hydration until the solution is relatively dilute.