Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Garabato on 12/09/2007 05:03:04

Title: ¿Why does sulphuric acid raises the temperature of water?
Post by: Garabato on 12/09/2007 05:03:04
I'm sure you all have done it, you add H2SO4to a bit of water and the Tº raises a little. could anyone explain how is it caused or anything about it?

BTW: I asked in Yahoo Answers and I got a lot of answers just saying "because it is an exothermic reaction", really, this is obvious  [:P] I need something a bit more..."college degree".

Thanks
Title: ¿Why does sulphuric acid raises the temperature of water?
Post by: eric l on 12/09/2007 07:56:20
Concentrated sulphuric acid is so hygroscopic that it can char a piece of sugar dropped into it.  Sugar is a carbohydrate, meaning that the formula can be written as Cn(H2O)m, and it is a very stable compound, so this just shows how hygroscopic concentrated sulphuric acid is if it can extract water from such a stable compound.
Generally speaking, you have to add energy to go from a more stable compound or mix to a less s stable one; the other way around you deliver energy.
The reaction of H2SO4 with water is actually something like :
H2SO4 + 2 H2O --> 2 H3O+ + SO42-
(in water or aqueous solutions you will find the H+-ion only bound to water as H3O+)
The mix with the ionized forms is much more stable than the mix on the left side; in fact; to make concentrated sulphuric acid stating from "tower acid" (= the solution you get with the industrial production of sulphuric acid), you have to put in much more energy than what is needed to evaporate the water.
Check also : http://en.wikipedia.org/wiki/Sulfuric_acid ; it does a little more than just confirm that the reaction is exothermic.
Title: ¿Why does sulphuric acid raises the temperature of water?
Post by: lightarrow on 12/09/2007 17:16:12
Quote from: Garabato on 12/09/2007 05:03:04
I'm sure you all have done it, you add H2SO4to a bit of water and the Tº raises a little. could anyone explain how is it caused or anything about it?

BTW: I asked in Yahoo Answers and I got a lot of answers just saying "because it is an exothermic reaction", really, this is obvious  [:P] I need something a bit more..."college degree".

Thanks

Just a little bit in addition to what eric I said:

H2SO4 is a strong acid, at least in its first ionization:
H2SO4 + H2O --> H3O+ + HSO4- (the right reaction in acid solution),
that is, it gives away one proton (H+) with very little need of energy (the reaction of one proton loss is only slightly endothermic);
on the other hand, the reaction:
H+ + H2O --> H3O+ is Very exothermic, so the net result is a very exothermic reaction.
Title: ¿Why does sulphuric acid raises the temperature of water?
Post by: DrDick on 14/09/2007 16:49:05
The above answers only tell part of the story.

First, there is the reaction of H2SO4 with water.

      H2SO4 + H2O → H3O+ + HSO4-

This is an exothermic reaction because the O-H bond in H3O+ is stronger than the O-H bond found in H2SO4.

There's also a solvation aspect to this problem.  The more strongly the ions interact with water, the more heat is given off when they interact.  The sulfate ion has four oxygen atoms that can accept hydrogen bonds from water, resulting in a very high hydration enthalpy for that ion (magnesium sulfate, or Epsom's salt, also heats up water quite a bit).

Any acid going into water will heat up the water because of the first effect.  Sulfuric acid shows an exceptionally strong effect because of the extra solvation energy.  If you pour pure sulfuric acid into water, you can actually cause the water to boil.

Dick