[Rutherford]

Why the electrical conductivity of a H₂SO₄ solution, after increasing the concentration of H₂SO₄, first increases and then decreases?
H2SO4 2H⁺+SO4^2-
When the concentration of H₂SO₄ increaces, the concentration of the ions increases too, but why does the electrical conductivity decrease if the concentration of H₂SO₄ keeps increasing?

[Arkcon]

The arrow you used only points one way.  Does that describe what really happens?

[Rutherford]

But H₂SO₄ is a strong acid. If I add more H₂SO₄ more ions will be produced and the electrical conductivity raises, I think.

[Borek]

But H₂SO₄ is a strong acid.

How strong? What about HSO₄^-?

[Rutherford]

Ok, then:
H₂SO₄ HSO₄^-+H⁺
HSO4^- SO₄^2-+H⁺
Is this the solution: if I add more H₂SO₄ more H⁺ ions are produced so they shift the 2nd reaction to left?

[Arkcon]

Yes.  Does pure H₂SO₄ conduct electricity?  Is it an ionic compound?

[Rutherford]

It shouldn't, but I suppose that the electrical conductivity decrease is very small. Thank you both for help.

[Borek]

Be ready for a surprise. Pure sulfuric acid is amphiprotic:

2H₂SO₄ <-> HSO₄^- + H₃SO₄⁺

Ionic product for this reaction is defined as

K = [HSO₄^-][H₃SO₄⁺]

and not that small. With pK=2.9 pure acid must be quite ionized.

[Rutherford]

Wow, never heard about that. So there are HSO₄^-,SO₄^2-,H⁺ and H₃SO₄⁺ ions in the solution and 3 reactions are occuring.

[Borek]

When you add water it is water that gets protonated, you will not see H₃SO₄⁺ in the solution, but in pure acid.

[Rutherford]

And what would happen with the electrical conductivity if I used HCl? Same as H₂SO₄?

[Borek]

Is HCl a diprotic acid? Can HCl be obtained as a pure acid (not as a water solution)?

[Rutherford]

I think it can be obtained. Isn't diprotic, so the electrical conductivity should always increase?

[Borek]

I think it can be obtained.

Don't think - check. This is one of a basic properties of the hydrochloric acid.

Isn't diprotic, so the electrical conductivity should always increase?

Diproticity is only a part of the problem, I mentioned it just to show you you can't directly compare both acids.

Initially the more concentrated the sulfuric acid, the more ions in the solution, so the lower the resistance. However, once concentration of water in acid gets low, amount of ions gets lower as well, so the conductivity falls and resistance goes up.

Now check how concentrated HCl gets and try to apply similar reasoning - will it work?

[Rutherford]

The more concentrated HCl is, the more ions in the solution are (bigger conductivity). You said that pure HCl can't be obtained, so the conductivity would increase to a level, then after increasing the concentration of HCl it should start to decrease (because there are less ions) and then it remains at a point (because pure HCl can't be obtained). Correct?
Why pure HCl doesn't exist? If there is no water, HCl won't dissociate and I thought that it should exist. But HCl is initially a gass, so it needs to be dissolved in water to make the acid. Is this the reason?