[Fzang]

When you need to use an acid catalyst in a reaction, when is it "suitable" to write either

H₃O⁺, HCl, H⁺, H₂SO₄, etc etc etc...

To me it seems like you roll a dice and decide whether you want to write HCl sometimes or just H⁺. Is there any sense in this at all, besides "I use H₂SO₄ because obviously I need a strong acid."

[Fzang]

So.. uh, *Ignore me, I am impatient*.

[justgladtobeme]

It actually is important to distinguish between the acid catalyst.  It determines the other molecules in solution available for other reactions or as by-products.  For example:

H₃O⁺ : Water that has been acidified; water available as base.
HCl : H⁺ source; H₃O⁺ won't be present unless water is; Cl^- anion available as weak base.
H⁺ : This designates any acid source, HCl, H₂SO₄, etc.
H₂SO₄ : See HCl.

So, writing H₃O⁺ in a situation where only H⁺ is needed, and it would be detrimental to have H₂O as a base in solution, would be incorrect.

Hope this helps.

[stewie griffin]

If you used anhydrous acid (as jstgladtobeme points out), then your choice of acid should be clearly indicated.
For example, suppose you are doing a reaction in diethyl ether and you need some anhydrous acid. HCl is more soluble in diethyl ether than is H2SO4. So one would most likely choose HCl for their reaction, and that should be made clear.