What are H+ ions?

1 Answer
Dec 5, 2015

H^+ ions represent the characteristic cation of water. I will try to explain what I mean.

Explanation:

There are 2 common representations of autoprotolysis in water:

H_2O rightleftharpoons H^+ + OH^- And,

2H_2O rightleftharpoons H_3O^+ + OH^-

I write representations because these are precisely what they are: a conception of water at the molecular level, which are chemically reasonable and certainly useful but might or might not represent reality.

We know that at 298 K, K_w = 10^(-14) = [OH^-][H^+], and for most acid/base problems this will suffice. But what is this species, which is also represented as H_3O^+, the acidium ion? As far as anyone knows this is a cluster of water molecules with an extra proton associated; so H_7O_3^+ or H_9O_4^+, 3-4 water molecules with an extra H^+ tacked on.

If you have ever played rugby, think of a maul where the forwards bind together and pass the pill from hand to hand. This may not be realistic, in that within a cluster of water molecules (unlike within a pack of forwards), proton transfer can tunnel across water clusters; it is the movement of charge rather than protons. This explains the ionic mobility of H^+ and OH^- in comparison with other ions.

At undergraduate level, it is probably useful to think of the acidium ion as an actual entity. You must be able to use K_w in some form in your calculations, and use [H^+] and [H_3O^+] to determine pH and pOH.

Apologies if I have written info I have written elsewhere. I could not find the link.