Question

What are H+ ions?

Answer 1

`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.