What are H+ ions?

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.