What characterizes a substance as an acid?

1 Answer
Oct 10, 2016

An acid is any species that increases concentrations of the characteristic cation of the solvent.


What does the above spray mean?

Well, water undergoes an equilibrium reaction, that has been extensively measured:

#H_2O rightleftharpoons H_3O^+ + HO^-#.

Now, clearly, this is an equilibrium reaction, and at #298*K#, the ion product #[H_3O^+][HO^-]=10^(-14)#.

At neutrality, #[H_3O^+]=[HO^-]=10^(-7)*mol*L^-1#.

An #"acid"# is conceived to be any species that increases the concentration of #[H_3O^+]# to a level greater than this equilibrium value.

Typical acids are the so-called mineral acids, #"hydrogen halides"#, #"HX#, which tend to be room temperature gases, but which are prodigiously soluble in water to give stoichiometric solutions of acids, i.e.:

#HX(g) + H_2O(l) rarr H_3O^+ + X^-#

Now water is a very important solvent system, yet it is not the only solvent system. I could perform my reactions in say acetic acid or hydrogen fluoride, if I wanted a more acidic solvent than water; or could perform my reactions in liquid ammonia, if I wanted a more basic solvent system.

In liquid ammonia, (which is much harder to study than the water-based solvent system), we conceive the acid/base equilibrium reaction as:

#2NH_3(l) rightleftharpoons NH_4^+ + NH_2^-#

Here, our definition of an acid is the same as previous, but we have moved to a different solvent. The characteristic cation of the ammonia solvent is ammonium ion, whereas the characteristic anion is #"amide ion"#, #NH_2^-#, which is unknown in water. But note that this behaviour is distinct from the behaviour of ammonia IN WATER, where ammonium ion would protonate the solvent to some extent:

#NH_4^+ + H_2O rightleftharpoonsNH_3(aq) + H_3O^+#

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