Question

What characterizes a substance as an acid?

Answer 1

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

Explanation:

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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