# What characterizes a substance as an acid?

Oct 10, 2016

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:

${H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H {O}^{-}$.

Now, clearly, this is an equilibrium reaction, and at $298 \cdot K$, the ion product $\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{- 14}$.

At neutrality, $\left[{H}_{3} {O}^{+}\right] = \left[H {O}^{-}\right] = {10}^{- 7} \cdot m o l \cdot {L}^{-} 1$.

An $\text{acid}$ is conceived to be any species that increases the concentration of $\left[{H}_{3} {O}^{+}\right]$ to a level greater than this equilibrium value.

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

$H X \left(g\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{3} {O}^{+} + {X}^{-}$

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:

$2 N {H}_{3} \left(l\right) r i g h t \le f t h a r p \infty n s N {H}_{4}^{+} + N {H}_{2}^{-}$

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 $\text{amide ion}$, $N {H}_{2}^{-}$, 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:

$N {H}_{4}^{+} + {H}_{2} O r i g h t \le f t h a r p \infty n s N {H}_{3} \left(a q\right) + {H}_{3} {O}^{+}$

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