What is an acid?

1 Answer
Sep 6, 2017

Answer:

What is an acid?

Explanation:

Well, it is a species that increases the concentration of the characteristic cation of the solvent. And since the normal solvent is water, we would say that an acid is a species that increases concentration of the hydronium ion, #H_3O^+#, the #"acidium species...."#

In water we know that #"autoprotolysis"# occurs.....

#2H_2O(l) rightleftharpoons H_3O^+ + HO^-#

And under standard conditions we can quantify this equilibrium, i.e.

#K_w=[H_3O^+][HO^-]=10^(-14)#; and we can take #log_10# of both sides to give......

#log_10K_w=log_10[H_3O^+]+log_10[HO^-]=log_10(10^-14)=-14#

And on rearrangement......

#14=underbrace(-log_10[H_3O^+])_(pH)underbrace(-log_10[HO^-])_(pOH)#

And so (finally!), our working relationship.....

#14=pH+pOH#

And so at high acidity #pH# is low or negative; at neutrality #pH=pOH=7#, and under alkaline conditions, #pOH# is low or negative.

Anyway a lot to digest here....I would expect a 1st or 2nd year undergrad to be able to reproduce this......A level students should know that #14=pH+pOH#.