Does acid and base react directly, or do they react first with water?

Question

If an acid $\ce{AH}$ reacts with a base $\ce{B}$ in aqueous solution, in textbook they write

$$\ce{AH + B -> A- + BH+}\label{rxn:1}\tag{1}$$

But first I add acid to water and then base, so why we don't write the equations like this:

• When I add acid to water: $$\ce{AH + H2O -> A- + H3O+}$$

• When I add base to the acidic solution: $$ \begin{align} \ce{B + H2O &-> BH+ + OH-}\\ \ce{B + H3O+ &-> BH+ + H2O} \end{align} $$

So the equation $\eqref{rxn:1}$ isn't true because the base doesn't react directly with the acid, but with $\ce{H3O+}$ formed by the acid reaction with water. Is what I have written true?

Answer 1

With strong bases and acids, only $\ce{OH-}$ and $\ce{H3O+}$ react, right, because $\ce{AH}$ and $\ce{B}$ (practically) don't exist in aqueous solution.

For weak acids and bases, that's not true, but still there the water can "transport" protons much faster between AH and B than direct exchange after diffusion. As was noted below, that's called the Grotthuss mechanism.

So yes, you are right, but the formula (1) is a valid simplification which is, for most uses, as good.