Question

How do you write an acid/base reactions with phases? For example, including phases, write the chemical equation for the reaction of the weak acid HCN with water.

Answer 1

`HC-=N(aq) + H_2O(l) rightleftharpoons H_3O^+ + ""^(-)C-=N`

`K_a=([H_3O^+][""^(-)C-=N])/([HC-=N]) = 10^(-9.21)" at "298K`

Explanation:

Of course, the anions and the cations are solvated by, or aquated by, several water molecules. The acidium ion in water, which we represent as `H_3O^+` is likely to be a cluster of several water molecules, with an EXTRA `H^+` ion, i.e. `{H_9O_4}^+` or something similar. The representation of this as `H^+` or `H_3O^+` may even be naive, but it is still useful and practical for calculation.