# 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.

##### 1 Answer
Aug 7, 2016

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 ${H}_{3} {O}^{+}$ is likely to be a cluster of several water molecules, with an EXTRA ${H}^{+}$ ion, i.e. ${\left\{{H}_{9} {O}_{4}\right\}}^{+}$ or something similar. The representation of this as ${H}^{+}$ or ${H}_{3} {O}^{+}$ may even be naive, but it is still useful and practical for calculation.