Question

Na_2CO_3 and K_3PO_4 are expected to form basic solutions. Write a reaction showing how either CO_3^2- or PO_4^3- reacts with water to form hydroxide ions.

Answer 1

CO₃^2- + H₂O $\rightarrow$ HCO₃^- + OH^-

In this reaction water is the proton donor (Bronsted-Lowry acid) and carbonate ion is the proton acceptor (Bronsted-Lowry base), forming the bicarbonate ion. This is a classic Bronsted-Lowry acid/base reaction. The Arrhenius theory requires that an acid dissociates to produce H⁺ and a base dissociates to produce OH^-. That is certainly not the case in the above reaction.

The OH^- ion that is formed in this reaction actually comes from the water. The H⁺ that combines with CO₃^2- comes from the water molecule, leaving behind the OH^-.

PO₄^3- + H₂O $\rightarrow$ HPO₄^2- + OH^-

The OH^- ion that is formed in this reaction actually comes from the water. The H⁺ that combines with PO₄^3- comes from the water molecule, leaving behind the OH^-.