The Chemistry of Phosphoric Acid: A Deep Dive into Polyprotic Acids
What makes phosphoric acid a polyprotic acid?
Given that phosphoric acid is a polyprotic acid, what are the balanced chemical equations for the sequence of reactions that phosphoric acid can undergo when dissolved in water?
Phosphoric Acid as a Polyprotic Acid
Phosphoric acid is classified as a polyprotic acid due to its ability to donate more than one proton (hydrogen ion) when dissolved in water. This characteristic sets it apart from monoprotic acids, which can only donate one proton.
Balanced Chemical Equations
When phosphoric acid (H3PO4) is dissolved in water, it undergoes a stepwise dissociation process, resulting in the formation of dihydrogen phosphate (H2PO4^-), hydrogen phosphate (HPO4^2-), and phosphate (PO4^3-) ions. Each step of the dissociation process has its own equilibrium constant (Ka).
The balanced chemical equations for the dissociation of phosphoric acid in water are as follows:
H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4^-(aq) with Ka1
H2PO4^-(aq) + H2O(l) → H3O+(aq) + HPO4^2-(aq) with Ka2
HPO4^2-(aq) + H2O(l) → H3O+(aq) + PO4^3-(aq) with Ka3
Phosphoric acid (H3PO4) is a polyprotic acid that dissociates in a stepwise manner when dissolved in water, forming dihydrogen phosphate, hydrogen phosphate, and eventually phosphate anions, with each step having a distinct ionization constant.
Explanation:
Phosphoric acid (H3PO4) is a polyprotic acid because it can donate more than one proton (hydrogen ion) when dissolved in water. It dissociates in water in a stepwise manner, with each step having its own dissociation constant (Ka). The balanced chemical equations for the dissociation of phosphoric acid in water are as follows:
H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4^-(aq) with Ka1
H2PO4^-(aq) + H2O(l) → H3O+(aq) + HPO4^2-(aq) with Ka2
HPO4^2-(aq) + H2O(l) → H3O+(aq) + PO4^3-(aq) with Ka3
The ionization constants decrease with each subsequent step, reflecting the decreased acidity of the anions. As a triprotic acid, phosphoric acid contributes to the formation of phosphate buffer systems in different applications.