Hydrogen phosphate, H3PO4(s), is a chemical substance, which, in pure anhydrous form, exists as white solid. It melts at 42.35oC to form a viscous colourless liquid. In aqueous solution, it ionizes as follows:
H3PO4 (aq) + H2O(l) → H3O+(aq) + H2PO4– (aq)
H2PO4–(aq) + H2O(l)→ H3O+(aq) + HPO42–(aq)
HPO42– (aq) + H2O(l)→ H3O+(aq) + PO43–(aq)
In the above equilibrium reactions, H3PO4(aq) acts as a(n)
acid.
what kind of acid polyprotic acid or strong acid?
H3PO4 is a polyprotic acid. It can donate multiple protons (H+) in aqueous solution and undergo multiple ionization reactions.
In the given equilibrium reactions, H3PO4(aq) acts as a weak acid.
To understand why H3PO4 acts as an acid, let's break down the reactions. In aqueous solution, H3PO4 initially donates a proton (H+) to water (H2O) to form H3O+ (hydronium ion) and H2PO4- (dihydrogen phosphate ion). This is the first ionization step:
H3PO4 (aq) + H2O(l) → H3O+(aq) + H2PO4–(aq)
The H2PO4- ion, acting as a conjugate base, can also donate a proton to water, resulting in the formation of another hydronium ion (H3O+) and HPO42- (monohydrogen phosphate ion). This is the second ionization step:
H2PO4– (aq) + H2O(l) → H3O+(aq) + HPO42–(aq)
Finally, HPO42- can also donate a proton to water, yielding another hydronium ion (H3O+) and PO43- (phosphate ion). This is the third ionization step:
HPO42– (aq) + H2O(l) → H3O+(aq) + PO43–(aq)
Based on these reactions, we can see that H3PO4 donates more than one proton (H+) in a stepwise manner. Each ionization step represents the loss of one proton and the formation of a new ion, with the resulting species acting as a conjugate base for the next ionization.
Since H3PO4 donates protons to water, it behaves as an acid. However, it is important to note that H3PO4 is a weak acid, meaning it only partially ionizes in water and does not fully dissociate into ions.