#"H"_3"PO"_4#, #"H"_2"PO"_4^(-)#, #"HPO"_4^(2-)#: which species acts as an acid?

1 Answer
Sep 24, 2016

Answer:

See explanation.

Explanation:

Out of the three compounds you listed, phosphoric acid, #"H"_3"PO"_4#, is always an acid. On the other hand, the other two compounds can act both as an acid and as a base.

So, phosphoric acid will give off one of its three acidic protons to form #"H"_2"PO"_4^(-)#. This polyatomic ion is called dihydrogen phosphate because it contains two acidic protons and one phosphate anion, #"PO"_4^(3-)#.

https://en.wikipedia.org/wiki/Phosphate#Chemical_properties

DIhydrogen phosphate can give off one of its two acidic protons to form #"HPO"_4^(2-)#. This polyatomic ion is called hydrogen phosphate because it contains one acidic proton and one phosphate anion.

https://en.wikipedia.org/wiki/Phosphate#Chemical_properties

Finally, hydrogen phosphate can give off its acidic proton to form the phosphate anion,

https://en.wikipedia.org/wiki/Phosphate#Chemical_properties

Now, dihydrogen phosphate and hydrogen phosphate can act both as an acid and a base because they can either release or accept a proton.

For example, you can have

#"H"_ 3"PO"_ (4(aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 2"PO"_ (4(aq))^(-) + "H"_ 3"O"_ ((aq))^(+)#

and

#"H"_ 2"PO"_ (4(aq))^(-) + "H"_ 3"O"_ ((aq))^(+) rightleftharpoons "H"_ 3"PO"_ (4(aq)) + "H"_ 2"O"_ ((l))#

In the reverse reaction, dihydrogen phosphate accepts a proton to reform phosphoric acid, and so it acts as a base.

Similarly, you can have

#"H"_ 2"PO"_ (4(aq))^(-) + "H"_ 2"O"_ ((l)) rightleftharpoons "HPO"_ (4(aq))^(2-) + "H"_ 3"O"_ ((aq))^(+)#

and

#"HPO"_ (4(aq))^(2-) + "H"_ 3"O"_ ((aq))^(+) rightleftharpoons "H"_ 2"PO"_ (4(aq))^(-) + "H"_ 2"O"_ ((l))#

In this case, dihydrogen phosphate acts as an acid in the forward reaction because it donates a proton to form hydrogen phosphate. In the reverse reaction, hydrogen phosphate acts as a base because it accepts a proton to reform dihydrogen phosphate.

For hydrogen phosphate, you can have

#"HPO"_ (4(aq))^(2-) + "H"_ 2"O"_ ((l)) rightleftharpoons "PO"_ (4(aq))^(3-) + "H"_ 3"O"_ ((aq))^(+)#

Here hydrogen phosphate acts as an acid because it donates a proton to form the phosphate anion.