Question

1. Why is the pKa₂ of H₃PO₄ greater than pKa₁ and why is pKa₃ greater than pKa₂?

Answer 1

Look at basic definitions

The larger the value of Ka, the stronger the acid.

now we know that pKa = -log (Ka) ( pKa is the negative logarithm of Ka )

Therefore, the smaller value of pKa implies stronger the acid or the higher the pKa value, the weaker the acid.

Phosphoric acid

H3PO4

Ka1 = 6.9×10–3 Ka2 = 6.2×10–8 Ka3 = 4.8×10–13

pKa1 is the disassociation constant for the first ionization

pKa2 is the disassociation constant for the second ionization

pKa3 is the disassociation constant for the third ionization.

pKa1 < pKa2 < pKa3

Once a proton is lost it is ok to stabilize the additional electron density through delocalization over two oxygen atoms in dihydrogen phosphate ion (H₂PO₄^-), which is a weak acid compared to phosphoric acid. But if one more proton is lost from dihydrogen phosphate ion it develops another negative charge (-2) and creates a electrostatic repulsion between these two negative charges, moreover this charge can be delocalized on only one oxygen atom. The same concept also applicable to monohydrogen phosphate ion (HPO₄^2-), which is also weaker acid than dihydrogen phosphate ion, leading to form more unstable phosphate ion (PO₄^3-). The successive ionization can be shown as below

Hope this helped you!

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