Question

Is boric acid a protic acid?Explain.

Answer 1

Here we should try to focus on the term protic acid,which means donating or releasing protons or hydrogen ions,so by checking the electronic configuration of boric acid,we could answer this question.

Complete step by step solution: Let us try to understand the Lewis concept of acids and bases:
Lewis acids are referred to those which can accept a pair of electrons to form a covalent bond.Lewis bases are referred to those substances that can donate it's pair of electrons to form a covalent bond.
So, now let's try to understand the structure of boric acid:
${{\text{H}}_{\text{3}}}\text{B}{{\text{O}}_{\text{3}}}$ known as orthoboric acid,or commonly boric acid,has an electronic configuration of $\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{1}}}$and in an excited state,one of the electron from 2s is promoted to 2p orbital,giving rise to three hybridised orbitals,which overlaps individually to form three B-O bonds.
Now we are aware that boric acid is a weak monobasic acid,so it doesn't donate any proton,but rather behaves as a Lewis acid,which means it takes up a electron pair to form a bond.
So it actually takes up a pair of electrons from OH(hydroxyl) ion and doesn't behave as a protic acid.
Equation: $\text{HOH+B(OH}{{\text{)}}_{\text{3}}}\to {{\left[ \text{B}{{\left( \text{OH} \right)}_{\text{4}}} \right]}^{-}}\text{+}{{\text{H}}^{\text{+}}}$
Thus,it forms a complex of borohydride by taking hydroxyl ions.

So the statement above stated doesn't hold true.

Additional Information: Boric acid forms,soft white ,needle like crystals having a soapy touch.
When heated at higher temp of $\text{10}{{\text{0}}^{\text{o}}}\text{C}$, it loses water and forms metaboric acid.
And on further heating at $\text{16}{{\text{0}}^{\text{o}}}\text{C}$ it forms tetraboric acid.
And on further strong heating ${{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ is formed.

Note: Though boric acid is a weak monobasic acid,we might mistake it to be protic acid,but it's tendency to behave as a Lewis acid,alters it's proticity.