Is C6H5COOH an acid or a base? is it strong or weak?
The Correct Answer and Explanation is:
Correct Answer: C6H5COOH is a weak acid.
The chemical formula C6H5COOH represents benzoic acid. This substance is unequivocally classified as a weak acid. Its chemical behavior is determined by the presence of the carboxyl functional group (-COOH) attached to a phenyl ring (C6H5).
According to the Brønsted-Lowry theory, an acid is a species that can donate a proton (H⁺). The hydrogen atom of the hydroxyl portion of the carboxyl group is acidic and can be donated in an aqueous solution. When dissolved in water, benzoic acid establishes an equilibrium, donating its proton to a water molecule to form the hydronium ion (H₃O⁺) and the benzoate anion (C6H5COO⁻). The reaction is:
C₆H₅COOH(aq) + H₂O(l) ⇌ C₆H₅COO⁻(aq) + H₃O⁺(aq)
The classification of benzoic acid as “weak” pertains to the extent of this reaction. Unlike strong acids such as hydrochloric acid (HCl), which dissociate completely in water, benzoic acid only partially dissociates. The equilibrium for the reaction above lies significantly to the left, meaning that in a solution of benzoic acid, the majority of the molecules remain in their undissociated, molecular form (C₆H₅COOH) rather than as benzoate ions.
The acidic nature of the molecule, despite being weak, can be attributed to the stability of its conjugate base, the benzoate ion. After the proton is lost, the resulting negative charge is not confined to a single oxygen atom. Instead, it is delocalized through resonance across both oxygen atoms of the carboxylate group. This spreading of the negative charge stabilizes the anion, making the parent molecule more likely to release the proton than a simple alcohol would be. However, this stabilization is not extensive enough to drive the reaction to completion, hence its classification as a weak acid, with an acid dissociation constant (Ka) of approximately 6.5 x 10⁻⁵.
