JEE Challenger
More from p-Block Elements
ChemMedAdv 2014 • Paper 1

Determine Correct Statements Regarding Orthoboric Acid Structure and Acidity

Select the correct statement(s) concerning orthoboric acid:

Options

A

It acts as a weak acid in aqueous solution owing to self-ionization.

B

The acidity of its aqueous solution increases when ethylene glycol is added.

Correct
C

It possesses a three-dimensional network structure sustained by hydrogen bonding.

D

It functions as a weak electrolyte in aqueous media.

Correct

Topics & Concepts

p-Block Elementsp-Block Elements

Step-by-Step Solution

To determine the correct statement(s) regarding orthoboric acid (H3BO3\text{H}_3\text{BO}_3), let us analyze each option individually:

  1. Analysis of Option (A): Orthoboric acid does not act as a weak acid by self-ionization or by releasing a proton directly (i.e., it is not a Brønsted-Lowry protic acid). Instead, it behaves as a weak monobasic Lewis acid by accepting a hydroxyl ion (OH−\text{OH}^-) from a water molecule: B(OH)3+2H2O⇌[B(OH)4]−+H3O+\text{B(OH)}_3 + 2\text{H}_2\text{O} \rightleftharpoons [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+ Since its acidity in water arises from accepting an OH−\text{OH}^- ion rather than self-ionization, Option (A) is incorrect.

  2. Analysis of Option (B): When a 1,2-diol (cis-diol) such as ethylene glycol (HO-CH2-CH2-OH\text{HO-CH}_2\text{-CH}_2\text{-OH}) is added to an aqueous solution of orthoboric acid, it reacts with [B(OH)4]−[\text{B(OH)}_4]^- to form a highly stable, cyclic chelate complex:

B(OH)3+2 HO-CH2-CH2-OH⇌(CH2-OO-CH2∣B−∣CH2-OO-CH2)+H++4H2O\text{B(OH)}_3 + 2\,\text{HO-CH}_2\text{-CH}_2\text{-OH} \rightleftharpoons \begin{pmatrix} \text{CH}_2\text{-O} & & \text{O-CH}_2 \\ \vert & \text{B}^- & \vert \\ \text{CH}_2\text{-O} & & \text{O-CH}_2 \end{pmatrix} + \text{H}^+ + 4\text{H}_2\text{O}

The formation of this stable chelate shifts the equilibrium strongly to the right (according to Le Chatelier's principle), significantly increasing the degree of ionization and making the solution distinctly more acidic. Thus, Option (B) is correct.

  1. Analysis of Option (C): Orthoboric acid possesses a two-dimensional planar layered structure rather than a three-dimensional network structure. Discrete triangular BO33−\text{BO}_3^{3-} units are linked together in two-dimensional sheets via hydrogen bonding (O-H⋯O\text{O-H}\cdots\text{O}). These layers slide past one another, giving boric acid a slippery, soapy feel. Thus, Option (C) is incorrect.

  2. Analysis of Option (D): Because orthoboric acid ionizes only partially in aqueous media (Ka≈5.8×10−10K_a \approx 5.8 \times 10^{-10}), it produces a low concentration of free ions in solution, thereby functioning as a weak electrolyte. Thus, Option (D) is correct.

Conclusion: The correct statements are (B) and (D).

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