Question

Ionisation of boric acid in aqueous medium gives which one of the following? (a) \(\left[\mathrm{BO}_{3}\right]^{3-}\) (b) \(\left[\mathrm{B}(\mathrm{OH})_{4}\right]\) (c) \(\left[\mathrm{B}(\mathrm{OH})_{2} \mathrm{O}\right]^{-}\) (d) \(\left[\mathrm{B}(\mathrm{OH}) \mathrm{O}_{2}\right]^{2-}\)

Short answer

The ion formed is \\(\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}\\), which is option (b).

Step-by-step solution

Understanding Boric Acid Ionization

Boric acid, with the chemical formula \(\text{H}_3\text{BO}_3\), is a weak monobasic acid. It does not release protons directly but instead, it accepts hydroxide ions (\(\text{OH}^-\)) from water, forming tetrahydroxyborate ions in solution.

Reaction of Boric Acid with Water

When boric acid ionizes in water, it reacts with water molecules to form \(\text{B(OH)}_4^-\). The reaction is: \[\text{H}_3\text{BO}_3 + 2\text{H}_2\text{O} \rightleftharpoons \text{B(OH)}_4^- + \text{H}_3\text{O}^+.\] This shows that the resulting ion from the ionization process is \(\text{B(OH)}_4^-\).

Identifying the Correct Ion

Comparing the tetrahydroxyborate ion (\(\text{B(OH)}_4^-\)) formed from boric acid ionization, we match it to option (b). This means the correct answer is \(\text{B(OH)}_4^-\).

Key concepts

Weak Monobasic Acid

Boric acid (\(\text{H}_3\text{BO}_3\)) is classified as a weak monobasic acid. This classification means that although it has only one acidic hydrogen atom that can be involved in the ionization process, it behaves differently from typical acids. Unlike strong acids that release protons directly into solution, boric acid doesn't dissociate into protons (\(\text{H}^+\)). Instead:

• It acts by accepting hydroxide ions (\(\text{OH}^-\)) from water.
• The process forms a complex ion known as tetrahydroxyborate.

This unique behavior stems from its structure and affinity for hydration, making it an atypical example in the world of monobasic acids. Due to its weak acidic nature, boric acid has a limited ability to change the pH of the solution significantly.

Tetrahydroxyborate Ion

Tetrahydroxyborate is the ion formed when boric acid interacts with water. Its chemical notation is \(\text{B(OH)}_4^-\). Let's go through the formation process:

• When boric acid is in solution, it reacts with water molecules rather than releasing hydrogen ions.
• This reaction involves the uptake of hydroxide ions (\(\text{OH}^-\)) from water.
• This results in the formation of the tetrahydroxyborate ion (\(\text{B(OH)}_4^-\)), which is stable in aqueous solutions.

The ability of boric acid to form this tetrahydroxyborate complex is due to its molecular structure that allows bonding with four hydroxyl groups. This ion stabilizes the solution by balancing the charges, making it an essential concept in boric acid chemistry.

Water Reaction with Acids

Reactions between water and acids can vary significantly depending on the acid's nature. In the case of boric acid:

• Water is not only a solvent but also a reactant.
• Boric acid, instead of releasing protons, interacts with water to establish equilibrium.
• This process involves the coordination with hydroxyl ions derived from water molecules.

The resultant interaction forms the hydronium ion (\(\text{H}_3\text{O}^+\)) and the tetrahydroxyborate ion (\(\text{B(OH)}_4^-\)). The reaction equilibrium can be expressed by the equation:
\[\text{H}_3\text{BO}_3 + 2\text{H}_2\text{O} \rightleftharpoons \text{B(OH)}_4^- + \text{H}_3\text{O}^+.\]This interaction showcases how water can facilitate the ionization processes within weak acids, leading to a dynamic equilibrium in solution, rather than straightforward proton donation seen with strong acids. Understanding these reactions allows students to anticipate how different acids will behave in solution.