Question

The correct statement (s) for orthoboric acid is/are (A) It behaves as a weak acid in water due to self ionization. (B) \Lambdacidity of its aqueous solution increases upon addition of ethylene glycol. (C) It has a three dimensional structure due to hydrogen bonding. (D) It is a weak electrolyte in water.

Short answer

(B) and (D) are correct. Orthoboric acid becomes more acidic upon the addition of ethylene glycol and it is a weak electrolyte in water.

Step-by-step solution

Understanding Orthoboric Acid Behavior

Orthoboric acid, known as H3BO3, tends to act as a Lewis acid by accepting a hydroxide ion (OH-) rather than donating a proton (H+). This is because it accepts electrons from the hydroxide ion forming a tetrahydroxyborate ion.

Effect of Ethylene Glycol on Acidity

The addition of ethylene glycol, a polyalcohol, can enhance the acidity of orthoboric acid because it interacts with the hydroxyl groups of the boric acid, helping it to release H+ ions more easily. This is a concept known as 'complexation', which can lead to an increase in the solubility of boric acid and its dissociation.

Analyzing the Structure due to Hydrogen Bonding

Orthoboric acid molecules can form hydrogen bonds with each other. However, in the crystalline form, this does not result in a three-dimensional network; rather, it forms layers. So, the statement that it has a three-dimensional structure due to hydrogen bonding is misleading.

Determining the Electrolyte Nature

Weak electrolytes are substances that do not completely ionize in solution. Since orthoboric acid dissociates to only a small extent in water, releasing few ions, it can be classified as a weak electrolyte.

Key concepts

Lewis Acid

In chemistry, a Lewis acid is a substance that can accept an electron pair to form a covalent bond. Contrary to the more popular Brønsted-Lowry definition of an acid, which involves the donation of a proton (H+), a Lewis acid relies on accepting electrons.

Orthoboric acid (H3BO3) demonstrates its Lewis acid behavior not by donating protons, but rather by accepting a hydroxide ion (OH−), which has a pair of electrons to share. When this acceptance occurs, orthoboric acid undergoes a transformation to create a tetrahydroxyborate ion. This unique behavior is crucial for students to understand as it demonstrates the versatile nature of acids in chemistry beyond the typical proton donation.

Hydrogen Bonding

Hydrogen bonding is a type of dipole-dipole interaction that occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences an attraction to another electronegative atom with a lone pair of electrons.

Orthoboric acid molecules leverage hydrogen bonding to interact with each other. While some might assume this leads to a three-dimensional structure, in reality, orthoboric acid forms layers due to these bonds, not a 3D network. Appreciating the nuances of hydrogen bonding in orthoboric acid helps clarify misconceptions about its molecular structure.

Weak Electrolyte

A weak electrolyte is a solute that partially dissociates into ions in solution. The term 'partial' is key because it means the substance does not fully ionize, distinguishing it from strong electrolytes, which completely disassociate.

In the case of orthoboric acid in water, we observe that it only slightly dissociates, releasing a limited number of ions. This qualifies it as a weak electrolyte. Such behavior impacts the conductivity of the solution; since fewer ions are available to carry an electrical current, the conductivity is lower compared to solutions of strong electrolytes. It also has implications for the acid's reactivity and solubility, important factors for students to consider in practical applications.

Complexation

Complexation refers to the chemical process where molecules or ions form a complex by attaching to a central atom or ion. This attachment is typically through coordinate covalent bonds.

In our context, when ethylene glycol is added to an aqueous solution of orthoboric acid, it enhances the acid's ability to release H+ ions, effectively increasing its acidity. The glycol molecules 'complex' with the hydroxyl groups on the orthoboric acid, affecting its behavior as an acid. Understanding complex formation is essential as it plays a critical role in various biological and industrial processes, including medication interaction with the body and the stabilization of metal ions in solution.