Problem 260 - A Complete Resource Book in Chemistry for JEE Main

Question

One of the following species acts as both Bronsted acid and base (a) \(\mathrm{H}_{2} \mathrm{PO}_{2}\) (b) \(\mathrm{HPO}_{3}^{2}\) (c) \(\mathrm{HPO}_{4}^{2}\) (d) all of the above

Step-by-Step Solution

Answer

(c) HPO_4^{2-} acts as both Bronsted acid and base.

Step 1: Understanding Bronsted Acid and Base

A Bronsted acid is a substance that can donate a proton ( ext{H}^{+}), whereas a Bronsted base is a substance that can accept a proton. For the given species, we need to identify which can act as both based on the possibility of losing and gaining a proton.

Step 2: Analyzing Each Species

For (a) \(H_2PO_2^-\), the possible proton donation creates \(HPO_2^{2-}\), not very favorable. Acceptance seems unlikely. For (b) \(HPO_3^{2-}\), it can accept a proton to become \(H_2PO_3^-\) or donate a proton to form \(PO_3^{3-}\). For (c) \(HPO_4^{2-}\), it can accept a proton becoming \(H_2PO_4^-\) and donate to form \(PO_4^{3-}.\)

Key Concepts

Proton Donation

In Bronsted acid-base theory, the concept of proton donation is central. A Bronsted acid is defined by its ability to donate a proton, denoted as \( \text{H}^+ \). This is a key process where the acid undergoes a transformation by releasing a proton from its molecular structure.
For instance, when analyzing chemical species like \( \mathrm{HPO}_{4}^{2-} \), proton donation can occur when it loses a hydrogen ion, transforming into another species, \( \mathrm{PO}_{4}^{3-} \). This specific action of releasing a proton makes \( \mathrm{HPO}_{4}^{2-} \) a Bronsted acid.
Among the different species, not all are likely to donate a proton effectively. Some species might be stable in their current form and show reluctance in parting with a proton, thus exhibiting weak acidic properties or being unsuitable for proton donation. When determining the potential for proton donation, consider the stability of the resulting product if it were to release a proton.

Proton Acceptance

Proton acceptance is the other side of the Bronsted acid-base interaction. Here, a Bronsted base is the chemical species capable of accepting a proton. This acceptance transforms the base into a protonated form.
For example, \( \mathrm{HPO}_{4}^{2-} \) can act as a Bronsted base by accepting an \( \text{H}^+ \), thus converting into \( \mathrm{H}_2PO_{4}^{-} \). Through this reaction, the species demonstrates its ability to accept protons.
A good candidate for proton acceptance generally has characteristics favorable for stabilization after adding a proton. One factor that influences the ease of proton acceptance is the electron-rich nature of the species which can stabilize additional positive charges. Hence, if a species has lone pairs or electron-rich sites, it is more likely to successfully accept a proton.

Chemical Species Analysis

Conducting a chemical species analysis involves understanding the dual role certain species can play within the Bronsted framework of acids and bases. Not every compound can donate and accept protons; only certain species can act as both an acid and a base, making them amphoteric.
For \( \mathrm{HPO}_{4}^{2-} \), we find a perfect example of an amphoteric species as it can donate a proton to turn into \( \mathrm{PO}_{4}^{3-} \) and accept a proton to become \( \mathrm{H}_2PO_{4}^{-} \). By assessing the transformation pathways through analysis,we can establish which paths are chemically viable.
When performing this analysis, consider details such as molecular structure, charge, and the environment that the species might encounter. The ability to both donate and accept protons renders such species significant in chemical reactions, as they can help in buffering systems and maintaining pH balance.