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-} can act as both an acid and a base.

Step 1: Understand Bronsted Acid and Base

A Bronsted acid is a species that can donate a proton (H^+) to another species, whereas a Bronsted base is a species that can accept a proton. Therefore, a molecule acting as both must be able to donate and accept a proton.

Step 2: Evaluate H_{2}PO_{2}

The species H_{2}PO_{2} can donate a proton, forming HPO_{2}^{-}, but cannot accept a proton as it has no negative charge or easily accessible lone pairs for accepting protons. Thus, it can only act as an acid.

Step 3: Evaluate HPO_{3}^{2-}

The species HPO_{3}^{2-} is a dianion and likely substantial negative charge could accept a proton, acting as a base. However, donating a proton to form H_{2}PO_{3}^{-} is unlikely because it already has a substantial negative charge. It is primarily a base.

Step 4: Evaluate HPO_{4}^{2-}

This species can both donate and accept a proton. It can donate a proton to form PO_{4}^{3-}, acting as an acid. It can also accept a proton to form H_{2}PO_{4}^{-}, acting as a base. Hence, it can act as both a Bronsted acid and base.

Step 5: Compare All Options

Among the given options, only HPO_{4}^{2-} can act as both a Bronsted acid and base. The others can only predominantly act as either an acid or a base, not both.

Key Concepts

Proton Donation

In the context of Bronsted-Lowry theory, proton donation is a core characteristic of acids. When an acid donates a proton, it typically becomes a new species with one less positive charge. For example,

• The species \(H_{2}PO_{2}^{-}\) can donate a proton, creating \(HPO_{2}^{2-}\).
• Similarly, \(HPO_{4}^{2-}\) can donate a proton to form \(PO_{4}^{3-}\).

The act of donating a proton is what defines a substance as a Bronsted acid. This transition between species highlights the reversible nature of acid-base reactions, showcasing how a single molecule can change its role in a reaction.

Proton Acceptance

Proton acceptance is the defining action of a Bronsted base. Bases typically have lone pairs of electrons or a negative charge that facilitates the acceptance of protons. Let's consider a few examples:

• The dianion \(HPO_{3}^{2-}\) has sufficient negative charge and can act as a base by accepting a proton to form \(H_{2}PO_{3}^{-}\).
• Also, \(HPO_{4}^{2-}\) can accept a proton to create \(H_{2}PO_{4}^{-}\).

This ability to accept protons makes a substance a Bronsted base. Essentially, it transforms the molecule into a more positively charged species, emphasizing the adaptable nature of these reagents.

Amphoteric Species

An amphoteric species is a versatile player in chemistry, capable of acting both as an acid and a base. This adaptability allows such species to donate and accept protons, depending on the surrounding chemical environment:

• \(HPO_{4}^{2-}\) is an excellent example of an amphoteric ion. It can donate a proton to form \(PO_{4}^{3-}\), behaving as an acid.
• Conversely, it can accept a proton to become \(H_{2}PO_{4}^{-}\), functioning as a base.

Amphoterism is a valuable characteristic for maintaining balance in chemical reactions and is pivotal in processes where pH conditions vary.

Acid-Base Reactions

Acid-base reactions are pivotal exchanges between proton donors (acids) and acceptors (bases). The interaction primarily results in the transfer of protons, which shapes the reaction dynamics. In the Bronsted-Lowry mode,

• An acid such as \(HPO_{4}^{2-}\) donates a proton to become \(PO_{4}^{3-}\), aligning with its acidic role.
• Simultaneously, this ion can accept a proton, transforming into \(H_{2}PO_{4}^{-}\), illustrating its basic side.

These reactions are essential in many natural and industrial processes. Understanding the dualistic roles of ions like \(HPO_{4}^{2-}\) helps in predicting the outcome and direction of chemical reactions.