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) \(\mathrm{HPO}_{4}^{2-}\) can act as both an acid and a base.

Step 1: Identify Bronsted Acid and Base

A Bronsted acid is a substance that can donate a proton (H extsuperscript{+}), and a Bronsted base is a substance that can accept a proton. We need to determine which option can act as both a proton donor and a proton acceptor.

Step 2: Evaluate Option (a)

The species \(\mathrm{H}_{2} \mathrm{PO}_{2}\) can donate a proton to become \(\mathrm{HPO}_{2}^{-}\), but it cannot accept a proton since it has no more hydrogen atoms to add; hence, it cannot act as both.

Step 3: Evaluate Option (b)

The species \(\mathrm{HPO}_{3}^{2-}\) can accept a proton to become \(\mathrm{H}_2\mathrm{PO}_{3}^{-}\), but cannot donate a proton because it can only accept them as a base. As such, this species cannot act as both.

Step 4: Evaluate Option (c)

The species \(\mathrm{HPO}_{4}^{2-}\) can donate a proton to form \(\mathrm{PO}_{4}^{3-}\) and can accept a proton to become \(\mathrm{H}_2\mathrm{PO}_{4}^{-}\). This means it can act as both a Bronsted acid and a base.

Step 5: Conclusion

After evaluating all options, only \(\mathrm{HPO}_{4}^{2-}\) fulfills the role of acting as both a Bronsted acid and base according to definition.

Key Concepts

Understanding Proton Donors

In the Bronsted acid-base theory, a proton donor is an essential concept to grasp. Perhaps a bit more formally, a proton donor is known as a Bronsted acid. This means it has the ability to donate a proton ( H^{+} ) to another substance. By shedding a proton, the acid becomes its conjugate base. For instance, consider H_2 ext{PO}_2 . If it donates a proton, it transforms into HPO_2^{-} .

• Proton donors are defined by their ability to give up protons.
• Once the proton is released, the original compound becomes its conjugate base.
• It's vital to understand the readiness of a molecule to give away a proton to define it as a Bronsted acid.

Grasping this idea can help you differentiate between which compounds in a chemical equation might act as acids. Each time a species donates a proton, it plays the role of an acid in that reaction.

Comprehending Proton Acceptors

The counterpart to proton donors, proton acceptors are what we describe as Bronsted bases. These are species that can gain or accept a proton ( H^{+} ) from another molecule. Once a base accepts a proton, it becomes its conjugate acid. Take HPO_3^{2-} as an example. Upon accepting a proton, it transforms into H_2PO_3^{-} .

• Proton acceptors participate in reactions by acquiring protons from donors.
• After proton addition, bases become conjugate acids.
• Understanding their role helps identify bases in chemical reactions.

It's crucial in any acid-base reaction to recognize which molecules can act as acceptors to predict reaction products. Seeing this behavior in equations helps explain how substances increase their complexity by gaining protons.

Exploring Conjugate Acid-Base Pairs

When discussing Bronsted acids and bases, the concept of conjugate acid-base pairs is central. These pairs consist of two compounds that transform into each other by the gain or loss of a proton. For example, when HPO_4^{2-} donates a proton, it turns into PO_4^{3-} , creating a conjugate acid-base pair with H_2PO_4^{-} , should HPO_4^{2-} later accept a proton.

• Each acid has a corresponding conjugate base, and vice versa.
• The conversion between them involves either donating or accepting a proton.
• This relationship is essential for balance in chemical reactions.

Conjugate acid-base pairs help maintain equilibrium in reactions, allowing us to predict how substances will interact under certain conditions. Recognizing these pairs enhances understanding of how molecules can interchange roles within reactions.