Question

Which of the following species can act both as an acid as well as a base? (a) \(\mathrm{SO}_{4}^{2-}\) (b) \(\mathrm{HSO}_{4}^{-}\) (c) \(\mathrm{PO}_{4}^{3-}\) (d) \(\mathrm{OH}\)

Short answer

\(\mathrm{HSO}_{4}^{-}\) can act as both an acid and a base, making it the amphoteric species among the given options.

Step-by-step solution

Understand the concept of Amphoterism

A species that can act as both an acid and a base is called amphoteric. According to the Bronsted-Lowry theory, an acid is a proton (hydrogen ion, H+) donor, and a base is a proton acceptor. An amphoteric species must be able to both donate and accept a proton.

Analyze each option for amphoteric behavior

Examine each species to determine if it can donate and accept a proton. (a) \(\mathrm{SO}_{4}^{2-}\) cannot accept a proton because it is a fully deprotonated sulfate ion. (b) \(\mathrm{HSO}_{4}^{-}\) can donate a proton to form \(\mathrm{SO}_{4}^{2-}\) and can accept a proton to form \(\mathrm{H}_{2}\mathrm{SO}_{4}\). (c) \(\mathrm{PO}_{4}^{3-}\) is similar to \(\mathrm{SO}_{4}^{2-}\) and does not typically accept a proton but can donate protons to form \(\mathrm{HPO}_{4}^{2-}\) and further down to \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\). (d) The hydroxide ion \(\mathrm{OH}^{-}\) is a strong base and not known for donating protons; it accepts protons to form water.

Identify the amphoteric species

Out of the given options, \(\mathrm{HSO}_{4}^{-}\) is the only species that can act both as an acid and as a base. It can donate a proton to become \(\mathrm{SO}_{4}^{2-}\) and accept a proton to become sulfuric acid, \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Key concepts

Bronsted-Lowry Theory

The Bronsted-Lowry theory of acids and bases is essential to understanding a wide range of chemical processes, including the behavior of amphoteric species. In this theory, an acid is defined as any substance that can donate a proton (a hydrogen ion, denoted as \(H^+\)), while a base is any substance that can accept a proton.

What makes the Bronsted-Lowry perspective distinctive is its focus on the transfer of protons between molecules or ions during a chemical reaction. This approach to acids and bases extends beyond the traditional concepts which limited acids and bases to substances that produce \(H^+\) or hydroxide ions \(OH^-\) in water.

For instance, in the case of \(HSO_4^-\), it can act as a Bronsted-Lowry acid when it donates a proton to become \(SO_4^{2-}\), and as a Bronsted-Lowry base when it accepts a proton to become sulfuric acid \(H_2SO_4\). Understanding this dual role is central to recognizing the versatile nature of amphoteric species.

Acids and Bases

Acids and bases are two fundamental categories of chemical species that have opposing characteristics and are capable of neutralizing each other in a reaction. The Bronsted-Lowry theory provides a more detailed understanding of the behavior of these substances by describing acid-base reactions as proton transfer processes.

An acid is a proton donor - it possesses a hydrogen atom that can be released as a positively charged hydrogen ion. This hydrogen ion plays a pivotal role in the acid's ability to react with other substances. On the other hand, a base is a proton acceptor - it has the ability to take in a hydrogen ion and form a bond with it.

Substances like \(HSO_4^-\) demonstrate the dynamic nature of acids and bases. Not all substances are restricted to being solely an acid or a base; some have the unique capacity to function as both, depending on the reaction’s context. This versatility is central to the concept of amphoteric species.

Proton Donor and Acceptor

Proton donors and acceptors are key terms in the understanding of acid-base chemistry within the framework of the Bronsted-Lowry theory. A proton donor is any species that can release a proton, while a proton acceptor is a species that can bind to an incoming proton.

When analyzing a chemical reaction, identifying the proton donors and acceptors can reveal the direction of proton transfer and, consequently, the role each species plays in the reaction. Water \(H_2O\), for example, can act as both a proton donor and acceptor, depending on the other species present in the reaction mixture.

In the case of \(HSO_4^-\), it is amphoteric because it can function as a proton donor by giving up a proton to become \(SO_4^{2-}\), and as a proton acceptor by taking in a proton to turn into \(H_2SO_4\). The ability of a species to act as both a donor and acceptor of protons is a hallmark of amphoterism, enabling the substance to engage in a wider variety of chemical reactions.