Question

Identify each of the following species as a Bronsted acid, base, or both: (a) HI, (b) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\), (c) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) (d) \(\mathrm{HSO}_{4}^{-}\)

Short answer

HI is a Bronsted Acid, \(\mathrm{CH}_{3} \mathrm{COO}^{-}\) is a Bronsted Base, and both \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) and \(\mathrm{HSO}_{4}^{-}\) are amphoteric (Bronsted Acid and Base).

Step-by-step solution

Classification of HI

Hydroiodic acid (HI) is a compound containing hydrogen. It can donate its single proton (H+) to a different compound, hence it acts as a Bronsted acid.

Classification of \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)

The compound \(\mathrm{CH}_{3} \mathrm{COO}^{-}\) (acetate ion) has a negative charge and can accept a proton. Hence, it is termed a Bronsted base.

Classification of \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)

\(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) (dihydrogen phosphate ion) can donate a proton considering its hydrogen content, hence acts as a Bronsted acid. Additionally, it also holds a negative charge that allows it to accept a proton, hence it acts as a Bronsted base as well. Thus, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) is a Bronsted acid and base, also termed amphoteric.

Classification of \(\mathrm{HSO}_{4}^{-}\)

Similarly, \(\mathrm{HSO}_{4}^{-}\) (hydrogen sulphate ion) has both characteristics. It can donate a proton due to the presence of hydrogen, making it a Bronsted acid. Its ability to accept a proton, due to its negative charge, makes it a Bronsted Base. Hence, \(\mathrm{HSO}_{4}^{-}\) is also amphoteric.

Key concepts

Understanding Acid-Base Chemistry

Acid-base chemistry involves understanding how substances interact by either donating or accepting protons (H extsuperscript{+}). The Bronsted-Lowry theory provides a useful framework to identify acids and bases. According to this theory:

• Bronsted Acids are substances that can donate a proton to another substance.
• Bronsted Bases are substances that can accept a proton from another substance.

When an acid donates a proton, it transforms into a conjugate base, whereas a base becomes a conjugate acid upon accepting a proton. In the exercise, HI is classified as a Bronsted acid as it donates a proton. Meanwhile, CH extsubscript{3}COO extsuperscript{-} acts as a base by accepting a proton. This interaction is a fundamental concept in understanding how acid-base reactions occur in various chemical contexts. The ability to categorize substances into acids or bases is crucial for predicting the behavior of chemical reactions, especially in aqueous solutions.

Amphoteric Behavior and Its Role

Amphoteric substances are molecules or ions that can behave both as acids and bases depending on the environment. They have dual functionality based on the reactions they are involved in. The examples provided in the exercise, H extsubscript{2}PO extsubscript{4} extsuperscript{-} and HSO extsubscript{4} extsuperscript{-}, showcase amphoteric behavior:

• As an Acid: These substances can donate a proton because of the presence of a hydrogen atom.
• As a Base: Their negative charge enables them to accept a proton in a reaction.

These properties allow them to adapt based on the existing conditions, making them versatile in various chemical reactions. Understanding the amphoteric nature of compounds helps in predicting their role in different reaction setups, especially in biochemical pathways and industrial processes.

The Importance of Proton Transfer

Proton transfer is a key concept in many chemical reactions, especially those involving acid and base chemistry. It involves the movement of protons (H extsuperscript{+}) from an acid to a base. This transfer is the primary driver of acid-base reactions, facilitating the transformation of particles and influencing the equilibria of reactions. Consider the substance H extsubscript{2}PO extsubscript{4} extsuperscript{-}, which can donate a proton, turning into HPO extsubscript{4} extsuperscript{2-}, or can accept a proton to become H extsubscript{3}PO extsubscript{4}. Such proton exchanges are fundamental:

• They are responsible for maintaining the pH balance in biological systems.
• They play a critical role in metabolic processes.
• They help in stabilizing the structure of organic and inorganic compounds.

Mastering the concept of proton transfer is essential for students to understand complex biochemical processes and industrial chemical reactions, providing insights into reaction mechanisms and product formation.