Question

Which of the following species are amphiprotic in aqueous solution? For such a species, write one equation showing it acting as an acid, and another equation showing it acting as a base. \(\mathrm{OH}^{-}, \mathrm{NH}_{4}^{+}, \mathrm{H}_{2} \mathrm{O}, \mathrm{HS}^{-}\) \(\mathrm{NO}_{2}^{-}, \mathrm{HCO}_{3}^{-}, \mathrm{HBr}\)

Short answer

The amphiprotic species are: \(OH^-\), \(H_2O\), \(HS^-\) and \(HCO_3^-\). As acids, their reactions are: \(OH^- + H^+ \rightarrow H_2O\), \(H_2O + H^+ \rightarrow H_3O^+\), \(HS^- + H^+ \rightarrow H_2S\), and \(HCO_3^- + H^+ \rightarrow H_2CO_3\). As bases, their reactions are: \(OH^- + H_2O \rightarrow HO^- + H_2O\), \(H_2O + H_2O \rightarrow OH^- + H_3O^+\), \(HS^- + H_2O \rightarrow OH^- + H_2S\), and \(HCO_3^- + H_2O \rightarrow OH^- + H_2CO_3\)

Step-by-step solution

Amphiprotic Identification

The amphiprotic species are: \(OH^-\), \(H_2O\), \(HS^-\) and \(HCO_3^-\). They have the ability to both accept and donate a proton.

Action as Acids

As acids, these species donate a proton (H+). The equations describing this are: \(OH^- + H^+ \rightarrow H_2O\), \(H_2O + H^+ \rightarrow H_3O^+\), \(HS^- + H^+ \rightarrow H_2S\), and \(HCO_3^- + H^+ \rightarrow H_2CO_3\)

Action as Bases

As bases, these species accept a proton (H+). The equations describing this are: \(OH^- + H_2O \rightarrow HO^- + H_2O\), \(H_2O + H_2O \rightarrow OH^- + H_3O^+\), \(HS^- + H_2O \rightarrow OH^- + H_2S\), and \(HCO_3^- + H_2O \rightarrow OH^- + H_2CO_3\)

Key concepts

Proton Transfer

Proton transfer is a fundamental concept in chemistry, especially in the study of acids and bases. It involves the movement of a hydrogen ion, or proton ( H^+ ), from one molecule to another. This movement can either be from an acidic species that donates the proton, or to a basic species that accepts it. In acid-base reactions, identifying which species donate and which accept protons is essential. Amphiprotic substances play a unique role here. They can behave as either acids or bases, depending on the reaction environment. For instance, in water, amphiprotic substances like OH^- or H_2O can donate a proton to form H_2O and H_3O^+ respectively, or they can accept a proton from another substance. This dual capability allows for flexible chemistry and explains why specific species react differently under varying conditions.
Proton transfer is at the heart of many reactions and illustrates the dynamic nature of chemical processes.

Acid-Base Reactions

Acid-base reactions are a central theme in chemistry, showcasing the dynamic interaction of substances. At a fundamental level, these reactions involve acids donating protons and bases accepting them. What makes amphiprotic species fascinating is their ability to participate in these reactions flexibly. For example, HS^- and HCO_3^- can act as acids by donating a proton to form H_2S and H_2CO_3 , or as bases by accepting a proton from water, leading to reactions like HS^- + H_2O ightarrow OH^- + H_2S . This adaptability allows them to maintain balance in reactions and buffer systems.
Understanding the mechanisms of acid-base reactions helps us predict the behavior of substances in various conditions, from controlling pH in solutions to facilitating biological processes.

Aqueous Solution Chemistry

When studying chemistry in aqueous solutions, water acts as a quintessential medium for reactions due to its polar nature and ability to dissolve a wide range of substances. Aqueous solution chemistry often involves amphiprotic substances, which can both donate and accept protons in water. This behavior is particularly crucial in determining the pH of solutions. For example, H_2O acting as a base accepting a proton can form H_3O^+ , affecting the acidity of the solution.
Amphiprotic species like H_2O , OH^- , and HCO_3^- help moderate shifts in pH, making them vital components in biological and chemical systems where pH is a critical factor. Aqueous solution chemistry underpins much of our understanding of processes ranging from industrial chemical reactions to the functioning of biochemical systems in living organisms.