Question

Of the following, the amphiprotic ion is (a) \(\mathrm{HCO}_{3}^{-}\) (b) \(\mathrm{CO}_{3}^{2-} ;\) (c) \(\mathrm{NH}_{4}^{+} ;\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+} ;\) (e) \(\mathrm{ClO}_{4}^{-}\).

Short answer

The amphiprotic ion among the options is \(\mathrm{HCO}_{3}^{-}\).

Step-by-step solution

Understand the meaning of amphiprotic

An amphiprotic ion can act as both an acid and a base. This means it can donate a proton (H+) to become a conjugate base or accept a proton (H+) to become a conjugate acid, depending on the environment.

Examine each ion

(a) \(\mathrm{HCO}_{3}^{-}\): This ion can lose a H+ ion to become \(\mathrm{CO}_{3}^{2-}\) or gain a H+ ion to become \(\mathrm{H}_{2}\mathrm{CO}_{3}\). Therefore, it is amphiprotic. \n(b) \(\mathrm{CO}_{3}^{2-}\): This ion can only accept a H+ to become \(\mathrm{HCO}_{3}^{-}\). Therefore, it is not amphiprotic. \n(c) \(\mathrm{NH}_{4}^{+}\): This ion can only lose a H+ ion to become \(\mathrm{NH}_{3}\). Therefore, it is not amphiprotic. \n(d) \(\mathrm{CH}_{3}\mathrm{NH}_{3}^{+}\): This ion can only lose a H+ ion to become \(\mathrm{CH}_{3}\mathrm{NH}_{2}\). Therefore, it is not amphiprotic. \n(e) \(\mathrm{ClO}_{4}^{-}\): This ion is a stable anion and cannot lose or accept a proton. Therefore, it is not amphiprotic.

Conclusion

Only one ion shows the capability of both accepting and donating a proton, which makes it amphiprotic. That ion is \(\mathrm{HCO}_{3}^{-}\).

Key concepts

Proton transfer

Proton transfer is a fundamental process in chemistry, particularly in acid-base reactions. When we talk about proton transfer, we're essentially discussing the movement of a hydrogen ion ( H^+ ) from one molecule or ion to another. This process is central to the behavior of amphiprotic ions. Amphiprotic ions, like HCO_3^- , can both donate and accept protons, which means they are capable of undergoing proton transfer in both directions.
In an acid-base reaction, a proton transfer can determine the direction and extent of the reaction. For instance, when HCO_3^- acts as a base, it accepts a proton to form H_2CO_3 . Conversely, when it acts as an acid, it donates a proton to become CO_3^{2-} . This dual capability allows amphiprotic ions to participate in complex chemical environments, adapting their behavior based on the surrounding conditions.
Understanding proton transfer is key to mastering many areas of chemistry, from biochemical pathways to industrial processes, where acids and bases play critical roles.

Conjugate acid-base pairs

Conjugate acid-base pairs are a vital concept in understanding how acids and bases interact. They are essentially two species that transform into each other by the gain or loss of a proton ( H^+ ).
When an acid donates a proton, it becomes its conjugate base. Similarly, when a base accepts a proton, it transforms into its conjugate acid. This interconversion helps us understand the dynamic nature of acid-base reactions. Take bicarbonate ion ( HCO_3^- ) as an example:

• As an acid, HCO_3^- donates a proton to form CO_3^{2-} , its conjugate base.
• As a base, HCO_3^- accepts a proton to become H_2CO_3 , its conjugate acid.

This relationship not only explains the amphiprotic nature of HCO_3^- but also sheds light on the broader acid-base balance in solutions. Conjugate acid-base pairs allow for the maintenance of pH levels and the facilitation of various chemical reactions that are essential for life and industrial chemistry.

Bicarbonate ion

The bicarbonate ion, HCO_3^- , is a versatile and essential component in various chemical and biological systems. One of its key characteristics is being amphiprotic, which allows it to interact flexibly with acids and bases. This flexibility makes bicarbonate an important buffer in biological pH regulation.
In the human body, bicarbonate plays a critical role in maintaining the acid-base balance in blood and other fluids. It acts as a buffer that helps neutralize acids or bases introduced into the system. When the pH level in the blood falls, bicarbonate ions can react with hydrogen ions to form carbonic acid ( H_2CO_3 ), thus minimizing the decrease in pH.
Moreover, engineering uses bicarbonate in various applications such as wastewater treatment and carbon capture, leveraging its ability to engage in proton transfer reactions. Understanding bicarbonate's amphiprotic nature allows scientists and engineers to harness its properties effectively for environmental and health purposes.