Question

Anions containing hydrogen (for example, \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-} )\) usually show amphoteric behavior. Write equations illustrating the amphoterism of these two anions.

Short answer

The amphoteric behavior of the given anions, \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), can be illustrated through the following equations: \( \mathrm{HCO}_{3}^{-} \): Acidic behavior: \[ \mathrm{HCO}_{3}^{-} \rightarrow \mathrm{CO}_{3}^{2-} + \mathrm{H}^{+} \] Basic behavior: \[ \mathrm{HCO}_{3}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} \] \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\): Acidic behavior: \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \rightarrow \mathrm{HPO}_{4}^{2-} + \mathrm{H}^{+} \] Basic behavior: \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{H}_{3}\mathrm{PO}_{4} \]

Step-by-step solution

Identify acidic and basic behavior for \(\mathrm{HCO}_{3}^{-}\)

An acidic species can donate a proton (H\(^{+}\)), while a basic species can accept a proton (H\(^{+}\)). We will start with the \(\mathrm{HCO}_{3}^{-}\) anion, which is derived from carbonic acid (H\(_{2}\)CO\(_{3}\)). Acidic behavior: When \(\mathrm{HCO}_{3}^{-}\) donates a proton, it becomes CO\(_{3}^{2-}\). \[ \mathrm{HCO}_{3}^{-} \rightarrow \mathrm{CO}_{3}^{2-} + \mathrm{H}^{+} \] Basic behavior: When \(\mathrm{HCO}_{3}^{-}\) accepts a proton, it becomes H\(_{2}\)CO\(_{3}\). \[ \mathrm{HCO}_{3}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} \]

Identify acidic and basic behavior for \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)

Next, we will examine the \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) anion, which is derived from the dihydrogen phosphate ion (H\(_{2}\)PO\(_{4}^{-}\)). Acidic behavior: When \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) donates a proton, it becomes HPO\(_{4}^{2-}\). \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \rightarrow \mathrm{HPO}_{4}^{2-} + \mathrm{H}^{+} \] Basic behavior: When \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) accepts a proton, it becomes H\(_{3}\)PO\(_{4}\). \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{H}_{3}\mathrm{PO}_{4} \] Now we have equations illustrating the amphoteric behavior of both anions.

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is essential when exploring amphoteric behavior. An acid-base reaction involves the transfer of protons (H\(^{+}\)). Acids are substances that can donate protons, while bases can accept them.
Anions like bicarbonate (\(\mathrm{HCO}_{3}^{-}\)) and dihydrogen phosphate (\(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\)) can act as both acids and bases. This dual behavior lets them participate in various chemical reactions, helping maintain pH balance.
For example, in the acidic reaction, \(\mathrm{HCO}_{3}^{-}\) releases a proton to form \(\mathrm{CO}_{3}^{2-}\), while in a basic reaction, it accepts a proton to re-form \(\mathrm{H}_{2}\mathrm{CO}_{3}\). Similarly, \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) can donate or accept protons, showing its role in acid-base chemistry.

Anion Chemistry

Anion chemistry focuses on negatively charged ions, such as \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\). These are called anions because they gain electrons and hold a negative charge.
These anions often arise from weak acids losing a proton. For example, carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) loses a hydrogen ion to form the bicarbonate anion (\(\mathrm{HCO}_{3}^{-}\)).
In aqueous solutions, anions impact the pH and can interact with cations to form salts. Their behavior is significant in buffering systems, which resist changes in pH, making anion chemistry vital in processes like metabolism and photosynthesis.

Proton Transfer

Proton transfer is a key concept in understanding how substances like \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) exhibit amphoteric behavior. Proton transfer involves moving a hydrogen ion (H\(^{+}\)) from one molecule to another.
When \(\mathrm{HCO}_{3}^{-}\) acts as an acid, it transfers a proton to become \(\mathrm{CO}_{3}^{2-}\). Conversely, it can become a base by accepting a proton to create \(\mathrm{H}_{2}\mathrm{CO}_{3}\).
This dual nature underlines amphoterism, wherein a substance can both donate and accept protons, adapting to different chemical environments. Understanding proton transfer is crucial for grasping chemical reactions, equilibrium, and pH changes.

Carbonic Acid

Carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) plays an important role in buffering systems and in the carbon cycle. As a weak acid, it partially dissociates in water to form bicarbonate (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen ions (H\(^{+}\)).
This reversible reaction allows \(\mathrm{H}_{2}\mathrm{CO}_{3}\) to help maintain the pH balance in blood and oceans. For example, when the blood pH becomes too acidic, bicarbonate can absorb excess protons, reducing acidity.
Moreover, carbonic acid is crucial in exhalation. It helps transport carbon dioxide, derived from metabolism, from cells to the lungs, where it's expelled as a gas. Understanding carbonic acid's roles and reactions enriches our knowledge of biological and environmental chemistry.

Dihydrogen Phosphate

Dihydrogen phosphate (\(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\)) is an important anion in biochemistry and environmental systems. It is derived from phosphoric acid and participates actively in buffering solutions.

• In biological systems, it's crucial in energy storage and transfer, as seen in ATP (adenosine triphosphate).
• With its ability to donate and accept protons, \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) plays a significant role in maintaining pH levels in cells and in soil.

The equilibrium between \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) and \(\mathrm{HPO}_{4}^{2-}\) compensates for pH changes, supporting processes such as photosynthesis and respiration.
This dynamic behavior shows how dihydrogen phosphate exemplifies concepts like buffer solutions and the biogeochemical cycle.