Question

Anions containing hydrogen (for example, \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-}\) ) show amphoteric behavior when reacting with other acids or bases. Write equations illustrating the amphoterism of these anions.

Short answer

The amphoteric behavior of \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-}\) can be shown through their reaction with acids and bases: When reacting with acids: 1. \(\mathrm{HCO}_{3}^{-} + \mathrm{HA} \rightarrow \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2}\mathrm{A}\) 2. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-} + \mathrm{HA} \rightarrow \mathrm{PO}_{4}^{3-}+ 2\mathrm{H}\mathrm{A}\) When reacting with bases: 1. \(\mathrm{HCO}_{3}^{-} + \mathrm{B}^- \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} + \mathrm{BO}\) 2. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-} + \mathrm{B}^- \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{BO}\) These equations illustrate the ability of these anions to act as both acids and bases, demonstrating their amphoteric behavior.

Step-by-step solution

Reaction with Acids

In reactions with acids, the given anions will act as bases. They will accept a proton (H⁺) from the acid. Let's write the balanced chemical equations for both anions reacting with a general acid (HA): 1) Reaction for the bicarbonate ion (HCO₃⁻): \[ \mathrm{HCO}_{3}^{-} + \mathrm{HA} \rightarrow \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2}\mathrm{A} \] 2) Reaction for the hydrogen phosphate ion (H₂PO₄²⁻): \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{2-} + \mathrm{HA} \rightarrow \mathrm{PO}_{4}^{3-}+ 2\mathrm{H}\mathrm{A} \] In these reactions, both anions accept a proton from the general acid (HA), and they produce a new acid (H₂A) along with their respective conjugate base.

Reaction with Bases

In reactions with bases, the given anions will act as acids. They will donate a proton (H⁺) to the base. Let's write the balanced chemical equations for both anions reacting with a general base (B): 1) Reaction for the bicarbonate ion (HCO₃⁻): \[ \mathrm{HCO}_{3}^{-} + \mathrm{B}^- \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} + \mathrm{BO} \] 2) Reaction for the hydrogen phosphate ion (H₂PO₄²⁻): \[ \mathrm{H}_{2} \mathrm{PO}_{4}^{2-} + \mathrm{B}^- \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{BO} \] In these reactions, both anions donate a proton to the general base (B), and they produce a new weaker acid (H₂CO₃ and H₃PO₄) along with a conjugate base (BO). Thus, we have illustrated the amphoteric behavior of the given anions, i.e., their ability to react with both acids and bases in different chemical reactions.

Key concepts

Anions

Anions are negatively charged ions, formed when an atom or molecule gains one or more electrons. This gain in electrons gives them extra negative charge. What's fascinating about anions is their role in various chemical reactions, including oxidation-reduction (redox) reactions, where electrons are transferred between substances. In the context of acid-base reactions, anions can influence how substances interact with acids or bases based on their charge and composition.

For example, some anions like the bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen phosphate ion (\(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-}\)) exhibit amphoteric behavior. This means they can act both as acids and bases, depending on the reacting environment. When in the presence of stronger acids, they behave as bases by accepting protons. Conversely, in the presence of bases, they donate protons, acting as acids.

Amphoteric anions are particularly interesting because they illustrate the dual nature of certain ions, and understanding them can shed light on how chemical systems can adapt and react under varying conditions.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. These equations show the reactants starting the reaction and the products formed, along with their respective quantities. When writing chemical equations, it's crucial to ensure they are balanced, meaning the number of each type of atom is the same on both sides of the equation. This reflects the conservation of mass principle, where matter is neither created nor destroyed.

Balanced chemical equations help us understand the stoichiometry of a reaction – essentially the relative amounts of reactants and products. For amphoteric anions, we see their reactions with acids and bases illustrated through balanced equations.

Consider the reaction of the bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)) with a general acid (HA):
\[ \mathrm{HCO}_{3}^{-} + \mathrm{HA} \rightarrow \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2}\mathrm{A} \] Here, the bicarbonate ion has accepted a proton, demonstrating its basic nature in this scenario. Observing and writing such equations allows chemists to predict and interpret the behavior of substances in reactions.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of protons (\(\mathrm{H}^{+}\)) between reactants. An acid is a proton donor and a base a proton acceptor. The reaction between an acid and a base can lead to the formation of water and salts, commonly known as neutralization. However, the behavior gets more complex with amphoteric ions.

In the case of bicarbonate (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen phosphate (\(\mathrm{H}_{2} \mathrm{PO}_{4}^{2-}\)) ions, they can switch roles based on their environment, showing their amphoteric nature. When reacting with acids, these anions accept protons, behaving as bases. Conversely, when they encounter bases, they donate protons, acting as acids.

For instance, consider the reaction of the hydrogen phosphate ion with a base (B):
\[ \mathrm{H}_{2} \mathrm{PO}_{4}^{2-} + \mathrm{B}^- \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{BO} \] Here, the hydrogen phosphate ion donates a proton, transitioning to a phosphoric acid and forming a conjugate base (BO).

These reactions display the flexibility and dual nature of amphoteric substances in chemical processes.