Question

Identify each of the following species as a Bronsted acid, base, or both: (a) \(\mathrm{PO}_{4}^{3-},\) (b) \(\mathrm{ClO}_{2}^{-},\) (c) \(\mathrm{NH}_{4}^{+}\), (d) \(\mathrm{HCO}_{3}^{-}\).

Short answer

(a) Base, (b) Base, (c) Acid, (d) Both.

Step-by-step solution

Identify PO₄³⁻ Properties

To identify whether \( \mathrm{PO}_4^{3-} \) acts as a Bronsted acid, base, or both, remember that a Bronsted acid donates protons (H⁺) and a Bronsted base accepts protons. \( \mathrm{PO}_4^{3-} \) can accept protons to form \( \mathrm{HPO}_4^{2-} \), so it acts as a Bronsted base.

Determine ClO₂⁻ Functionality

For \( \mathrm{ClO}_2^{-} \), evaluate whether it can gain or lose protons. It can accept a proton to become \( \mathrm{HClO}_2 \), indicating it acts as a Bronsted base.

Analyze NH₄⁺ Characteristics

Check if \(\mathrm{NH}_4^{+}\) can donate a proton. \(\mathrm{NH}_4^{+}\) can donate a proton to revert to \(\mathrm{NH}_3\). Therefore, \(\mathrm{NH}_4^{+}\) acts as a Bronsted acid.

Assess HCO₃⁻ Dual Behavior

\(\mathrm{HCO}_3^{-}\) can both accept a proton to become \(\mathrm{H}_2\mathrm{CO}_3\) and donate a proton to turn into \(\mathrm{CO}_3^{2-}\). Thus, \(\mathrm{HCO}_3^{-}\) functions as both a Bronsted acid and a base.

Key concepts

Proton Donor

In the world of Bronsted acids and bases, a proton donor is essentially a substance that can release a hydrogen ion, or proton ( H^+ ), into a solution. This characteristic defines it as a Bronsted acid. When an acid dissolves in water, it donates a proton to water molecules, forming hydronium ions ( H_3O^+ ). For example, the substance NH_4^+ acts as a classic proton donor. It has the ability to shed a proton, transforming itself into NH_3 . This process illustrates how NH_4^+ behaves as a Bronsted acid in chemical reactions.

Key points to remember about proton donors are:

• They release protons ( H^+ ) into the solution.
• They are identified as Bronsted acids.
• Examples include substances like NH_4^+ and acids such as HCl and H_2SO_4 .

Recognizing proton donors is essential to understanding how acid-base reactions proceed and the roles substances play in these reactions.

Proton Acceptor

On the flip side, a proton acceptor is a substance that can receive a proton from another molecule. This makes the substance a Bronsted base. Proton acceptors play a crucial role in neutralizing acids and forming new compounds by incorporating protons. Take the example of PO_4^{3-} , which acts as a proton acceptor. It readily accepts a proton to become HPO_4^{2-} , showcasing its basic nature.

Essential characteristics of proton acceptors include:

• They can accept protons ( H^+ ) from the solution.
• They serve as Bronsted bases.
• They include ions like PO_4^{3-} and bases like OH^- and NH_3 .

By acknowledging the role of proton acceptors, students can better understand how bases interact with acids and the resulting products from these chemical interactions.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are intimately linked in an acid-base reaction, where the loss of a proton from an acid forms its conjugate base, and the gain of a proton by a base forms its conjugate acid. This relationship is central to the dual functionality of substances in acid-base chemistry.

Take HCO_3^{-} for example. This substance can act as both an acid and a base, making it amphoteric. When it donates a proton, it transforms into CO_3^{2-} , its conjugate base. Conversely, when it accepts a proton, it becomes H_2CO_3 , its conjugate acid. This dual behavior demonstrates how HCO_3^{-} and its related species interact in various environments.

Some important points about conjugate acid-base pairs are:

• Each acid has a corresponding conjugate base formed by loss of a proton.
• Each base has a corresponding conjugate acid formed by gain of a proton.
• Amphoteric species like HCO_3^{-} have both conjugate acids and bases.

Understanding these pairs is crucial in predicting the outcomes of acid-base reactions and the roles different compounds play.