Question

Describe amphoteric behavior and give an example.

Short answer

Amphoteric behavior is the ability to act as both acid and base. Water and aluminum hydroxide are examples, reacting as an acid (donating H^+) or base (accepting H^+) depending on the reactants.

Step-by-step solution

Understanding Amphoteric Behavior

Amphoteric behavior refers to a substance's ability to react both as an acid and as a base. This duality is highlighted in the Bronsted-Lowry theory where an acid is a proton donor and a base is a proton acceptor. Amphoteric substances can donate or accept protons depending on the reaction they undergo.

Identify an Amphoteric Substance

Water (H_2O) is a classic example of an amphoteric substance. It can act as an acid by donating a proton to form hydroxide (OH^-), or as a base by accepting a proton to form hydronium (H_3O^+). Another example is aluminum hydroxide (Al(OH)_3), which can react with both acids and bases.

Example of Acidic Reaction

When aluminum hydroxide reacts with hydrochloric acid (HCl), it acts as a base and forms aluminum chloride (AlCl_3) and water, showing its ability to accept protons from HCl.

Example of Basic Reaction

Conversely, when aluminum hydroxide reacts with sodium hydroxide (NaOH), it acts as an acid and forms sodium aluminate (NaAlO_2) and water, illustrating its capacity to donate protons to NaOH.

Key concepts

Bronsted-Lowry Theory

The Bronsted-Lowry theory is a cornerstone concept in the understanding of acid-base chemistry. It redefines acids and bases in terms of their ability to donate or accept protons (H+ ions), a significant departure from the earlier Arrhenius definition which based these properties on the production of hydrogen or hydroxide ions in solution.

According to this theory, an acid is a 'proton donor' and will release a proton during a chemical reaction, while a base is a 'proton acceptor' and will gain a proton. This proton transfer process is the basis of an acid-base reaction. It's crucial to note that the Bronsted-Lowry concept also introduces the idea of conjugate acid-base pairs; when an acid donates a proton, it becomes a conjugate base, and when a base accepts a proton, it becomes a conjugate acid.

Acid-Base Reactions

Acid-base reactions are chemical processes in which an acid donates a proton to a base. In the context of the Bronsted-Lowry theory, these reactions are viewed as proton transfer reactions.

To conceptualize this, consider the reaction between hydrochloric acid (HCl) and water (H2O). HCl donates a proton to water, transforming into its conjugate base, chloride (Cl-), while water, after accepting a proton, becomes its conjugate acid, hydronium (H3O+). The general formula for an acid-base reaction is: \( \text{Acid} + \text{Base} \rightarrow \text{Conjugate Base} + \text{Conjugate Acid} \).

It's important for students to recognize that such reactions are equilibrium processes, which means the reaction can occur in both forward and reverse directions. This dual nature emphasizes the dynamicity of acid-base interactions and the importance of equilibrium constants in predicting the extent of a reaction.

Aluminum Hydroxide

Aluminum hydroxide, \( \text{Al(OH)_3} \), exemplifies the interesting property of amphoteric behavior, which means it can act both as an acid and as a base. Its amphoteric nature allows it to react with both acidic and basic substances.

When aluminum hydroxide encounters a strong acid like hydrochloric acid (HCl), it behaves as a base, accepting protons and forming aluminum chloride (AlCl3) and water. On the other hand, if aluminum hydroxide is mixed with a strong base like sodium hydroxide (NaOH), it demonstrates its acidic characteristics by donating protons, resulting in the formation of sodium aluminate (NaAlO2) and water.

Understanding the amphoteric behavior of aluminum hydroxide not only solidifies the concept of acid-base chemistry but also illustrates the versatile reactions such substances can undergo. It's an excellent example of how chemical species can exhibit dual characteristics depending on their reaction environment, making it an essential study in chemistry education.