Question

Aluminum hydroxide is an amphoteric substance. It can act as either a Brönsted-Lowry base or a Lewis acid. Write a reaction showing Al(OH) \(_{3}\) acting as a base toward \(\mathrm{H}^{+}\) and as an acid toward OH \(^{-}\)

Short answer

As a Brönsted-Lowry base, aluminum hydroxide, Al(OH)\(_{3}\), accepts a proton (H\(^{+}\)) in the reaction: \[ Al(OH)_{3} + H^{+} \to Al(OH)_{4}^{-} \] As a Lewis acid, aluminum hydroxide, Al(OH)\(_{3}\), accepts an electron pair from the OH\(^{-}\) ions in the reaction: \[ Al(OH)_{3} + OH^{-} \to Al(OH)_{4}^{-} \]

Step-by-step solution

Brönsted-Lowry base

A Brönsted-Lowry base is a substance that can accept a proton (H\(^{+}\)). In this reaction, aluminum hydroxide, Al(OH)\(_{3}\) will act as a base by accepting a proton from the H\(^{+}\) ions. As a result, there will be an increase in hydroxide ions (OH\(^{-}\)) in the molecule. Reaction: Al(OH)\(_{3}\) + H\(^{+}\) → Al(OH)\(_{4}^{-}\)

Lewis acid

A Lewis acid is a substance that can accept an electron pair. In this reaction, aluminum hydroxide, Al(OH)\(_{3}\) will act as an acid by accepting an electron pair from the OH\(^{-}\) ions. As a result, there will be a change in the molecule's structure and charge distribution. Reaction: Al(OH)\(_{3}\) + OH\(^{-}\) → Al(OH)\(_{4}^{-}\)

Key concepts

Brönsted-Lowry base

A Brönsted-Lowry base is a substance that can accept a proton, which is symbolized as H\(^{+}\). Aluminum hydroxide (Al(OH)\(_{3}\)) is an excellent example of this kind of base. To understand why, consider that a Brönsted-Lowry base typically has one or more lone pairs of electrons. These lone pairs are essential because they make it possible for the substance to "receive" or "bond with" a proton.

When Al(OH)\(_{3}\) interacts with an H\(^{+}\) ion, it acts as a Brönsted-Lowry base. The reaction that exemplifies this concept is:
Al(OH)\(_{3}\) + H\(^{+}\) → Al(OH)\(_{4}^{-}\).
Here, Al(OH)\(_{3}\) accepts a proton, leading to the formation of Al(OH)\(_{4}^{-}\). This demonstrates aluminum hydroxide's ability to act as a Brönsted-Lowry base by gaining a hydrogen ion.

Lewis acid

In contrast to a Brönsted-Lowry base, a Lewis acid is defined as a substance that can accept a pair of electrons. This definition revolves around the transfer of electrons rather than protons. Aluminum hydroxide can also act as a Lewis acid, showing its versatile, amphoteric nature.

When looking at Al(OH)\(_{3}\) in this capacity, it is important to note that it can accept an electron pair from an OH\(^{-}\) ion. This kind of reaction demonstrates how aluminum hydroxide behaves as an acid under Lewis's theory:
Al(OH)\(_{3}\) + OH\(^{-}\) → Al(OH)\(^{4-}\).
In this scenario, Al(OH)\(_{3}\) is "accepting" electrons, signifying its role as a Lewis acid. So, even the same substance can have different roles depending on whether it's interacting with protons or electrons.

Aluminum Hydroxide

Aluminum hydroxide, represented as Al(OH)\(_{3}\), is an amphoteric substance. This means it has the unique ability to behave as both an acid and a base, depending on the circumstances. Its dual characteristics make it a versatile compound in various chemical reactions.

Some key properties of aluminum hydroxide include:

• Amphoteric behavior: Can act as either a Brönsted-Lowry base or a Lewis acid, depending on the reacting agents.
• Chemical formation: Composed of aluminum, hydrogen, and oxygen in a balanced proportion.
• Reactivity: Can participate in reactions involving protons and electron pairs, showcasing its ability to adapt to different roles.

By understanding how it functions in different scenarios, we can see the broader implications of amphoteric substances like aluminum hydroxide in chemistry.
These complexities illustrate why substances like aluminum hydroxide are so exciting for chemists to study—they adapt and interact in multiple, interesting ways!