Problem 140 - Chemistry An Atoms First Approach

Question

Zinc hydroxide is an amphoteric substance. Write equations that describe \(\mathrm{Zn}(\mathrm{OH})_{2}\) acting as a Brönsted-Lowry base toward \(\mathrm{H}^{+}\) and as a Lewis acid toward \(\mathrm{OH}^{-}\).

Step-by-Step Solution

Answer

When Zn(OH)₂ acts as a Brönsted-Lowry base, it accepts a proton (H⁺) and forms a complex ion, as shown in the equation: \(Zn(OH)_{2} + H^{+} \rightarrow Zn(OH)(H_{2}O)^{2+}\). When Zn(OH)₂ acts as a Lewis acid, it accepts an electron pair from the OH⁻ ion, forming Zn(OH)₃⁻, with the equation: \(Zn(OH)_{2} + OH^{-} \rightarrow Zn(OH)_{3}^{-}\).

Step 1: Zn(OH)₂ as a Brönsted-Lowry base

When Zn(OH)₂ acts as a Brönsted-Lowry base, it will accept a proton (H⁺) from an acid. To write the equation for this reaction, we can start by writing the reactants on the left side and the products on the right side. In this case, the reactants are Zn(OH)₂ and a source of H⁺, which we can represent as H⁺. Zn(OH)₂ + H⁺ → After accepting the proton, the hydroxide ion (OH⁻) in Zn(OH)₂ is replaced by a water molecule (H₂O), resulting in the formation of Zn(OH)H₂O or Zn(OH)(H₂O). This product is a complex ion, so we can write the equation as: Zn(OH)₂ + H⁺ → Zn(OH)(H₂O)²⁺

Step 2: Zn(OH)₂ as a Lewis acid

When Zn(OH)₂ acts as a Lewis acid, it will accept an electron pair, which in this case is donated by an OH⁻ ion. To write the equation for this reaction, we can again start by writing the reactants on the left side and the products on the right side. In this case, the reactants are Zn(OH)₂ and OH⁻. Zn(OH)₂ + OH⁻ → After accepting the electron pair from the OH⁻ ion, the Zn(OH)₂ will incorporate this extra OH⁻ ion, forming Zn(OH)₃⁻. We can now write the final equation as: Zn(OH)₂ + OH⁻ → Zn(OH)₃⁻

Key Concepts

Brönsted-Lowry Base

Brönsted-Lowry theory is one of the foundational concepts in acid-base chemistry. It defines acids as substances that can donate a proton (\( ext{H}^+ \)), while bases are substances that can accept a proton. In this case, zinc hydroxide, \( ext{Zn(OH)}_2 \), plays the role of a Brönsted-Lowry base. This means it acts as a proton acceptor.

When \( ext{Zn(OH)}_2 \) interacts with a proton donor (acid), it accepts a \( ext{H}^+ \) ion. Initially, the zinc hydroxide structure holds hydroxide ions (\( ext{OH}^- \)). When these ions accept a proton, a water molecule is formed in place of each. This transition leads to a new complex, \( ext{Zn(OH)}( ext{H}_2 ext{O})^2^+ \). This exchange demonstrates the versatility of zinc hydroxide to act as a base by intercepting protons during reactions.

Lewis Acid

The Lewis acid-base concept expands the definition beyond protons to include electron pairs. According to Lewis, an acid is any substance that can accept a pair of electrons. This ability reflects zinc hydroxide's role as a Lewis acid.

A notable feature of Lewis acids is their electron pair acceptance capability. In the context of \( ext{Zn(OH)}_2 \), it accepts an electron pair from \( ext{OH}^- \) ions. Unlike the Brönsted-Lowry model, this theory highlights the Lewis acid's role in forming bonds via electron pair acquisition. When \( ext{Zn(OH)}_2 \) absorbs this additional \( ext{OH}^- \), it forms \( ext{Zn(OH)}_3^- \), showcasing a clear path of interaction as a Lewis acid.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, where acids donate protons and bases accept them. Alternatively, acids accepting electron pairs and bases donating them can define these reactions using the Lewis description.

Zinc hydroxide, as an amphoteric substance, illustrates the dual nature allowing it to play roles in these reactions. In the case of Brönsted-Lowry, it acts through proton acceptance, while in Lewis acid-base reactions, it displays acceptance of electron pairs.

Such reactions encourage a balance where substances like \( ext{Zn(OH)}_2 \) flexibly adjust roles based on the reacting partners. Observing these interactions enriches understanding of chemical behavior across different scenarios.

Complex Ions

Complex ions represent molecules formed by central atoms bonded to surrounding ligands via coordinate covalent bonds. In zinc hydroxide’s reactions, the resulting species are complex ions when it either accepts protons or electron pairs.

Acting as a Brönsted-Lowry base with acids, \( ext{Zn(OH)}_2 \) transforms to \( ext{Zn(OH)}( ext{H}_2 ext{O})^2^+ \), a complex ion. Conversely, as a Lewis acid with hydroxide ions, it turns into \( ext{Zn(OH)}_3^- \), which is also a complex ion.

Understanding complex ions includes recognizing the role of coordinate bonds in creating stable structures that allow versatile functions in different chemical environments.