Problem 77 - Chemistry The Molecular Science

Question

Write chemical equations to illustrate the amphoteric behavior of (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\). (b) \(\mathrm{Sb}(\mathrm{OH})_{3}\)

Step-by-Step Solution

Answer

Zn(OH)₂ and Sb(OH)₃ can both act as acids and bases, forming complex ions and salts.

Step 1: Understanding Amphoteric Behavior

In chemistry, an amphoteric substance can react both as an acid and as a base. We need to write two reactions for each compound: one showing it acting as an acid and the other as a base.

Step 2: Zn(OH)₂ as an Acid

When zinc hydroxide, \( \mathrm{Zn(OH)}_2 \), acts as an acid, it can react with a strong base, such as \( \mathrm{NaOH} \), to form a complex ion. The balanced chemical equation is: \[ \mathrm{Zn(OH)}_2 + 2 \mathrm{OH}^- \rightarrow \mathrm{[Zn(OH)}_4]^{2-} \]

Step 3: Zn(OH)₂ as a Base

When \( \mathrm{Zn(OH)}_2 \) acts as a base, it can react with a strong acid, such as \( \mathrm{HCl} \), to form zinc chloride and water. The balanced chemical equation is: \[ \mathrm{Zn(OH)}_2 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + 2\mathrm{H}_2\mathrm{O} \]

Step 4: Sb(OH)₃ as an Acid

Antimony(III) hydroxide, \( \mathrm{Sb(OH)}_3 \), acts as an acid by reacting with a strong base. It can dissolve in excess \( \mathrm{NaOH} \) to form \( \mathrm{Na}_3\mathrm{SbO}_3 \). The balanced equation is: \[ \mathrm{Sb(OH)}_3 + 3 \mathrm{NaOH} \rightarrow \mathrm{Na}_3\mathrm{SbO}_3 + 3\mathrm{H}_2\mathrm{O} \]

Step 5: Sb(OH)₃ as a Base

When \( \mathrm{Sb(OH)}_3 \) acts as a base, it reacts with acids like \( \mathrm{HCl} \) to form \( \mathrm{SbCl}_3 \) and water. The balanced equation is: \[ \mathrm{Sb(OH)}_3 + 3 \mathrm{HCl} \rightarrow \mathrm{SbCl}_3 + 3\mathrm{H}_2\mathrm{O} \]

Key Concepts

Zinc Hydroxide

Zinc hydroxide, represented chemically as \( \mathrm{Zn(OH)}_2 \), is a fascinating compound due to its amphoteric nature. This means it can behave both as an acid and a base. Let's dive into its behavior in both scenarios.
When \( \mathrm{Zn(OH)}_2 \) acts as an acid, it typically reacts with strong bases. For instance, when it meets sodium hydroxide \( \mathrm{NaOH} \), \( \mathrm{Zn(OH)}_2 \) forms a complex ion through the reaction:

• \( \mathrm{Zn(OH)}_2 + 2 \mathrm{OH}^- \rightarrow \mathrm{[Zn(OH)}_4]^{2-} \)

In this reaction, \( \mathrm{Zn(OH)}_2 \) effectively donates a proton to the \( \mathrm{OH}^- \) ions, forming a tetrahydroxozincate(II) ion.
Conversely, when \( \mathrm{Zn(OH)}_2 \) functions as a base, it reacts with acids like hydrochloric acid \( \mathrm{HCl} \):

• \( \mathrm{Zn(OH)}_2 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + 2\mathrm{H}_2\mathrm{O} \)

Here, the zinc hydroxide accepts protons from the acidic \( \mathrm{HCl} \), turning into \( \mathrm{ZnCl}_2 \) and releasing water. This duality in behavior highlights its versatility in reactions.

Antimony Hydroxide

Antimony(III) hydroxide, with the chemical formula \( \mathrm{Sb(OH)}_3 \), is another great example of an amphoteric compound. It exhibits dual behavior, manifesting as either an acid or base under different conditions.
In its acidic role, antimony hydroxide reacts with strong bases. When it encounters sodium hydroxide \( \mathrm{NaOH} \), the reaction is:

• \( \mathrm{Sb(OH)}_3 + 3 \mathrm{NaOH} \rightarrow \mathrm{Na}_3\mathrm{SbO}_3 + 3\mathrm{H}_2\mathrm{O} \)

In this process, \( \mathrm{Sb(OH)}_3 \) acts as an acid, providing hydroxide ions to combine with \( \mathrm{NaOH} \), leading to the formation of sodium antimonate \( \mathrm{Na}_3\mathrm{SbO}_3 \).
Similarly, when \( \mathrm{Sb(OH)}_3 \) is exposed to an acid like hydrochloric acid \( \mathrm{HCl} \), it takes on the role of a base:

• \( \mathrm{Sb(OH)}_3 + 3 \mathrm{HCl} \rightarrow \mathrm{SbCl}_3 + 3\mathrm{H}_2\mathrm{O} \)

Here, \( \mathrm{Sb(OH)}_3 \) accepts protons from \( \mathrm{HCl} \), resulting in the formation of \( \mathrm{SbCl}_3 \). This amphoteric nature is essential in various industrial applications of antimony compounds.

Acid-Base Reactions

Acid-base reactions are at the heart of understanding the behavior of amphoteric compounds like zinc hydroxide and antimony hydroxide. These reactions involve the transfer of protons between reactants, showcasing how substances can swap roles between acids and bases.
An acid is a substance that can donate protons, while a base is one that accepts them. Amphoteric compounds, due to their balanced chemical characteristics, can function as both, reacting with both acids and bases.
Understanding these reactions:

• When acting with acids, amphoteric substances take in protons to neutralize the acid, often resulting in water and a salt.
• When interacting with bases, they expel hydroxide ions, forming complex reactions that produce different ions or salts.

These versatile reactions are pivotal in chemistry, enabling countless processes in biological systems and industrial manufacturing. By studying amphoteric behavior, chemists harness the unique capabilities of compounds like \( \mathrm{Zn(OH)}_2 \) and \( \mathrm{Sb(OH)}_3 \) for innovative solutions.