Question

\(\mathrm{Zn}(\mathrm{OH})_{2}\) is an amphoteric hydroxide. Write balanced ionic equations to show its reaction with (a) \(\mathrm{HCl}\), (b) \(\mathrm{NaOH}\) [the product is \(\left.\mathrm{Zn}(\mathrm{OH})_{4}^{2-}\right]\).

Short answer

Zn(OH)₂ reacts with HCl to form Zn²⁺ and water; with NaOH to form Zn(OH)₄²⁻.

Step-by-step solution

Understanding Amphoteric Behavior

Zinc hydroxide, \( \mathrm{Zn(OH)}_2 \), is amphoteric, meaning it can react with both acids and bases. This property will guide us in writing the balanced ionic equations.

Reaction (a): Zn(OH)₂ with HCl

When \( \mathrm{Zn(OH)}_2 \) reacts with hydrochloric acid (\( \mathrm{HCl} \)), being acidic, the reaction involves \( \mathrm{Zn(OH)}_2 \) dissolving to form a soluble zinc chloride salt and water. The balanced ionic equation is: \[ \mathrm{Zn(OH)}_2(s) + 2 \mathrm{H^+(aq)} \rightarrow \mathrm{Zn^{2+}(aq)} + 2 \mathrm{H_2O(l)} \] This shows the zinc hydroxide hydrogens being replaced by the protons from HCl.

Reaction (b): Zn(OH)₂ with NaOH

In a reaction with a strong base like \( \mathrm{NaOH} \), \( \mathrm{Zn(OH)}_2 \) reacts to form \( \mathrm{Zn(OH)}_4^{2-} \). The balanced ionic equation is: \[ \mathrm{Zn(OH)}_2(s) + 2 \mathrm{OH^- (aq)} \rightarrow \mathrm{Zn(OH)}_4^{2-} (aq) \] Here, \( \mathrm{Zn(OH)}_2 \) acts as an acid in the presence of a strong base, accepting two additional hydroxide ions.

Key concepts

Ionic Equations

Ionic equations are crucial in understanding reactions at the molecular level. They highlight only the particles participating directly in a chemical reaction. This means we exclude the "spectator ions" that do not undergo any change.
For example, in the case of zinc hydroxide, when it reacts with hydrochloric acid or sodium hydroxide, we focus solely on zinc-related ions and the changes they undergo.
Ionic equations not only simplify complex reactions but also help students visualize the transformation of substances during chemical changes. They are particularly handy in showcasing amphoteric behavior, as seen with zinc hydroxide, which can act as both an acid and a base.

Zinc Hydroxide

Zinc hydroxide, written as \( \mathrm{Zn(OH)}_2 \), is an important compound due to its amphoteric nature. This means it can react with both acids and bases, highlighting its dual character.
As a solid compound, zinc hydroxide doesn't dissolve readily in water. However, its interactions with strong acids like HCl or strong bases like NaOH enable it to form soluble ions.

• In acidic solutions, it acts like a base, accepting protons and dissolving.
• In basic solutions, it acts as an acid, accepting hydroxide ions and forming a complex ion.

This characteristic is not just fascinating but also useful in various industrial and laboratory processes.

Acid-Base Reactions

Acid-base reactions are essential in chemistry. They involve the transfer of protons (in the Bronsted-Lowry sense) or the formation of water (in the Arrhenius sense). Amphoteric substances like zinc hydroxide can participate in these reactions in unique ways.
When zinc hydroxide reacts with hydrochloric acid, the transfer of protons leads to the formation of zinc ions in solution and water as a product. Conversely, when reacting with sodium hydroxide, zinc hydroxide accepts hydroxide ions, showcasing its capacity to behave like an acid in a basic medium.

Hydrochloric Acid

Hydrochloric acid (HCl) is a strong acid commonly used in laboratory and industrial settings. It is known for its ability to provide protons or hydrogen ions (\( \mathrm{H^+} \)) when dissolved in water. This property makes it an ideal candidate for reacting with substances like zinc hydroxide.
During the reaction with zinc hydroxide, HCl provides protons that convert \( \mathrm{Zn(OH)}_2 \) into dissolved zinc ions, creating water in the process. This reaction illustrates how HCl effectively neutralizes basic compounds, highlighting its role as a strong acid.

Sodium Hydroxide

Sodium hydroxide (NaOH) is a strong base that is widely used in various chemical reactions. It readily dissociates in water, releasing \( \mathrm{OH^-} \) ions. These ions can react with amphoteric substances.
In the reaction with zinc hydroxide, sodium hydroxide causes \( \mathrm{Zn(OH)}_2 \) to accept hydroxide ions, resulting in the formation of a soluble complex, \( \mathrm{Zn(OH)}_4^{2-} \). This illustrates the strong base's ability to alter the nature of a compound through the acceptance of additional hydroxide ions.
By understanding the role of NaOH in these reactions, students gain insights into the behavior of bases towards compounds with amphoteric characteristics.