Question

Explain why zinc hydroxide, \(\mathrm{Zn}(\mathrm{OH})_{2}\), is more soluble in hydrochloric acid than in water.

Short answer

Zinc hydroxide is more soluble in hydrochloric acid than in water due to its reaction with HCl, forming more soluble zinc chloride and water.

Step-by-step solution

Identify the compound

First, identify the compound involved: zinc hydroxide, \(\mathrm{Zn}(\mathrm{OH})_{2}\), which is an amphoteric compound capable of reacting with both acids and bases.

Understand solubility in water

In water, \(\mathrm{Zn}(\mathrm{OH})_{2}\) partially dissociates slightly into zinc ions, \(\mathrm{Zn}^{2+}\), and hydroxide ions, \(\mathrm{OH}^{-}\), due to its low solubility.

Reaction with hydrochloric acid

When \(\mathrm{Zn}(\mathrm{OH})_{2}\) is placed in hydrochloric acid (HCl), it reacts with the acid. The \(\mathrm{H}^{+}\) ions from HCl combine with the \(\mathrm{OH}^{-}\) ions to form water, according to the equation: \[ \mathrm{Zn}(\mathrm{OH})_{2} + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + 2 \mathrm{H}_{2}\mathrm{O} \].

Le Chatelier's principle

Applying Le Chatelier's principle, the removal of \(\mathrm{OH}^{-}\) ions through the formation of water shifts the equilibrium towards the dissolution of more \(\mathrm{Zn}(\mathrm{OH})_{2}\), increasing its solubility.

Conclusion on solubility

Thus, \(\mathrm{Zn}(\mathrm{OH})_{2}\) dissolves more in HCl than in water due to the reaction producing more soluble products and the shift of equilibrium favoring dissolution.

Key concepts

Zinc Hydroxide

Zinc hydroxide, with the chemical formula \( \mathrm{Zn(OH)}_{2} \), is an interesting compound because it is amphoteric. This means it can react with both acids and bases, making it a versatile chemical in various reactions.
In pure water, zinc hydroxide has minimal solubility. It slightly dissociates into zinc ions \( \mathrm{Zn}^{2+} \) and hydroxide ions \( \mathrm{OH}^{-} \). However, because it has low solubility, this dissociation is not complete.
Due to its amphoteric nature, significant changes occur when zinc hydroxide is exposed to different pH conditions. This amphoteric behavior sets the stage for interesting reactions particularly when it interacts with acids like hydrochloric acid.

Le Chatelier's Principle

Le Chatelier's principle is a fundamental concept in chemistry used to predict the effect of changes in concentration, temperature, and pressure on systems at equilibrium.
When applied to zinc hydroxide in hydrochloric acid, Le Chatelier's principle helps us understand why zinc hydroxide becomes more soluble. In the presence of hydrochloric acid, the \( \mathrm{H}^{+} \) ions from the acid react with the \( \mathrm{OH}^{-} \) ions from zinc hydroxide to form water.
According to Le Chatelier's principle, the removal of \( \mathrm{OH}^{-} \) ions from the solution (as they are converted into water) results in the system shifting to counteract this change. It moves the equilibrium towards further dissociation of zinc hydroxide, thus increasing its solubility.

• This principle makes it easier to predict and control chemical reactions in various industrial and laboratory processes.
• In this case, it explains the increased solubility of zinc hydroxide when dissolved in an acidic solution compared to pure water.

Hydrochloric Acid Reaction

The reaction of zinc hydroxide with hydrochloric acid is a classic example of an acid-base reaction. Hydrochloric acid, or HCl, is a strong acid that dissociates completely in water to produce hydrogen ions \( \mathrm{H}^{+} \) and chloride ions \( \mathrm{Cl}^{-} \).
When you introduce HCl to zinc hydroxide, a chemical reaction occurs:
\[ \mathrm{Zn(OH)}_{2} + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + 2 \mathrm{H}_{2}\mathrm{O} \]
This equation reveals that zinc chloride \( \mathrm{ZnCl}_{2} \), a significantly more soluble compound, and water are formed.
This process results in more of the solid zinc hydroxide dissolving because the products of this reaction are much more soluble in aqueous forms.

• Through consuming \( \mathrm{OH}^{-} \) ions, the reaction shifts to dissolve more zinc hydroxide, further demonstrating Le Chatelier's principle.
• This reaction essentially transforms an insoluble compound into soluble ionic forms, thus increasing the overall solubility of zinc hydroxide in an acidic solution compared to pure water.