Question

Both \(\mathrm{Ag}^{+}\) and \(\mathrm{Zn}^{2+}\) form complex ions with \(\mathrm{NH}_{3}\). Write balanced equations for the reactions. However, \(\mathrm{Zn}(\mathrm{OH})_{2}\) is soluble in \(6 \mathrm{M} \mathrm{NaOH},\) and \(\mathrm{AgOH}\) is not. Explain.

Short answer

\(\mathrm{Ag}^{+}\) forms \([\mathrm{Ag}(\mathrm{NH}_{3})_2]^{+}\), \(\mathrm{Zn}^{2+}\) forms \([\mathrm{Zn}(\mathrm{NH}_{3})_4]^{2+}\), \(\mathrm{Zn(OH)}_2\) is amphoteric and dissolves in NaOH, \(\mathrm{AgOH}\) is not.

Step-by-step solution

Write Ag+ and NH3 Complex Equation

Silver ion, \(\mathrm{Ag}^{+}\), reacts with ammonia, \(\mathrm{NH}_{3}\), to form the diamminesilver complex ion. The balanced equation for this reaction is:\[ \mathrm{Ag}^{+} + 2\ \mathrm{NH}_{3} \rightarrow [\mathrm{Ag}(\mathrm{NH}_{3})_2]^{+} \]

Write Zn2+ and NH3 Complex Equation

Zinc ion, \(\mathrm{Zn}^{2+}\), reacts with ammonia to form the complex ion tetraamminezinc. The balanced equation is:\[ \mathrm{Zn}^{2+} + 4\ \mathrm{NH}_{3} \rightarrow [\mathrm{Zn}(\mathrm{NH}_{3})_4]^{2+} \]

Zn(OH)2 Solubility in NaOH

Zinc hydroxide, \(\mathrm{Zn(OH)}_2\), is amphoteric and dissolves in both acids and bases. In \(6\ \mathrm{M}\ \mathrm{NaOH}\), it further reacts to form the soluble zincate ion:\[ \mathrm{Zn(OH)}_2 + 2\ \mathrm{OH}^{-} \rightarrow [\mathrm{Zn(OH)}_4]^{2-} \]

AgOH Insolubility in NaOH

Silver hydroxide, \(\mathrm{AgOH}\), is not amphoteric and does not form complex ions with hydroxide. It remains insoluble under basic conditions and precipitates out as \(\mathrm{AgOH}\).

Conclusion

The solubility difference is because \(\mathrm{Zn(OH)}_2\) is amphoteric and can form soluble complexes with hydroxide ions, whereas \(\mathrm{AgOH}\), lacking amphoteric behavior, remains insoluble.

Key concepts

silver-ammonia complex

Silver ions can interact with ammonia in an interesting way. When \(\mathrm{Ag}^{+}\) ions are mixed with ammonia \(\mathrm{NH}_{3}\), they form a compound known as a complex ion. Specifically, this creates the diamminesilver complex ion. This transformation can be expressed in the balanced chemical equation: \[ \mathrm{Ag}^{+} + 2\, \mathrm{NH}_{3} \rightarrow [\mathrm{Ag}(\mathrm{NH}_{3})_2]^{+} \]Here's how it works:

• Formation: Two ammonia molecules link with one silver ion.
• Stability: This complex ion is quite stable in solution.

Understanding the behavior of silver ions with ammonia is crucial, especially in understanding solubility and reactions in different chemical environments.

zinc-ammonia complex

Similar to silver, zinc ions \(\mathrm{Zn}^{2+}\) can also react with ammonia. However, zinc forms a different kind of complex ion. The reaction produces the tetraamminezinc complex, as shown by the equation:\[\mathrm{Zn}^{2+} + 4\, \mathrm{NH}_{3} \rightarrow [\mathrm{Zn}(\mathrm{NH}_{3})_4]^{2+}\]Key aspects of this reaction:

• Formation: Four ammonia molecules coordinate around the zinc ion.
• Solubility: These complex ions are soluble in water, impacting the overall solubility of zinc compounds.

Zinc's ability to form complex ions helps to explain its different reactivity patterns, especially when comparing with other metals like silver.

amphoterism

Amphoterism is a unique characteristic of certain compounds and metals, like zinc hydroxide. An amphoteric substance can react with both acids and bases. This is a key idea to understand how zinc compounds behave in different environments.

Why is Zinc Hydroxide Amphoteric?

• Dual reactivity: Zinc hydroxide can dissolve in acids to form zinc salts, and in strong bases like \(6\ \mathrm{M}\ \mathrm{NaOH}\) to form zincate ions.
• Chemical reaction: In the presence of bases, zinc hydroxide converts to zincate, as shown:\[ \mathrm{Zn(OH)}_2 + 2\, \mathrm{OH}^{-} \rightarrow [\mathrm{Zn(OH)}_4]^{2-} \]

This property has wide-ranging implications for zinc's chemical applications and highlights a big difference compared to other non-amphoteric compounds.

solubility

Solubility refers to the ability of a substance to dissolve in a solvent, typically water for most chemistry problems. It's fascinating because solubility can vastly change based on chemical properties, like amphoterism.

Comparing Zinc and Silver Hydroxide

• Zinc Hydroxide: Thanks to its amphoteric nature, it readily dissolves in basic solutions, forming soluble zincate ions.
• Silver Hydroxide: Lacks amphoterism and won't dissolve in a strong base, staying as a precipitate.

Understanding solubility helps predict the behavior of various substances in different chemical conditions, and can even influence how they're used in industrial and laboratory settings.

balanced chemical equations

A balanced chemical equation is crucial for accurately representing a chemical reaction. It ensures that the law of conservation of mass is respected, meaning the same number of atoms are seen on each side of the equation.

Importance in Chemistry

• Predicting Products: Balanced equations help determine the products formed in a reaction.
• Quantification: They allow chemists to calculate how much of each substance is involved.

For instance, knowing the balance in the formation of complex ions helps understand reaction mechanisms and possible applications thereof.