Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

Zn reacts with concentration \(\mathrm{NaOH}\) solution to produce (a) \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{H}_{2}\) (b) \(\mathrm{ZnO}_{2}^{2-}\) only (c) \(\mathrm{ZnO}_{4}^{2-}\) and \(\mathrm{H}_{2}^{2}\) (d) \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

Option (a): \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{H}_{2}\) are produced.

Step 1: Understand the Reaction

Zinc (Zn) reacts with a concentrated sodium hydroxide (NaOH) solution. This is an example of a reaction between a metal and a strong base, where zinc acts as an amphoteric metal, meaning it can react with both acids and bases to form different compounds.

Step 2: Write the Balanced Equation

The reaction between zinc and concentrated NaOH yields zincate (\(\mathrm{ZnO}_{2}^{2-}\)), which is a complex ion, and hydrogen gas (\(\mathrm{H}_{2}\)). The balanced chemical equation is: \[ \mathrm{Zn} + 2\mathrm{NaOH} + 2\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Na}_{2}\mathrm{ZnO}_{2} + 3\mathrm{H}_{2} \] Here, \(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\) dissociates into \(\mathrm{2Na}^{+}\) and \(\mathrm{ZnO}_{2}^{2-}\) in the solution.

Step 3: Analyze the Products

From the balanced equation, the products of the reaction are sodium zincate (\(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\)) and hydrogen gas (\(\mathrm{H}_{2}\)). The presence of \(\mathrm{ZnO}_{2}^{2-}\) in the zincate indicates that \(\mathrm{ZnO}_{2}^{2-}\) is indeed produced in the reaction.

Step 4: Decide the Correct Option

Since the reaction produces \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{H}_{2}\), the correct answer is option (a). This matches with the products listed: \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{H}_{2}\).

Key Concepts

Zinc Reaction with NaOH

Zinc is an amphoteric metal, which means it can interact with both acids and bases. When zinc reacts with a concentrated sodium hydroxide (NaOH) solution, it demonstrates this amphoteric nature by combining with the base in an interesting way. In this reaction, zinc reacts with NaOH to form sodium zincate and hydrogen gas.

• Amphoteric metals, like zinc, are capable of undergoing chemical reactions with both acids and bases.
• The interaction of zinc with NaOH illustrates this dual reactive property.

During the reaction, zinc reacts with NaOH and water to eventually form sodium zincate and liberates hydrogen gas. The occurrence of hydrogen gas production indicates the liberation of hydrogen ions from water molecules, further demonstrating the amphoteric behavior.

Balanced Chemical Equations

A balanced chemical equation is essential for understanding a chemical reaction fully. It shows the exact quantities of reactants that react and the products that form. For the reaction between zinc and NaOH, the balanced chemical equation is:\[ \mathrm{Zn} + 2\mathrm{NaOH} + 2\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Na}_{2}\mathrm{ZnO}_{2} + 3\mathrm{H}_{2} \]To balance a chemical equation correctly, one should ensure that there are equal numbers of each type of atom on both sides of the equation.

• Every element's atom count on the left (reactants) should match with that on the right (products).
• This balance maintains the law of conservation of mass, indicating matter can neither be created nor destroyed.

For this equation, there are two Na atoms, two O atoms from the OH groups, and one Zn atom on each side, while hydrogen atoms balance out as three molecules of hydrogen gas on the right.

Complex Ion Formation

Complex ion formation is a process where simple ions combine to form a more complex structure. In the zinc reaction with NaOH, we see the formation of sodium zincate, a type of complex ion.

• A complex ion typically consists of a central metal ion bonded to one or more molecules or ions.
• The metal ion is usually bonded through coordinate covalent bonds with ligands or other ions.

In the case of sodium zincate, \(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\), zinc forms an ion complex with oxygen ions derived from the hydroxides and water molecules. This formation of complex ions allows the reaction to yield stable products that are soluble in water, like the sodium zincate which further dissociates into \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{2Na}^{+} \) ions in solution. Understanding complex ion formation is crucial to grasp how amphoteric metals react in basic solutions.