Understanding the Reaction of Sodium Hydroxide with Zinc

Let's break down the chemical reaction that occurs when sodium hydroxide ($\text{NaOH}$), a strong base, reacts with zinc ($\text{Zn}$), a metal. Zinc is known to be an amphoteric metal, which means it can react with both acids and strong bases.

When zinc reacts with a concentrated solution of sodium hydroxide, a chemical change takes place, resulting in the formation of new substances. This is a redox reaction where zinc is oxidized and water is reduced to form hydrogen gas.

The reaction can be represented by the following balanced chemical equation:

$\text{Zn(s)} + \text{2NaOH(aq)} + \text{2H}_2\text{O(l)} \rightarrow \text{Na}_2[\text{Zn(OH)}_4]\text{(aq)} + \text{H}_2\text{(g)}$

In this equation:

• $\text{Zn}$ is solid zinc metal.
• $\text{NaOH(aq)}$ is aqueous sodium hydroxide solution.
• $\text{H}_2\text{O(l)}$ is liquid water.
• $\text{Na}_2[\text{Zn(OH)}_4]\text{(aq)}$ is aqueous sodium tetrahydroxozincate(II), commonly referred to as sodium zincate. This is a soluble complex salt.
• $\text{H}_2\text{(g)}$ is hydrogen gas, which is released during the reaction.

Alternatively, at higher temperatures or under specific conditions, the reaction can sometimes be represented as:

$\text{Zn(s)} + \text{2NaOH(aq)} \rightarrow \text{Na}_2\text{ZnO}_2\text{(aq)} + \text{H}_2\text{(g)}$

Here, $\text{Na}_2\text{ZnO}_2$ is anhydrous sodium zincate, also a form of sodium zincate.

In both common representations, the products are a form of sodium zincate and hydrogen gas.

Analyzing the Options for Sodium Hydroxide and Zinc Reaction

Let's look at the given options based on our understanding of the reaction:

• Option 1: Zinc hydroxide and hydroxide gas
  • Zinc hydroxide ($\text{Zn(OH)}_2$) is an intermediate precipitate sometimes formed first, but it dissolves in excess sodium hydroxide to form the soluble zincate. "Hydroxide gas" is not a standard chemical term for a product in this reaction. This option is incorrect.
• Option 2: Sodium Zincate and hydrogen gas
  • As shown by the chemical equations, sodium zincate ($\text{Na}_2[\text{Zn(OH)}_4]$ or $\text{Na}_2\text{ZnO}_2$) and hydrogen gas ($\text{H}_2$) are the main products formed when zinc reacts with excess sodium hydroxide. This option correctly identifies both products.
• Option 3: Zinc hydroxide and sodium
  • Zinc hydroxide is an intermediate, not the final product in excess $\text{NaOH}$. Sodium metal ($\text{Na}$) is a very reactive metal and is not produced in this reaction; sodium remains as part of the sodium zincate compound. This option is incorrect.
• Option 4: Sodium Zincate and water
  • Sodium zincate is a product. However, water is a reactant in the formation of the hydrated zincate complex ($\text{Na}_2[\text{Zn(OH)}_4]$). Hydrogen gas, not water, is the gaseous product released. This option is incorrect.

Based on the chemical reaction, the reaction of sodium hydroxide with zinc produces sodium zincate and hydrogen gas.

Revision Table: Key Concepts of Metal-Base Reactions

Additional Information: Amphoteric Nature of Zinc

Zinc's ability to react with both acids and strong bases is a key characteristic of its amphoteric nature. With acids, zinc reacts to form a salt and hydrogen gas (e.g., $\text{Zn} + \text{2HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$). With strong bases like sodium hydroxide, it forms a soluble complex salt, sodium zincate, and also releases hydrogen gas, as discussed above. This behavior distinguishes amphoteric metals from metals that only react with acids.

The reaction of zinc with sodium hydroxide is often used to demonstrate the amphoteric properties of zinc and is a common laboratory preparation method for hydrogen gas using this specific type of metal-base reaction.