Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

A white, water insoluble solid A turns yellow on heating and becomes white on cooling. A gives a clear solution B when treated with dilute \(\mathrm{HCl}\) or \(\mathrm{NaOH}\). When \(\mathrm{H}_{2} \mathrm{~S}\) is passed through solution \(\mathrm{B}\), and made neutral, a white precipitate \(\mathrm{C}\) is formed. Identify A? (a) \(\mathrm{ZnO}\) (b) \(\mathrm{CuO}\) (c) \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{Ag}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Compound A is \(\mathrm{ZnO}\).

Step 1: Analyzing the Heating and Cooling Reaction

The compound A turns yellow on heating and reverts to white upon cooling. This behavior is characteristic of zinc oxide \(\mathrm{ZnO}\). When \(\mathrm{ZnO}\) is heated, it changes from a white powder to a yellow color due to changes in the electronic structure, but it returns to white upon cooling.

Step 2: Solution Formation with \(\mathrm{HCl}\) and \(\mathrm{NaOH}\)

Compound A forms a clear solution B when treated with either dilute \(\mathrm{HCl}\) or \(\mathrm{NaOH}\). Zinc oxide \(\mathrm{ZnO}\) reacts with acids like \(\mathrm{HCl}\) to form \(\mathrm{ZnCl}_2\), a clear solution, and with bases like \(\mathrm{NaOH}\) to form soluble \(\mathrm{Na}_2\mathrm{ZnO}_2\), which is also a clear solution.

Step 3: Precipitate Formation with \(\mathrm{H}_2\mathrm{S}\)

Solution B (containing \(\mathrm{Zn}^{2+}\) ions) forms a white precipitate C when \(\mathrm{H}_2\mathrm{S}\) is bubbled through it and the solution is neutralized. The \(\mathrm{Zn}^{2+}\) ions react with \(\mathrm{S}^{2-}\) ions from \(\mathrm{H}_2\mathrm{S}\) under neutral conditions to form \(\mathrm{ZnS}\), which is a white precipitate.

Key Concepts

Chemical Properties

Zinc oxide, with the chemical formula \(\mathrm{ZnO}\), is an important compound in both industrial and laboratory settings. One of its distinguishing features is its color change upon heating. Normally, \(\mathrm{ZnO}\) appears as a white powder. When heated, it transitions to a yellow color. This change occurs due to alterations in its electronic structure, specifically the increase in oxygen vacancies at high temperatures, which affect its light absorption properties. However, when cooled back down, \(\mathrm{ZnO}\) reverts to its original white color.

• Color change: White to yellow on heating
• Insulating and insoluble in water

Another intrinsic property of zinc oxide is its insolubility in water, yet it reacts readily with acids and bases. This makes it a key amphoteric oxide, meaning it can react with both acids and bases to yield different products.
This dual reactive behavior plays a significant role in its application in various chemical reactions.

Zinc Sulfide Formation

Zinc sulfide, \(\mathrm{ZnS}\), is formed through a reaction where zinc ions \(\mathrm{Zn}^{2+}\) combine with sulfide ions \(\mathrm{S}^{2-}\). This happens when hydrogen sulfide gas \(\mathrm{H}_2\mathrm{S}\) is passed through a solution containing zinc ions. Under neutral conditions, the ions react to form a white precipitate of zinc sulfide.

• Zinc ions \(\mathrm{Zn}^{2+}\) originate from \(\mathrm{ZnO}\) dissolved in dilute \(\mathrm{HCl}\) or \(\mathrm{NaOH}\).
• Sulfide ions \(\mathrm{S}^{2-}\) are provided by \(\mathrm{H}_2\mathrm{S}\) gas.

This chemical transformation is useful for understanding precipitation reactions, where two solutes create an insoluble product. The resulting white precipitate serves as a visual confirmation of the chemical reaction.

Reaction with Acids and Bases

Zinc oxide's ability to react with acids and bases showcases its amphoteric nature. When zinc oxide is treated with an acid, such as dilute hydrochloric acid \(\mathrm{HCl}\), it reacts to form zinc chloride \(\mathrm{ZnCl}_2\), a soluble compound. The equation for this reaction is:\[ \mathrm{ZnO} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{O} \]In this process, the acidic protons interact with the oxide ions, neutralizing them and forming water.
Conversely, when \(\mathrm{ZnO}\) is treated with a base like sodium hydroxide \(\mathrm{NaOH}\), it forms sodium zincate \(\mathrm{Na}_2\mathrm{ZnO}_2\), another soluble compound:\[ \mathrm{ZnO} + 2\mathrm{NaOH} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Na}_2\mathrm{ZnO}_2 + 2\mathrm{H}_2\mathrm{O} \]

• Zinc oxide reacts with acids forming soluble salts.
• With bases, it forms complex soluble zincate ions.

This behavior reflects the dual ability of amphoteric substances to interact with both acidic and basic solutions, thus forming clear products in each case.