Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds on reaction with \(\mathrm{NaOH}\) and \(\mathrm{Na}_{2} \mathrm{O}_{2}\) gives yellow colour? (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (b) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) none of these

Step-by-Step Solution

Answer

(b) \( \mathrm{Cr(OH)_{3}} \) gives a yellow color.

Step 1: Understanding the Reaction Conditions

We are tasked to find out which compound turns yellow on reaction with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). These reagents create a strong oxidizing environment, favoring the formation of colored compounds.

Step 2: Considering Chromate Formation

\( \mathrm{Cr} \) from \( \mathrm{Cr(OH)_{3}} \) is a transition metal capable of forming complex oxides that have distinctive colors. Reacting \( \mathrm{Cr(OH)_{3}} \) with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \) should yield \( \mathrm{Na_2CrO_4} \), which is known for its yellow color.

Step 3: Analyzing Other Compounds

\( \mathrm{Zn(OH)_{2}} \) and \( \mathrm{Al(OH)_{3}} \) do not form colored compounds when exposed to \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). \( \mathrm{Zn} \) forms colorless \( \mathrm{ZnO_2^{2-}} \) and \( \mathrm{Al} \) forms colorless \( \mathrm{NaAlO_2} \).

Step 4: Concluding the Yellow Compound

Given the reaction conditions and analysis of possible products, \( \mathrm{Cr(OH)_3} \) is expected to produce a yellow solution due to the formation of \( \mathrm{Na_2CrO_4} \) when reacted with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \).

Key Concepts

Oxidizing Environment

An oxidizing environment is a chemical setting where substances accept electrons from reactions, generally causing an increase in the oxidation state of the atoms involved. In many chemical reactions, such conditions facilitate the transformation of compounds and are crucial in oxidation-reduction (redox) processes.

• In our specific scenario, reacting compounds with sodium hydroxide \( \mathrm{NaOH} \) and sodium peroxide \( \mathrm{Na_2O_2} \) creates this strong oxidizing environment.
• These substances will promote the change necessary for certain compounds to exhibit distinct color changes.
• This is particularly evident in chromate formation from chromic hydroxide, \( \mathrm{Cr(OH)_3} \), leading to the formation of \( \mathrm{Na_2CrO_4} \), which is yellow.

Introducing such an environment can have various effects on different compounds depending on their ability to undergo oxidation. The ability to predict these reactions requires a good grasp of both the nature of the compounds in question and the conditions in which they are reacted.

Transition Metal Chemistry

Transition metals are elements that possess partially filled d-electron subshells, granting them unique properties. These properties include the ability to form complex ions and exhibit various oxidation states.

• Their electron configurations enable them to partake in electron transitions that are often associated with colored compounds.
• Chromium \( \mathrm{Cr} \), found in \( \mathrm{Cr(OH)_3} \), is a classic example of a transition metal capable of forming complex ions, such as chromate \( \mathrm{CrO_4^{2-}} \), in oxidizing environments.
• This is made possible by chromium's ability to reach a higher oxidation state, such as \( +6 \) when forming \( \mathrm{CrO_4^{2-}} \).

Transition metal chemistry often involves a variety of reactions, some of which are central to industrial chemical processes. Their ability to display vibrant colors makes them valuable in dyes and pigments.

Complex Oxides

Complex oxides are compounds that contain more than one type of metal ion. Their formation often results in compounds with rich structural diversity and wide-ranging chemical properties. One prime example is chromate \( \mathrm{CrO_4^{2-}} \).

• When \( \mathrm{Cr(OH)_3} \) is treated under a strong oxidizing condition with \( \mathrm{NaOH} \) and \( \mathrm{Na_2O_2} \), it reacts to form chromate, a complex oxide recognized by its striking yellow color.
• These oxides are important in contexts like catalysis, electronic materials, and corrosion resistance.
• They are characterized by the bridging of oxygen atoms between metal centers, often leading to interesting magnetic and electrical properties.

Understanding complex oxides is crucial for fields such as material science and chemistry, where manipulating their unique properties opens up pathways for innovation in various technological applications.