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: Analyze the Reaction

We need to determine which compound reacts with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \) to give a yellow color. This reaction combination is known to test for the presence of chromium compounds as chromium can yield a chromate with a yellow hue.

Step 2: Consider Each Compound

First, \( \mathrm{Zn(OH)}_2 \) and \( \mathrm{Al(OH)}_3 \) do not form yellow products with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). They form different, colorless ions.However, \( \mathrm{Cr(OH)}_3 \) in the presence of \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \) gets oxidized to \( \mathrm{Na}_2\mathrm{CrO}_4 \), a chromate, which is yellow.

Step 3: Determine the Correct Answer

Since chromate ions (\( \mathrm{CrO}_4^{2-} \)) are yellow and are produced from \( \mathrm{Cr(OH)}_3 \) in this chemical reaction, \( \mathrm{Cr(OH)}_3 \) is the compound that gives the yellow color when reacted with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \).

Key Concepts

Chromium Compounds

Chromium compounds are fascinating and versatile in the field of chemistry. They have a range of oxidation states that allow them to participate in varied reactions. Among them, chromium(III) hydroxide, written as \( \mathrm{Cr(OH)}_3 \), is known for its ability to undergo transformations, leading to interesting visual changes.
When we discuss chromium compounds, their distinctive colors in various oxidation states often stand out. For students preparing for Chemistry JEE Main, it's vital to understand chromium's colored compounds, especially chromates and dichromates. These vibrant colors are not only visually striking but serve as important clues in chemical identification.
Here's the key point: chromium's oxidation state changes from +3 in chromium(III) hydroxide to +6 in chromates, which is crucial for its role in certain chemical tests. This shift to a higher oxidation state explains why certain reactions, such as with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \), result in vivid colors.

Oxidation Reactions

Oxidation reactions are a core element of chemical transformations, especially involving metals like chromium. In the case of chromium(III) hydroxide (\( \mathrm{Cr(OH)}_3 \)), when it reacts in the presence of \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \), it gets oxidized.
But what exactly does it mean to be oxidized? In simple terms, during an oxidation reaction, a substance loses electrons. Here, chromium in \( \mathrm{Cr(OH)}_3 \) transitions to a higher oxidation state, forming chromate ions \( \mathrm{CrO}_4^{2-} \). This reaction results in a color change to bright yellow, signifying a shift to the chromium(VI) state.

• Key Takeaway: Oxidation increases the oxidation state of a substance, which can often be identified by a change in color, as seen in this reaction where chromium changes from a +3 to a +6 state, forming chromates.

Understanding these processes is not just about recognizing color changes, but also grasping the electron transfer and transformations taking place.

Chromate Test

The chromate test is a simple yet effective chemical procedure used to detect the presence of chromium in a compound by producing a measurable color change. In educational settings, this is a practical application of oxidation principles.
During this test, you mix the compound with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). If chromium is present, the reaction will produce \( \mathrm{Na}_2\mathrm{CrO}_4 \), a compound recognized for its striking yellow color.

• Why the test works: Chromium in \( \mathrm{Cr(OH)}_3 \) gets oxidized, forming chromate ions.
• Visible indicator: The formation of a yellow hue confirms the presence of chromium.

The beauty of the chromate test lies in its straightforwardness. It's a visual and simple method used not only in labs but also in standardized tests like the Chemistry JEE Main to assess knowledge of chemical properties and reactions.