Problem 14 - Chemistry in 30 days

Question

When the vapours of chromyl chloride are passed through \(\mathrm{NaOH}\) solution, it turns yellow. This is due to formation of (a) \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (b) \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) (c) \(\mathrm{CrO}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{CrO}_{4}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{Na}_2 \mathrm{CrO}_4\)

Step 1: Understanding the Chemical Reaction

Chromyl chloride, \( ext{CrO}_2Cl_2\), when reacted with a sodium hydroxide solution, \( ext{NaOH}\), undergoes a chemical transformation. We need to identify which product is responsible for the yellow color.

Step 2: Chemical Equation Setup

The reaction of chromyl chloride with sodium hydroxide can be represented by the chemical equation: \[ \text{CrO}_2Cl_2 + 4\text{NaOH} \rightarrow \text{Na}_2\text{CrO}_4 + 2\text{NaCl} + 2\text{H}_2\text{O} \] This equation shows the complete reaction process.

Step 3: Identify the Yellow Product

From the above equation, the formation of \( ext{Na}_2 ext{CrO}_4\) is responsible for the yellow color. Sodium chromate, \( ext{Na}_2 ext{CrO}_4\), is known for its distinct yellow hue.

Step 4: Conclusion

Based on the process we've identified, the yellow color when chromyl chloride is passed through \( ext{NaOH}\) solution is due to \( ext{Na}_2 ext{CrO}_4\). Thus, the correct answer is (b) \( ext{Na}_2 ext{CrO}_4\).

Key Concepts

Chromyl Chloride Reaction

Chromyl chloride, represented chemically as \(\mathrm{CrO}_2\mathrm{Cl}_2\), is a compound that interacts vividly with various reagents. When vaporized chromyl chloride encounters a solution of sodium hydroxide, \(\mathrm{NaOH}\), an important reaction takes place. The reaction is a notable example in inorganic chemistry due to the distinct outcomes it produces. In this particular chemical reaction, chromyl chloride transforms under alkaline conditions provided by sodium hydroxide, leading to a complete rearrangement of its chemical structure. These reactions are crucial for understanding the behavior of transition metal compounds like chromium when exposed to different chemical environments. Such reactions are often studied in the context of inorganic synthesis and analytical chemistry.

Sodium Chromate Formation

Sodium chromate, \(\mathrm{Na}_2\mathrm{CrO}_4\), is the key product formed when chromyl chloride reacts with sodium hydroxide. This compound is notable for its vibrant yellow color, characteristic of chromate ions. The chemical reaction can be expressed with the equation:\[\mathrm{CrO}_2\mathrm{Cl}_2 + 4\mathrm{NaOH} \rightarrow \mathrm{Na}_2\mathrm{CrO}_4 + 2\mathrm{NaCl} + 2\mathrm{H}_2\mathrm{O}\]

• The reaction proceeds with the substitution of chloride ions in chromyl chloride with hydroxide ions from sodium hydroxide.
• This results in the formation of sodium chromate, which is soluble in water and gives the solution a yellow appearance.

Sodium chromate is not only important for this bright color indication, but it also plays a significant role in various industrial applications, including dyes and pigments.

Inorganic Chemistry

Inorganic chemistry encompasses the study of inorganic compounds, which includes metals, minerals, and organometallic compounds. The reaction between chromyl chloride and sodium hydroxide is a classic example within this field. Here, the focus is often on understanding and manipulating the chemistry of non-organic molecules and their transformations. Such reactions are vital to explore because they demonstrate the reactive nature of compounds, often forming the basis for various analytical techniques employed in labs. Complex interactions like these facilitate the synthesis of useful compounds like sodium chromate and extend our knowledge of reaction mechanisms and product formation.

Color Change Identification

Color change is a significant indicator in many chemical reactions, serving as a visual cue for the completion of a reaction or the formation of new products. In this scenario, the color change from clear or white in the initial state to a vivid yellow signifies the formation of sodium chromate. This yellow color arises due to the presence of chromate ions, \(\mathrm{CrO}_4^{2-}\), known for their distinct hue. Color changes are invaluable in educational settings as they allow students to witness chemical processes visually, aiding in the understanding of theoretical concepts. Moreover, colorimetric changes are employed as qualitative analytical methods to indicate the presence of particular ions or compounds in a solution.