Problem 22 - Chemistry in 30 days

Question

Which of the following statements is not correct when a mixture of \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is gently warmed with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) A deep red vapour is evolved (b) The vapour when passed into \(\mathrm{NaOH}\) solution gives a yellow solution of \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) (c) Chlorine gas is evolved (d) Chromyl chloride is formed

Step-by-Step Solution

Answer

Statement (c) is not correct; chlorine gas is not evolved.

Step 1: Understanding the Reaction

When a mixture of \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is warmed with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), several reactions occur. The key reaction is the formation of chromyl chloride (\(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\)), which manifests as a deep red vapor. This process involves the reaction: \[ \mathrm{NaCl} + \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CrO}_{2} \mathrm{Cl}_{2} + \mathrm{KHSO}_{4} + \mathrm{HCl} \] which shows that chromyl chloride is definitely formed.

Step 2: Analyzing the Conditions

Assess the by-products formed during the reaction. The major by-products are \(\mathrm{KHSO}_{4}\) and \(\mathrm{HCl}\). \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) is produced as expected, and it confirms the characteristic deep red vapor evolved.

Step 3: Exploring Solution Reactions

When chromyl chloride vapors are passed into \(\mathrm{NaOH}\) solution, the solution reacts to form \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\), a yellow chromate solution: \[ \mathrm{CrO}_{2} \mathrm{Cl}_{2} + \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{CrO}_{4} + \mathrm{NaCl} + \mathrm{H}_{2} \mathrm{O} \] This confirms that the yellow solution is formed.

Step 4: Verifying Chlorine Gas Evolution

There is no production of chlorine gas in the reactions described. Chlorine is not a product of any direct reaction mechanisms between \(\mathrm{NaCl}\), \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), and \(\mathrm{H}_{2} \mathrm{SO}_{4}\). The presence of \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) vapor might lead to some confusion, but no free chlorine \(\mathrm{Cl}_{2}\) is produced.

Key Concepts

NaCl and K2Cr2O7 reaction

When sodium chloride (NaCl) and potassium dichromate (\(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\)) are gently warmed with concentrated \(\mathrm{H}_2\mathrm{SO}_4\), a chemical reaction occurs. The interaction of these compounds leads to the formation of chromyl chloride (\(\mathrm{CrO}_2\mathrm{Cl}_2\)). This reaction can seem quite complex, but it breaks down into a series of straightforward steps that result in the identified products.

• Chromyl Chloride Formation: The key product, chromyl chloride, emerges from the active interplay of \(\mathrm{NaCl}\) and \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) in the acidic environment provided by concentrated sulfuric acid.
• By-products: Besides chromyl chloride, potassium bisulfate (\(\mathrm{KHSO}_4\)) and hydrogen chloride (\(\mathrm{HCl}\)) are formed as by-products.

This chemistry serves as a fascinating example of how intricate reactions yield distinctive compounds and physical changes, such as vapor emissions.

characteristic deep red vapor

One of the hallmark observations in the \(\mathrm{NaCl}\) and \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) reaction is the evolution of a deep red vapor. This visual clue is attributed to the evaporation of chromyl chloride (\(\mathrm{CrO}_2\mathrm{Cl}_2\)). The appearance of the red vapor is not just a captivating sight but also a practical indicator of the reaction's progress.

• Visual Indicator: The deep red color is a distinct marker confirming that chromyl chloride is present in the vapor phase.
• Chemical Confirmation: This vapor is identifiable due to its color, ensuring that the correct reaction pathway is being followed.

The presence of such a vivid color in the vapor phase makes it possible to visually monitor the reaction as it unfolds.

yellow sodium chromate solution

After obtaining chromyl chloride vapor from the initial reaction, passing it into a sodium hydroxide (\(\mathrm{NaOH}\)) solution results in another chemical change. This process leads to the formation of a sodium chromate (\(\mathrm{Na}_2\mathrm{CrO}_4\)) solution, notable for its yellow color.

• Solution Reaction: When chromyl chloride interacts with \(\mathrm{NaOH}\), the reaction produces sodium chromate, sodium chloride (\(\mathrm{NaCl}\)), and water (\(\mathrm{H}_2\mathrm{O}\)).
• Color Indicator: The yellow color of the sodium chromate solution acts as a visual indicator that the reaction has successfully proceeded.

This transformation underscores the versatility and predictability of chemical reactions under controlled conditions.

absence of chlorine gas evolution

A common misconception is that chlorine gas (\(\mathrm{Cl}_2\)) might be part of the evolved products in this reaction. However, no chlorine gas is generated during the process involving \(\mathrm{NaCl}\) and \(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\) with concentrated \(\mathrm{H}_2\mathrm{SO}_4\).

• Mistaken Identity: The chromyl chloride vapor may lead some to assume chlorine gas is present, but it is not. No substantiated reaction pathway leads to free \(\mathrm{Cl}_2\).
• Correct Product: The presence of \(\mathrm{HCl}\) as a by-product further confirms that elemental chlorine gas is not produced.

This absence of chlorine gas highlights the importance of understanding chemical reaction pathways to accurately predict and observe reaction products.