Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is correct when a mixture of \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is gently warmed with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) 1\. deep red coloured vapours are evolved 2\. the vapours when passed into \(\mathrm{NaOH}\) solution give a yellow solution of \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) 3\. chlorine gas is evolved 4\. chromyl chloride is formed (a) \(1,2,4\) (b) \(1,2,3\) (c) \(2,3,4\) (d) all are correct

Step-by-Step Solution

Answer

(a) 1,2,4.

Step 1: Identify the reactants

The mixture consists of \(\text{NaCl}\) and \(\text{K}_2\text{Cr}_2\text{O}_7\). When this mixture is warmed with concentrated \(\text{H}_2\text{SO}_4\), certain reactions will occur depending on the reagents.

Step 2: Understand the reaction with H2SO4

When \(\text{NaCl}\) and \(\text{K}_2\text{Cr}_2\text{O}_7\) are warmed with concentrated \(\text{H}_2\text{SO}_4\), \(\text{K}_2\text{Cr}_2\text{O}_7\) will form chromyl chloride \(\text{CrO}_2\text{Cl}_2\), which is observed as deep red vapors. This accounts for statement 1.

Step 3: Examine the formation of chromyl chloride

Chromyl chloride \(\text{CrO}_2\text{Cl}_2\), which forms from the reaction, is represented by \(\text{K}_2\text{Cr}_2\text{O}_7 + \text{H}_2\text{SO}_4 + \text{Cl}^- \rightarrow \text{CrO}_2\text{Cl}_2\text{(g)} + \text{H}_2\text{SO}_4 + \text{K}_2\text{SO}_4 + \text{H}_2\text{O}\). Thus, chromyl chloride is formed, confirming statement 4.

Step 4: Analyse the reaction of chromyl chloride in NaOH

The vapors of \(\text{CrO}_2\text{Cl}_2\) when passed through \(\text{NaOH}\), react to form \(\text{Na}_2\text{CrO}_4\) according to the equation \(\text{CrO}_2\text{Cl}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{CrO}_4 + 2\text{HCl}\). This supports statement 2.

Step 5: Consider chlorine gas evolution

Chlorine gas \(\text{Cl}_2\) is not typically evolved in these reactions when conducted correctly under these conditions. Therefore, statement 3 is not correct.

Key Concepts

Chromyl Chloride Formation

When a combination of \( \text{NaCl} \) (sodium chloride) and \(\text{K}_2\text{Cr}_2\text{O}_7\) (potassium dichromate) is gently warmed with concentrated \(\text{H}_2\text{SO}_4\) (sulfuric acid), chromyl chloride \(\text{CrO}_2\text{Cl}_2\) is formed. This transformation is important in inorganic chemistry because it explains how chromyl chloride, a compound that has deep red vapors, is evolved under these specific conditions. Understanding this reaction provides insight into various redox and acid-base chemistry processes.

During the reaction involving these substances, potassium dichromate acts as an oxidizing agent. The reaction equation can be depicted as:
\[ \text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 + 6\text{NaCl} \rightarrow 2\text{CrO}_2\text{Cl}_2 \text{(g)} + 3\text{Na}_2\text{SO}_4 + \text{K}_2\text{SO}_4 + 3\text{H}_2\text{O} \]While the sulfuric acid is highly concentrated and acts as a dehydrating agent to drive the reaction forward, the chlorine ions from \(\text{NaCl}\) assist in forming the chromyl chloride gas.

This specific reaction illustrates the principle of forming complex chromium compounds under controlled laboratory conditions. It is a classic example used in qualitative analysis to confirm the presence of chloride ions by the characteristic red vapors, thereby underpinning the broader principles of inorganic chemistry reaction schema.

Redox Reactions

Redox (reduction-oxidation) reactions play a crucial role in the formation of chromyl chloride when concentrated \(\text{H}_2\text{SO}_4\) interacts with a mixture of \(\text{NaCl}\) and \(\text{K}_2\text{Cr}_2\text{O}_7\). These reactions involve the transfer of electrons between chemical species, affecting their oxidation states.

In the context of chromyl chloride formation, the potassium dichromate, \(\text{K}_2\text{Cr}_2\text{O}_7\), serves as the oxidizing agent, meaning it undergoes reduction by gaining electrons. Simultaneously, \(\text{Cl}^-\) ions from sodium chloride are oxidized, losing their electrons and ultimately participating in forming \(\text{CrO}_2\text{Cl}_2\):

• Potassium dichromate facilitates the oxidation of chloride ions.
• The chloride ions assist in converting \(\text{Cr}^{6+}\), as part of \(\text{K}_2\text{Cr}_2\text{O}_7\), into chromyl chloride.

These reactions depict the essence of a redox process in a controlled experimental setup.

The systematic alteration in the oxidation states during the reaction promotes the evolution of the desired compounds, underlining the importance of electron transfer in chemical transformations. Comprehending such interactions paves the way for advanced explorations in electron transfer processes and paves a path for effective analysis of compound compositions through redox mechanisms. The knowledge of redox reactions is also essential for predicting the behavior and outcome of similar reactions that students may encounter in chemistry.

Chlorine Evolution

One might initially think that chlorine gas \(\text{Cl}_2\) is evolved during the reaction of sodium chloride \(\text{NaCl}\) and potassium dichromate \(\text{K}_2\text{Cr}_2\text{O}_7\) with concentrated sulfuric acid \(\text{H}_2\text{SO}_4\). However, in this specific reaction setup, chlorine gas is not typically released, as the conditions favor the formation of chromyl chloride instead.

The absence of chlorine evolution in this case is significant for students to understand because it exemplifies how specific reaction conditions can steer the reaction towards different products. Here’s why chlorine doesn’t mainly evolve:

• \(\text{NaCl}\), when oxidized in the presence of a strong oxidizing agent like \(\text{K}_2\text{Cr}_2\text{O}_7\), bonds preferentially to form \(\text{CrO}_2\text{Cl}_2\)
• Any chlorine gas that could potentially form is more likely bound within the chromyl chloride complex

Thus, creative application of reaction conditions and reagents effectively suppresses the free formation of chlorine gas, unlike reactions typically anticipated in general education.

This understanding helps inform students about the importance of manipulating reaction conditions and components to achieve desired outcomes in chemical reactions. It highlights the exploratory nature of chemistry where the simple tweaking of variables changes the results completely. Realizing why chlorine does not evolve in this situation builds a practical understanding of reaction dynamics and the complexities of predicting reaction paths.