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

The correct statements are 1, 2, and 4, making option (a) correct.

Step 1: Identify Reaction Components

First, let's identify the major components in the reaction mixture. We have sodium chloride \( \mathrm{NaCl} \), potassium dichromate \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \), and concentrated sulfuric acid \( \mathrm{H}_{2} \mathrm{SO}_{4} \). These reactants can undergo several reactions when combined.

Step 2: Understand Key Reaction Outcomes

When \( \mathrm{NaCl} \) and \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) react with concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \), chromyl chloride \( \mathrm{CrO}_{2} \mathrm{Cl}_{2} \) can form. This substance produces deep red vapors, which confirms point 1 from the options. These vapors, when passed into \( \mathrm{NaOH} \), can produce sodium chromate \( \mathrm{Na}_{2} \mathrm{CrO}_{4} \), a yellow solution, confirming point 2.

Step 3: Evaluate Evolution of Gases

The mixture of \( \mathrm{NaCl} \) and \( \mathrm{H}_{2} \mathrm{SO}_{4} \) can release chlorine gas \( \mathrm{Cl}_2 \). However, in this scenario, chromyl chloride formation is the dominant reaction, not chlorine gas evolution. Therefore, point 3 is incorrect.

Step 4: Conclude on Formation of Chromyl Chloride

As established, chromyl chloride is formed during the reaction, confirming point 4. The formation process involves the substitution of \( \mathrm{Cl}^- \) ions from \( \mathrm{NaCl} \) into the dichromate, facilitated by concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \).

Step 5: Compile Correct Points

Based on the reactions, points 1, 2, and 4 are correct because red vapors of chromyl chloride are formed, these vapors yield \( \mathrm{Na}_{2} \mathrm{CrO}_{4} \) in \( \mathrm{NaOH} \), and chromyl chloride is indeed formed. Point 3 is incorrect as chlorine gas is not the primary product.

Key Concepts

Chromyl Chloride Formation

Chromyl chloride formation is a fascinating process in inorganic chemistry. It occurs when a mixture containing sodium chloride \( \mathrm{NaCl} \) and potassium dichromate \( \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 \) is gently warmed with concentrated sulfuric acid \( \mathrm{H}_2 \mathrm{SO}_4 \). The chemical reaction produces chromyl chloride \( \mathrm{CrO}_2 \mathrm{Cl}_2 \), which is characterized by its deep red vapors.
The reaction can be understood as involving the chloride ions \( \mathrm{Cl}^- \) from sodium chloride interacting with the dichromate under the influence of sulfuric acid. It is a classic example of an oxidation-reduction reaction where the chromium species undergoes a change in oxidation state.

• First, the sulfates remove water and hydrogen to help drive the reaction.
• Next, dichromate ions interact with the chloride ions, leading to chromyl chloride formation.
• This reaction highlights the importance of using appropriate reaction conditions (such as concentrated \( \mathrm{H}_2 \mathrm{SO}_4 \)) to facilitate the formation of specific compounds like chromyl chloride.

Sodium Chromate Production

Sodium chromate \( \mathrm{Na}_2 \mathrm{CrO}_4 \) production is a subsequent step that occurs when the red vapors of chromyl chloride are bubbled into a sodium hydroxide \( \mathrm{NaOH} \) solution. The result is a yellow solution indicating the formation of sodium chromate. This process is another example of a chemical transformation where the chromium compound changes forms.
The chemical reaction can be represented as follows: when chromyl chloride reacts with sodium hydroxide, sodium chromate and other byproducts are formed. This is part of an acid-base reaction where hydroxide ions \( \mathrm{OH}^- \) help further oxidize the chromium present in chromyl chloride.

• The transition from chromyl chloride to sodium chromate involves rearranging oxygen and chlorine atoms around chromium.
• This reaction is a clear indicator of chromium's versatility and its ability to form different oxidation states and compounds under different conditions.
• The resulting sodium chromate is highly soluble in water, which accounts for the yellow solution observed in the reaction.

Chlorine Gas Reaction

While chlorine gas \( \mathrm{Cl}_2 \) is often a byproduct in reactions involving chlorides and strong acids, in the context of chromyl chloride formation, its evolution is not the main product. The potential for chlorine gas generation arises from the interaction between sodium chloride and sulfuric acid. However, the primary focus here is the production of chromyl chloride.

• The production of chlorine gas typically involves the decomposition or oxidation of chlorides, but under these conditions, chromyl chloride's formation is favored.
• This occurs because the relevant reaction conditions, such as temperature and the presence of strong oxidizing agents like \( \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 \), prioritize other pathways.
• Understanding these reaction dynamics is crucial to predicting and controlling the outcomes of chemical reactions in practical applications.

In summary, while chlorine might be generated under different circumstances, within this reaction context, its role is minimal and not prominent compared to the chromyl chloride pathway.