Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Oxalic acid reacts with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to give a mixture of two gases. When this mixture is passed through caustic potash, one of the gases is absorbed. What is the product formed by the absorbed gas with caustic potash? (a) \(\mathrm{KHCO}_{3}\) (b) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{KOH}\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

The product formed is \(\mathrm{K}_2\mathrm{CO}_3\) (potassium carbonate).

Step 1: Identify the Reaction

Oxalic acid (\( ext{H}_2 ext{C}_2 ext{O}_4)\) reacts with concentrated sulfuric acid (\( ext{H}_{2} ext{SO}_{4}\)) and produces a mixture of gases. The reaction is:\[\text{H}_2 ext{C}_2 ext{O}_4 + ext{H}_2 ext{SO}_4 \rightarrow ext{CO}_2 + ext{CO} + 2 ext{H}_2 ext{O}\] This gives carbon dioxide (\( ext{CO}_2\)) and carbon monoxide (\( ext{CO}\)) as the gaseous products.

Step 2: Determine Which Gas is Absorbed

When the mixture of carbon dioxide (\( ext{CO}_2\)) and carbon monoxide(\( ext{CO}\)) is passed through caustic potash(\( ext{KOH}\)), only carbon dioxide (\( ext{CO}_2\)) is absorbed. Carbon monoxide remains unabsorbed.

Step 3: Reaction of CO2 with Caustic Potash

The absorbed carbon dioxide reacts with caustic potash(\( ext{KOH}\)) to form potassium carbonate (\( ext{K}_2 ext{CO}_3\)) and water. The reaction is:\[2 ext{KOH} + ext{CO}_2 \rightarrow ext{K}_2 ext{CO}_3 + ext{H}_2 ext{O}\]

Step 4: Identify the Product

The product formed from the absorbed gas, carbon dioxide, reacting with caustic potash is potassium carbonate (\( ext{K}_2 ext{CO}_3\)). This matches option (b) in the provided choices.

Key Concepts

Reaction with sulfuric acid

Oxalic acid (\(\mathrm{H}_2\mathrm{C}_2\mathrm{O}_4\)) is an organic compound that reacts with concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) to produce gases. When these two chemicals come together, a decomposition reaction takes place. This reaction splits the oxalic acid to form carbon dioxide (\(\mathrm{CO}_2\)), carbon monoxide (\(\mathrm{CO}\)), and water (\(\mathrm{H}_2\mathrm{O}\)). A useful way to remember this is by considering that the \(\mathrm{CO}_2\) and \(\mathrm{CO}\) come from the breakdown of the carboxyl groups in oxalic acid.The simplified reaction can be written as follows:\[\text{H}_2\mathrm{C}_2\mathrm{O}_4 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{CO}_2 + \mathrm{CO} + 2 \mathrm{H}_2\mathrm{O}\]It is important to note that sulfuric acid acts as a dehydrating agent in this process, facilitating the removal of water from oxalic acid. This reaction underpins the formation of specific gases which can further interact with other compounds.

Gas absorption

Let's dive into the next part of the process, which involves understanding gas absorption. When we mention gas absorption, we are typically referring to a process where a gas phase component is transferred to a liquid phase and absorbed by it. In this context, the involvement of carbon dioxide (\(\mathrm{CO}_2\)) and carbon monoxide (\(\mathrm{CO}\)) is crucial.When the mixture of \(\mathrm{CO}_2\) and \(\mathrm{CO}\) formed from the reaction of oxalic acid and sulfuric acid is passed through caustic potash, only one of these gases gets absorbed. Caustic potash is actually a solution of potassium hydroxide (\(\mathrm{KOH}\)), which is capable of absorbing carbon dioxide due to its chemical properties.The carbon dioxide interacts more readily with \(\mathrm{KOH}\), while carbon monoxide passes through without being absorbed. This selective absorption is integral to the subsequent reactions that lead to new product formation.

Potassium carbonate formation

Once carbon dioxide (\(\mathrm{CO}_2\)) is absorbed by the caustic potash (\(\mathrm{KOH}\)), a chemical reaction occurs. This reaction involves the absorbed \(\mathrm{CO}_2\) combining with \(\mathrm{KOH}\) to form potassium carbonate (\(\mathrm{K}_2\mathrm{CO}_3\)) as well as water (\(\mathrm{H}_2\mathrm{O}\)).The chemical equation for this reaction is as follows:\[2 \mathrm{KOH} + \mathrm{CO}_2 \rightarrow \mathrm{K}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O}\]This reaction is a double replacement reaction where two molecules of potassium hydroxide (\(\mathrm{KOH}\)) react with one molecule of carbon dioxide (\(\mathrm{CO}_2\)). Through this interaction, the carbonate ion forms and bonds with potassium ions to produce potassium carbonate. Moreover, water is the byproduct of this process, highlighting the neutralization aspect of the reaction.

Caustic potash reaction

Caustic potash refers to an aqueous solution of potassium hydroxide (\(\mathrm{KOH}\)). Known for being a strong base, it is widely used in numerous chemical processes due to its capability to absorb acidic gases, among which carbon dioxide is a prominent example. The reaction between caustic potash and carbon dioxide is not only fundamental in laboratory settings but also in industrial processes where gas scrubbing is required. In such contexts, caustic potash serves as a convenient absorbent for acidic gases, leading to the formation of potassium carbonate in reactions similar to the following:- \(\mathrm{KOH}\) absorbs \(\mathrm{CO}_2\)- Forms \(\mathrm{K}_2\mathrm{CO}_3\)- Also produces \(\mathrm{H}_2\mathrm{O}\)The interactions of caustic potash underline its valuable role in controlling pH levels, capturing acidic gases, and generating useful byproducts such as potassium carbonate, demonstrating a converging point between organic chemistry and practical applications.