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 (b) ce{K2CO3}.

Step 1: Understanding the Reaction

Oxalic acid ( ce{H2C2O4} ) reacts with concentrated sulfuric acid ( ce{H2SO4} ) to produce a mixture of carbon monoxide ( ce{CO} ) and carbon dioxide ( ce{CO2} ) gases. This can be seen from the reaction: ce{H2C2O4 ->[H2SO4] CO2 + CO + H2O}.

Step 2: Role of Caustic Potash

Caustic potash refers to potassium hydroxide ( ce{KOH} ). When the mixture of ce{CO} and ce{CO2} is passed through caustic potash, only ce{CO2} is absorbed and reacts with ce{KOH}.

Step 3: Reaction of ce{CO2} with ce{KOH}

Carbon dioxide reacts with potassium hydroxide to form potassium carbonate ( ce{K2CO3} ) according to the equation: ce{2 KOH + CO2 -> K2CO3 + H2O}.

Step 4: Identifying the Product

Since ce{CO2} reacts with ce{KOH} to form ce{K2CO3} , the absorbed gas's product is potassium carbonate.

Key Concepts

Carbon Dioxide Absorption

In the given reaction, oxalic acid reacts with concentrated sulfuric acid, resulting in the formation of two gases: carbon monoxide \( \mathrm{CO} \) and carbon dioxide \( \mathrm{CO}_2 \). When these gases are introduced to a solution of caustic potash, which is chemically known as potassium hydroxide \( \mathrm{KOH} \), carbon dioxide gets absorbed. This means that \( \mathrm{CO}_2 \) dissolves into the solution, while \( \mathrm{CO} \) does not, due to its poor solubility in water. Hence, differential solubility plays a significant role in selectively absorbing \( \mathrm{CO}_2 \) and allowing \( \mathrm{CO} \) to be released.

Potassium Hydroxide Reaction

Once carbon dioxide is absorbed by potassium hydroxide, these two chemicals undergo a reaction. Potassium hydroxide, being a strong base, reacts with acids and certain gases such as \( \ce{CO2} \). This reaction is quite straightforward: the \( \mathrm{CO}_2 \) interacts with \( \ce{KOH} \) molecules in the solution. The basic idea here is that potassium hydroxide provides hydroxide ions \( \ce{OH^-} \) which then react with \( \mathrm{CO}_2 \) to form carbonate ions \( \ce{CO3^{2-}} \). This reaction is crucial in the conversion of gaseous \( \mathrm{CO}_2 \) into a less volatile compound, thus one way to effectively trap \( \mathrm{CO}_2 \) from a mixture of gases.

Potassium Carbonate Formation

Through the reaction of carbon dioxide with potassium hydroxide, potassium carbonate \( \mathrm{K}_2\mathrm{CO}_3 \) is produced. The chemical equation for this transformation is \( 2 \mathrm{KOH} + \mathrm{CO}_2 \rightarrow \mathrm{K}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O} \). During this process, the carbonate ions \( \ce{CO3^{2-}} \) that form after \( \mathrm{CO}_2 \) absorption combine with potassium ions \( \ce{K^+} \) from the dissociation of \( \ce{KOH} \), leading to the formation of potassium carbonate. Notably, potassium carbonate is a stable solid, usually left within the solution or as a precipitate, depending on the concentration. This chemical transformation highlights the efficient capture and stabilization of carbon dioxide by using a simple base such as \( \ce{KOH} \). It is a practical example of gas absorption leading to a tangible product, applicable in various industrial and environmental processes.