Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Oxalic acid is heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). When the resultant gases are passed over red hot carbon, \(\mathrm{X}\) is obtained. \(\mathrm{X}\) is (a) \(\mathrm{CO}\) (b) \(\mathrm{CO}_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{4}\) (d) \(\mathrm{CH}_{4}\)

Step-by-Step Solution

Answer

X is CO (carbon monoxide).

Step 1: Understand the Reaction

When oxalic acid ( C_2H_2O_4) is heated with concentrated H_2SO_4, it decomposes to form formic acid, CO, and CO_2.

Step 2: Identify the Second Reaction

The gases formed, which are CO and CO_2, are then passed over red hot carbon. Analyze the interaction of these gases with carbon.

Step 3: Determine the Role of Carbon in the Reaction

Red hot carbon reacts with CO_2 to form CO. The reaction, CO_2 + C → 2CO, causes any CO_2 to convert to additional CO.

Step 4: Conclude the Reaction Outcome

Since the reaction with red hot carbon converts all CO_2 to CO and CO remains unchanged, the final resulting gas is CO.

Key Concepts

Thermal Decomposition

Oxalic acid ( C_2H_2O_4) is a fascinating compound, primarily because of its behavior when heated. When we apply heat to oxalic acid in the presence of concentrated H_2SO_4, a thermal decomposition takes place. This means that the oxalic acid breaks down into simpler substances due to the heat energy applied. The process results in the formation of formic acid along with two gases, carbon monoxide ( CO ) and carbon dioxide ( CO_2 ).

The decomposition is initiated by the strong dehydrating nature of concentrated sulfuric acid. It removes water from the oxalic acid, thereby accelerating its breakdown. This step is crucial in understanding how oxalic acid transitions from a stable state to emitting gases upon heating.

To grasp this concept thoroughly, visualize thermal decomposition as an intricate dance of breaking bonds and forming new ones. Here, even without external oxygen, oxalic acid decomposes due to the heat and chemical influence of H_2SO_4 :

• Release of CO and CO_2 gases.
• Formation of formic acid as a byproduct.

This decomposition sets the stage for further reactions when these gases interact with elements like carbon.

Interaction with Carbon

Once oxalic acid has decomposed to release carbon monoxide ( CO ) and carbon dioxide ( CO_2 ), the next step is their interaction with carbon. When these gases are passed over a bed of red hot carbon, an interesting transformation happens.

Red hot carbon serves as a reducing agent, meaning it tends to donate electrons and engage in chemical reactions which reduce another substance. When CO_2 comes into contact with red hot carbon, it reacts to form additional carbon monoxide. This happens because carbon itself combines with the oxygen in the carbon dioxide to produce more CO .

This is essentially a reduction reaction where the carbon reduces CO_2 to CO :

• CO_2 + C ightarrow 2CO.

It's important to visualize this as the carbon taking away an oxygen from CO_2 to transform it into fresh CO . This interaction is pivotal to the resulting composition of the gas mixture.

Gas Reactions with Carbon

The interaction of gases like CO and CO_2 with red hot carbon is a classic example of gas-solid reactions in chemistry. These reactions are driven significantly by temperature, and the presence of red hot carbon accelerates the reaction processes. High temperature provides the kinetic energy necessary for the reactants to overcome potential energy barriers, facilitating the transformation of CO_2 into CO .

At this elevated temperature, the gases and carbon particles are able to collide with enough energy to break existing bonds, allowing the formation of new ones. This enables all available CO_2 molecules to participate in the redox reaction, further producing carbon monoxide. Because of this reaction:

• CO remains unreacted.
• All CO_2 is converted to CO.

Hence, the importance of understanding these gas reactions lies in their ability to alter the composition of the resultant gas mixture significantly.

Carbon Monoxide Formation

The culmination of these reactions — from the decomposition of oxalic acid to the interaction with carbon — results in the exclusive formation of carbon monoxide ( CO ). This process highlights the importance of each reaction step, as they transform initial reactants into desired products.

By converting all CO_2 into CO during its passage over red hot carbon, the final gas mixture produced contains CO exclusively. This process can be summarized as follows:

• Oxalic acid decomposes to release CO and CO_2.
• Red hot carbon converts CO_2 to additional CO.
• Ultimately resulting in the generation of pure CO.

Understanding this transformation is crucial, especially in industrial applications where carbon monoxide is an important gas. It exemplifies how chemical manipulation under specific conditions can achieve desirable chemical outputs. Such processes are foundational to chemical engineering and industrial chemistry.