Question

The Boudouard reaction (\( \text{C} + \text{CO}_2 \rightleftharpoons 2\text{CO} \)) in smelting is:

• A. Exothermic at all temperatures
• B. Endothermic at high temperatures, favouring CO formation
• C. Exothermic at high temperatures
• D. Irreversible

Hint:

At temperatures below \( 700^\circ\text{C} \), the equilibrium strongly favors \( \text{CO}_2 \) and carbon, whereas above \( 1000^\circ\text{C} \), the gas phase consists of more than \( 99\% \text{ CO} \).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks about the thermodynamic nature and temperature dependence of the Boudouard reaction inside an extraction reactor (like an iron blast furnace).
The Boudouard reaction represents the chemical equilibrium between solid carbon (coke) and carbon oxides (carbon dioxide and carbon monoxide).

Step 2: Key Formula or Approach:
The Boudouard reaction is written as:
\[ \text{C(s)} + \text{CO}_2\text{(g)} \rightleftharpoons 2\text{CO(g)} \]
At standard state, the reaction has a highly positive standard enthalpy of reaction:
\[ \Delta H^\circ \approx +172 \text{ kJ/mol} \]
This positive value of enthalpy indicates that the reaction is highly endothermic.

Step 3: Detailed Explanation:

• Thermodynamic Analysis: Because the standard enthalpy of reaction \( \Delta H^\circ \) is positive, Le Chatelier's principle dictates that an increase in temperature will shift the chemical equilibrium in the forward direction.
Thus, higher temperatures favor the forward, endothermic reaction, leading to a higher concentration of carbon monoxide (CO) gas relative to carbon dioxide (\( \text{CO}_2 \)).

• Entropy Effect: The reaction involves the conversion of one mole of gas into two moles of gas, resulting in a positive entropy change (\( \Delta S^\circ \gt 0 \)).
Since \( \Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \), a positive \( \Delta S^\circ \) causes the term \( -T\Delta S^\circ \) to become increasingly negative at higher temperatures, eventually overcoming the positive \( \Delta H^\circ \) and making \( \Delta G^\circ \) spontaneous (\( \Delta G^\circ \lt 0 \)) at high temperatures.

• Smelting Importance: In the blast furnace, this reaction is crucial because it regenerates the reducing gas (CO) required for reducing iron oxides.
However, due to its endothermic nature, it consumes a large amount of thermal energy in the lower zones of the furnace.


Step 4: Final Answer:
Therefore, the Boudouard reaction is endothermic at high temperatures, favoring CO formation, which corresponds to Option (B).