Question

Consider the following gas-phase reaction: \[ 2 {SO}_2 + {O}_2 \rightleftharpoons 2 {SO}_3 \] If the enthalpy of reaction is negative, which one of the following conditions promotes a higher equilibrium concentration of SO\(_3\)?

• A. Higher pressure and higher temperature
• B. Higher pressure and lower temperature
• C. Lower pressure and higher temperature
• D. Lower pressure and lower temperature

Hint:

For exothermic reactions, lowering the temperature and increasing the pressure generally favor the formation of products, according to Le Chatelier’s Principle.

Answer 1

The Correct Option is B

This reaction is exothermic (since the enthalpy is negative). According to Le Chatelier's Principle, to increase the equilibrium concentration of SO\(_3\), we need to favor the formation of SO\(_3\) in the reaction. 
Step 1: Effect of temperature - Since the reaction is exothermic, lowering the temperature will favor the production of SO\(_3\). At lower temperatures, the system will shift towards the products (SO\(_3\)) to release heat and restore equilibrium. 
Step 2: Effect of pressure - The reaction involves a reduction in the number of gas molecules (from 3 moles of reactants to 2 moles of products). According to Le Chatelier's Principle, increasing the pressure will shift the equilibrium to the side with fewer gas molecules, which is the product side in this case. 
Step 3: Conclusion The combination of higher pressure and lower temperature (Option B) will favor the production of SO\(_3\), as both conditions drive the reaction towards the product side.