### Correct Option: C. Concentration of SO₃ increases #### Detailed Explanation: To understand why the correct answer is C, we need to analyze the given exothermic reaction: \[ 2 \text{SO}_2(g) + \text{O}_2(g) \leftrightarrow 2 \text{SO}_3(g) \] 1. **Understanding Exothermic Reactions**: - An exothermic reaction is one that releases heat. In this case, the formation of sulfur trioxide (SO₃) from sulfur dioxide (SO₂) and oxygen (O₂) releases energy. - The reaction can be represented as: \[ \text{Reactants} \rightarrow \text{Products} + \text{Heat} \] 2. **Le Chatelier's Principle**: - This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. - In this scenario, reducing the temperature of the reaction shifts the equilibrium to the right (towards the products) because the system will try to produce more heat to counteract the decrease in temperature. 3. **Effect of Temperature Change**: - When the temperature is decreased from 800°C to 500°C, the equilibrium will shift towards the formation of more SO₃ to release heat and restore the temperature. - Therefore, the concentration of SO₃ will increase as the reaction shifts to the right. 4. **Conclusion**: - As a result of the temperature decrease, the concentration of SO₃ increases, making option C the correct answer. #### Why the Other Options are Incorrect: - **Option A: The reaction rate increases**: - This option is incorrect because lowering the temperature generally decreases the reaction rate. Reaction rates typically increase with temperature due to higher kinetic energy of the molecules, leading to more frequent and effective collisions. Thus, a decrease in temperature would slow down the reaction rate. - **Option B: Concentration of SO₃ decreases**: - This option is also incorrect. As explained, lowering the temperature shifts the equilibrium to the right, resulting in an increase in the concentration of SO₃, not a decrease. - **Option D: SO₂ gas becomes unreactive**: - This option is misleading. SO₂ does not become unreactive simply due to a temperature change. While the reaction rate may decrease, SO₂ remains a reactant and can still participate in the reaction. It does not become unreactive; rather, the equilibrium shifts to favor the production of SO₃. ### Summary for Revision: - **Exothermic reactions release heat**; lowering temperature shifts equilibrium towards products. - **Le Chatelier's Principle** states that equilibrium shifts to counteract changes in conditions. - **Lowering temperature** in this reaction increases the concentration of SO₃. - **Reaction rates decrease** with lower temperatures, and reactants do not become unreactive.