Consider these two statements regarding the equation [tex]$PCl _5(g) \rightleftharpoons PCl _3+ Cl _2+$[/tex] heat:
Statement 1: Adding heat to this reaction mixture would increase the reverse reaction.
Statement 2: Cooling this reaction mixture would increase the forward reaction.
A. Statements 1 and 2 are both true.
B. Statement 1 is true, and statement 2 is false.
C. Statements 1 and 2 are both false.
D. Statement 1 is false, and statement 2 is true.
Answer (2)
Adding heat favors the reverse reaction, as it consumes heat.
Cooling the reaction favors the forward reaction, as it produces heat.
Both statements are true.
The correct answer is A: Statements 1 and 2 are both true. A
Explanation
Understanding the Reaction Let's analyze the given chemical reaction and the two statements using Le Chatelier's principle. The reaction is: PC l 5 ( g ) ⇌ PC l 3 ( g ) + C l 2 ( g ) + heat This reaction is endothermic in the forward direction because heat is a product.
Analyzing Statement 1 Statement 1: Adding heat to this reaction mixture would increase the reverse reaction. According to Le Chatelier's principle, if we add heat to the system, the equilibrium will shift in the direction that consumes heat. In this case, the reverse reaction consumes heat, so adding heat will favor the reverse reaction. Therefore, statement 1 is true.
Analyzing Statement 2 Statement 2: Cooling this reaction mixture would increase the forward reaction. According to Le Chatelier's principle, if we cool the system (remove heat), the equilibrium will shift in the direction that produces heat. In this case, the forward reaction produces heat, so cooling the system will favor the forward reaction. Therefore, statement 2 is true.
Conclusion Since both statements 1 and 2 are true, the correct answer is A.
Examples
Le Chatelier's principle is widely used in industrial chemistry to optimize reaction conditions. For example, in the Haber-Bosch process for synthesizing ammonia ( N 2 + 3 H 2 ⇌ 2 N H 3 + heat ), controlling the temperature and pressure is crucial. Since the forward reaction is exothermic, lower temperatures favor ammonia production, but very low temperatures slow down the reaction rate. Therefore, a balance must be struck to achieve both a reasonable rate and a high yield. Similarly, increasing the pressure favors the forward reaction because it reduces the number of gas molecules. Understanding and applying Le Chatelier's principle allows chemists to maximize the efficiency of chemical processes.
Both statements about the reaction are true. Adding heat to the mixture favors the reverse reaction, while cooling the mixture favors the forward reaction. Thus, the correct answer is A: Statements 1 and 2 are both true.
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