Le Chatelier's Principle: Temperature and Catalysts
Advanced
Master the effects of temperature and catalysts on equilibrium. This stack clarifies how temperature uniquely alters both the equilibrium position and the constant K, and explains why catalysts affect reaction rate but not the final equilibrium state.
Le Chatelier's Principle: Temperature and Catalysts
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What is the only external stress that changes the numerical value of the equilibrium constant, \(K\)?
Temperature. Changes in concentration, pressure, or volume will cause the equilibrium to shift, but the value of \(K\) remains the same.
How can you predict the effect of a temperature change by treating heat as a reactant or product?
For an endothermic reaction (\(\Delta H > 0\)), heat is a reactant. For an exothermic reaction (\(\Delta H < 0\)), heat is a product. Then, apply Le Chatelier's principle as you would for a concentration change.
For an exothermic reaction $$ Reactants \rightleftharpoons Products + heat $$, what happens if the temperature is increased?
The system will shift to the left, toward the reactants. This is the endothermic direction, which consumes the added heat to relieve the stress.
For an endothermic reaction $$ Reactants + heat \rightleftharpoons Products $$, what happens if the temperature is increased?
The system will shift to the right, toward the products. This consumes the added heat, counteracting the temperature increase.
The Haber process $$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) $$ is exothermic. To maximize the yield of ammonia (\(NH_3\)), should you use a high or low temperature?
You should use a low temperature. Since the reaction is exothermic, lowering the temperature will shift the equilibrium to the right to produce more heat, thereby increasing the yield of \(NH_3\).
How does the value of \(K\) change when you increase the temperature of an *exothermic* reaction?
The value of \(K\) decreases. Increasing the temperature shifts the equilibrium left, favoring reactants. This decreases the product-to-reactant ratio, resulting in a smaller \(K\).
How does the value of \(K\) change when you increase the temperature of an *endothermic* reaction?
The value of \(K\) increases. Increasing the temperature shifts the equilibrium right, favoring products. This increases the product-to-reactant ratio, resulting in a larger \(K\).
What is the primary role of a catalyst in a reversible reaction?
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. It speeds up both the forward and the reverse reactions equally.
Does adding a catalyst change the position of equilibrium?
No. A catalyst does not affect the final concentrations of reactants and products at equilibrium. It only reduces the time it takes for the system to reach equilibrium.
Does adding a catalyst change the value of the equilibrium constant, \(K\)?
No. The equilibrium constant \(K\) is a function of temperature only and is not affected by the presence of a catalyst.
