14.9
勒夏特列原理（LeChatelier's Principle） ：温度变化
与质量作用定律一致，浓度变化所强调的平衡将转变为重新建立平衡，而不会改变平衡常数的值， K。 当平衡因温度变化而变化时， 然而，它以不同的相对成分重新建立，显示了平衡常数的不同值。
要了解这种现象，请考虑基本反应：
由于这是一种基本反应，正向和反向的速率定律可以直接从平衡方程的化学计量法中得出：
当…
温度会影响化学反应的速率；因此，对于一个平衡状态下的 反应，温度的变化对体系起着应力的作用。勒沙特列原理预测了系统将如何 响应，以最小化此类干扰。温度的变化会改变 平衡常数的值，这与浓度或体积的变化不同，它会移动平衡而不会改变 K 的值。考虑一下将气态五氯化磷分解为 三氯化磷和氯气气体的反应。对于这种吸热反应，吸收的热量 可以看作是一种反应物。温度的升高会给体系增加 热量，类似于添加更多的一种反应物。因此，平衡位置向产物移动，产生更多的三氯化磷和氯气 来消耗额外的热量，因为 平衡常数 K 的值增加了。另一方面，温度的降低 会从体系中带走热量，类似于除去一种反应物。平衡位置向反应物移动，产生更多的五氯化磷 来释放热量，因为 K 的值降低了。对于放热反应，例如二氧化硫和氧气 之间的气体反应 生成三氧化硫，释放的热量 可以看作是一种产物。温度的升高类似于添加 更多的一种产物。这会导致平衡位置 向反应物移动，产生更多的二氧化硫 和氧气来吸收一些额外的热量，因为 K 值降低了。相反地，降低这个放热反应 的温度会带走热量，就像除去一种产物一样。平衡位置向产物移动，产生更多的三氧化硫以释放热量，因为 K 增加了。因此，温度升高有利于 吸热反应中的产物，而温度降低 有利于放热反应中 的产物。
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Q1: Why does temperature change affect the equilibrium constant differently than concentration changes?
Temperature changes alter the equilibrium constant value itself, whereas concentration changes shift equilibrium without changing K. This occurs because temperature affects the rate constants for both forward and reverse reactions through the Arrhenius equation. Since the rate constants vary with temperature, the equilibrium constant—which depends mathematically on these rate constants—also changes with temperature.
Q2: How does increasing temperature affect an endothermic reaction at equilibrium?
For endothermic reactions, heat is treated as a reactant. Increasing temperature adds heat to the system, similar to adding more reactant. The equilibrium shifts toward products to consume the extra heat, and the equilibrium constant K increases. This produces more products like phosphorus trichloride and chlorine in the decomposition of phosphorus pentachloride.
Q3: What happens to an exothermic reaction when temperature decreases?
For exothermic reactions, heat functions as a product. Decreasing temperature removes heat, similar to removing a product. The equilibrium shifts toward products to release heat and restore the removed energy, and K increases. This produces more sulfur trioxide in the reaction between sulfur dioxide and oxygen.
Q4: How can you predict which direction an equilibrium will shift with a temperature change?
Consider the reaction's enthalpy change. For endothermic reactions, increasing temperature favors products; decreasing temperature favors reactants. For exothermic reactions, the opposite occurs: increasing temperature favors reactants, while decreasing temperature favors products. This prediction follows from treating heat as either a reactant or product depending on reaction type.
Q5: Why does decreasing temperature shift an exothermic equilibrium toward products?
In exothermic reactions, heat is a product. Removing heat by decreasing temperature is like removing a product, causing the equilibrium to shift right toward products. This shift produces more products to release additional heat and counteract the temperature decrease, while the equilibrium constant K increases.
Q6: How does temperature change differ from volume or pressure changes in affecting equilibrium?
Temperature changes alter the equilibrium constant value, while volume or pressure changes shift equilibrium without changing K. This fundamental difference occurs because temperature affects the rate constants underlying K through the Arrhenius equation. Understanding this distinction is essential for predicting how equilibrium systems respond to different stresses.
Q7: What role does the equilibrium constant play when temperature changes?
The equilibrium constant K is a mathematical function of the forward and reverse rate constants. Since temperature changes affect these rate constants differently through the Arrhenius equation, K itself changes. A new equilibrium is established with a different K value and different relative composition of reactants and products compared to the original equilibrium.