活化能Activation energy

阿伦尼乌斯方程 k = A·exp(−Ea/RT) 给出速率常数与温度的关系；以 ln k 对 1/T 作图得直线，斜率为 −Ea/R。按过渡态理论，活化能对应反应物与过渡态（活化络合物）之间的能量差。玻尔兹曼分布中能量超过 Ea 的分子比例随温度呈指数增长，粗略地说，室温附近温度每升高 10 °C 速率增至 2–4 倍。

The Arrhenius equation k = A·exp(−Ea/RT) relates the rate constant to temperature; a plot of ln k against 1/T is a straight line of slope −Ea/R. In transition-state theory Ea corresponds to the energy difference between reactants and the transition state (activated complex). The fraction of molecules with energy above Ea in the Boltzmann distribution rises exponentially with temperature, so near room temperature a 10 °C rise roughly doubles to quadruples the rate.

1884 年范特霍夫提出速率常数随温度变化的经验关系。1889 年瑞典化学家阿伦尼乌斯研究蔗糖水解速率时提出，只有获得足够能量的“活化分子”才能反应，引入了活化能概念。1935 年艾林、埃文斯和波兰尼建立过渡态理论，将活化能与势能面上的鞍点联系起来。

In 1884 van 't Hoff proposed an empirical relation between rate constants and temperature. In 1889 the Swedish chemist Arrhenius, studying the rate of sucrose hydrolysis, suggested that only 'activated' molecules with enough energy react, introducing activation energy. In 1935 Eyring, Evans and Polanyi developed transition-state theory, linking Ea to the saddle point on a potential energy surface.

催化剂提供活化能更低的途径；酶可把活化能降低 50 kJ/mol 以上。食品冷藏、药品稳定性加速试验、炸药安定性评价都基于活化能的温度效应。正反应活化能与逆反应活化能之差等于反应焓变：ΔH = Ea(正) − Ea(逆)。

Catalysts open lower-barrier pathways; enzymes can cut Ea by more than 50 kJ/mol. Refrigeration of food, accelerated stability testing of drugs and assessments of explosive stability all rely on the temperature effect of Ea. The difference between forward and reverse activation energies equals the reaction enthalpy: ΔH = Ea(forward) − Ea(reverse).