Equilibria and the Chemical Industry | 化学平衡与化工产业

📚 Equilibria and the Chemical Industry | 化学平衡与化工产业

Chemical equilibria lie at the heart of many industrial processes. In a reversible reaction, reactants form products while products simultaneously re-form reactants; when the forward and reverse rates become equal, the mixture reaches dynamic equilibrium. Industrial chemists manipulate temperature, pressure, concentration and catalysts to shift equilibrium positions, maximise yield, and control cost.

化学平衡是许多工业过程的核心。可逆反应中，反应物生成产物的同时产物又重新生成反应物；当正反应和逆反应速率相等时，混合物达到动态平衡。工业化学家通过调节温度、压力、浓度和催化剂来改变平衡位置，从而提高产率并控制成本。

1. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡

A reversible reaction is shown by the ⇌ sign. In a closed system, the forward reaction and the reverse reaction proceed at the same time.

可逆反应用 ⇌ 表示。在封闭体系中，正反应和逆反应同时进行。

Dynamic equilibrium means the rates of forward and reverse reactions are equal, and the macroscopic properties such as concentration and pressure stay constant. At the molecular level, both reactions continue.

动态平衡意味着正、逆反应速率相等，浓度和压强等宏观性质保持不变；在分子水平上，两个反应仍在持续进行。

Equilibrium can be approached from either direction. The same equilibrium mixture is obtained starting from pure reactants or pure products under the same conditions.

平衡可以从两个方向达到。相同条件下，无论从纯反应物还是纯产物出发，最终得到相同的平衡混合物。

2. The Equilibrium Constant Kc | 平衡常数 Kc

The square brackets represent equilibrium concentration in mol dm⁻³. For N₂ + 3H₂ ⇌ 2NH₃, the expression is:

方括号表示平衡浓度，单位为 mol dm⁻³。对于 N₂ + 3H₂ ⇌ 2NH₃，其表达式为：

Kc = [NH₃]² ÷ ([N₂][H₂]³)

The units are calculated from the concentration powers, in this case dm⁶ mol⁻². A large Kc means the equilibrium position lies to the right, giving a high yield of products. Kc changes only with temperature for a given reaction.

单位由浓度幂次计算，本例为 dm⁶ mol⁻²。Kc 值大说明平衡位置偏右，产物产率高。对指定反应，Kc 只随温度变化。

3. The Equilibrium Constant Kp | 平衡常数 Kp

For gas-phase equilibria, Kp uses partial pressures. Partial pressure = mole fraction × total pressure. For the Haber reaction, the expression is:

对气相平衡，Kp 使用分压。分压 = 摩尔分数 × 总压。对哈伯反应，其表达式为：

Kp = p(NH₃)² ÷ (p(N₂) × p(H₂)³)

Kp, like Kc, is constant at a fixed temperature and has units that depend on the equation. Its value indicates the position of equilibrium, but it is independent of total pressure.

Kp 与 Kc 一样在温度固定时为常数，其单位取决于方程式。Kp 的数值表示平衡位置，但与总压无关。

4. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s principle states that if a system at equilibrium is disturbed, the equilibrium position shifts to oppose the change. This principle is used to predict the effect of temperature, pressure and concentration.

勒夏特列原理指出，若平衡体系受到扰动，平衡位置会向削弱该扰动的方向移动。该原理用于预测温度、压力和浓度的影响。

For an exothermic forward reaction, increasing temperature shifts equilibrium to the left. For a reaction that produces fewer gas molecules, increasing pressure shifts equilibrium to the right.

对于放热正反应，升高温度使平衡向左移动；对于气体分子数减少的反应，增大压力使平衡向右移动。

5. Effect of Temperature on Equilibrium | 温度对平衡的影响

Temperature is the only condition that changes the value of Kc and Kp. For an exothermic reaction, increasing temperature decreases Kc because the reverse endothermic reaction is favoured.

温度是唯一能改变 Kc 和 Kp 数值的条件。对放热反应，升温使 Kc 减小，因为吸热的逆反应受到促进。

For an endothermic reaction, increasing temperature increases Kc and moves the equilibrium to the right. Industrial processes therefore choose a temperature that balances high yield with acceptable reaction rate.

对吸热反应，升温使 Kc 增大，平衡向右移动。因此工业过程选择的温度要在高产率和可接受的反应速率之间取得平衡。

6. Effect of Pressure and Concentration | 压力与浓度的影响

Pressure changes only affect equilibria involving

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