📚 Equilibrium Constant Kp for Homogeneous Systems | 均相体系的平衡常数 Kp
For AQA A-Level Chemistry, Topic 1.12 focuses on Kp for homogeneous gas-phase equilibria. Understanding partial pressure, mole fraction, and how the equilibrium constant Kp behaves under changing conditions is essential for both calculation and explanation questions.
在 AQA A-Level 化学中，1.12 主要考查均相气相平衡的 Kp。理解分压、摩尔分数以及平衡常数 Kp 在条件变化时的表现，对计算题和解释题都至关重要。
This guide rebuilds the key ideas step by step: homogeneous equilibria, Dalton's law of partial pressures, mole fractions, writing Kp expressions, calculating Kp from equilibrium amounts, and explaining why only temperature changes the value of Kp.
本文会逐步重建核心知识：均相平衡、道尔顿分压定律、摩尔分数、书写 Kp 表达式、由平衡物质的量计算 Kp，以及为什么只有温度会改变 Kp 的数值。
1. Homogeneous Equilibria and Partial Pressure | 均相平衡与分压
A homogeneous equilibrium is one in which all reactants and products are in the same physical state. In Kp questions on AQA Paper 1, this almost always means a gas-phase system, so every species appears in the Kp expression.
均相平衡是指所有反应物和生成物都处于同一相态。在 AQA 试卷 1 的 Kp 题目中，这几乎总是指气相体系，因此每种物质都会出现在 Kp 表达式中。
In a gas mixture, each gas exerts its own partial pressure. The partial pressure of a gas is the pressure it would exert if it alone occupied the container at the same temperature.
在气体混合物中，每种气体都有自己的分压。某种气体的分压是指它在相同温度下单独占据该容器时所施加的压力。
The total pressure is the sum of all partial pressures.
总压力等于所有分压之和。
P(total) = p(A) + p(B) + p(C) + ...
This relationship is Dalton's law of partial pressures. It is the foundation of every Kp calculation because equilibrium pressures are usually found from mole fractions and the total pressure.
这个关系就是道尔顿分压定律。它是所有 Kp 计算的基础，因为平衡分压通常由摩尔分数和总压求出。
Kp uses partial pressures rather than concentrations because the equilibrium constant is defined for gaseous systems in terms of the pressures of the gases present at equilibrium.
Kp 使用分压而不是浓度，因为该平衡常数是针对气相体系定义的，用平衡时各气体的压力表示。
2. Mole Fraction and Partial Pressure | 摩尔分数与分压
The mole fraction of gas A, written x(A), is the number of moles of A divided by the total number of moles of all gases in the mixture. It has no units, and its value is always between 0 and 1.
气体 A 的摩尔分数记作 x(A)，等于 A 的物质的量除以混合物中所有气体的总物质的量。它没有单位，数值始终在 0 到 1 之间。
x(A) = n(A) ÷ n(total)
Once the mole fraction is known, the partial pressure is calculated by multiplying it by the total pressure.
一旦知道摩尔分数，就可以用它乘以总压得到分压。
p(A) = x(A) × P(total)
4.0 ÷ 12.0 = 0.333. If the total pressure is 500 kPa, then p(NH₃) = 0.333 × 500 = 167 kPa.
例如，若混合物含有 2.0 mol N₂、6.0 mol H₂ 和 4.0 mol NH₃，总物质的量为 12.0 mol。NH₃ 的摩尔分数为 4.0 ÷ 12.0 = 0.333。若总压为 500 kPa，则 p(NH₃) = 0.333 × 500 = 167 kPa。
Always check that the mole fractions sum to 1 and the partial pressures sum to the total pressure. These checks catch most arithmetic errors in exam calculations.
务必检查摩尔分数之和等于 1，分压之和等于总压。这些检查能发现考试计算中的大多数算术错误。
= 0.500 × 500 = 250 kPa. The partial pressures sum to 167 + 83.3 + 250 = 500.3 kPa, which
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