平衡常数Equilibrium constant

对 aA + bB ⇌ cC + dD，Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ)，方括号表示平衡浓度（mol dm⁻³）。气相反应也可用分压表示：Kp = p(C)ᶜp(D)ᵈ / (p(A)ᵃp(B)ᵇ)。纯固体和纯液体（包括稀溶液中的溶剂水）不写入表达式。K 的单位由表达式中的浓度单位决定，指数相互抵消时 K 无单位。

For aA + bB ⇌ cC + dD, Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ), where square brackets are equilibrium concentrations in mol dm⁻³. For gases partial pressures can be used: Kp = p(C)ᶜp(D)ᵈ / (p(A)ᵃp(B)ᵇ). Pure solids and pure liquids (including water as solvent in dilute solution) are left out. The units of K follow from the expression; when the powers cancel, K has no units.

K 很大（如 > 10¹⁰）说明平衡时几乎全部为生成物，反应进行得很完全；K 很小说明平衡时主要是反应物。K 只反映反应能进行到什么程度，不反映反应快慢。把任意时刻的浓度代入同一表达式得到浓度商 Q：Q < K 时反应正向进行，Q > K 时逆向进行，Q = K 时处于平衡。

A very large K (say > 10¹⁰) means the equilibrium mixture is almost all products and the reaction goes essentially to completion; a very small K means mostly reactants remain. K shows how far a reaction goes, not how fast. Putting the concentrations at any moment into the same expression gives the reaction quotient Q: if Q < K the reaction moves forward, if Q > K it moves backward, and if Q = K it is at equilibrium.

改变浓度、压强或加入催化剂都不改变 K，只改变到达平衡的位置或快慢。只有温度能改变 K：对放热反应，升温使 K 减小；对吸热反应，升温使 K 增大。这与勒夏特列原理的预测一致。

Changing concentration or pressure, or adding a catalyst, does not change K; they change only the position reached or the time taken. Only temperature changes K: for an exothermic reaction K decreases as temperature rises; for an endothermic reaction it increases. This agrees with Le Chatelier's principle.

标准吉布斯自由能变与标准平衡常数的关系为 ΔG° = −RT ln K。ΔG° 为负时 K > 1；ΔG° 每变化约 5.7 kJ mol⁻¹，298 K 下 K 改变 10 倍。K 随温度的变化由范特霍夫方程描述。

The standard Gibbs energy change is related to the standard equilibrium constant by ΔG° = −RT ln K. A negative ΔG° gives K > 1; at 298 K, each change of about 5.7 kJ mol⁻¹ in ΔG° changes K tenfold. The temperature dependence of K is described by the van 't Hoff equation.