含义Meaning
一定温度下，可逆反应达到平衡时，产物活度（浓度、分压）幂之积与反应物幂之积的比为常数（古德贝格–瓦格，1864）。K 越大，正反应进行得越完全。
At a given temperature, the ratio of product to reactant activities (each raised to its coefficient) is constant at equilibrium (Guldberg–Waage, 1864). Larger K means the forward reaction goes further.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(K_c\) | 浓度平衡常数concentration equilibrium constant | (mol·L⁻¹)^{Δν} |
| \(K^\circ\) | 标准（热力学）平衡常数，无量纲standard (thermodynamic) constant, dimensionless | 1 |
| \(\nu_\mathrm J\) | 计量数（产物为正）stoichiometric number (+ for products) | 1 |
推导Derivation
动力学图像：基元反应正逆速率 \(k_+[\mathrm A]^a[\mathrm B]^b = k_-[\mathrm C]^c[\mathrm D]^d\) 时达平衡，\(K = k_+/k_-\)。
Kinetically, equal forward and reverse rates give K = k₊/k₋.
热力学：平衡时 \(\Delta_rG = \sum\nu_\mathrm J\mu_\mathrm J = 0\)，代入 \(\mu = \mu^\circ + RT\ln a\) 得 \(\prod a^\nu = e^{-\Delta_rG^\circ/RT}\) = 常数。
Thermodynamically ΔrG = Σνμ = 0 with μ = μ° + RT ln a gives a constant.
纯固体、纯液体、稀溶液中的溶剂活度为 1，不写入表达式。
Pure solids, liquids and solvent have activity 1.
适用条件与局限Assumptions & limitations
只随温度变化，与浓度、压强、催化剂无关；方程式写法改变（乘以 2 或倒写）时 K 相应平方或取倒数。
Depends only on temperature; scaling or reversing the equation squares or inverts K.
严格应用活度；高浓度或离子溶液中用浓度只是近似。
Strictly uses activities.
例题Worked example
H₂ + I₂ ⇌ 2HI，698 K 时 K = 54：若起始 [H₂] = [I₂] = 1.00 mol/L，平衡时 [HI] = 1.57 mol/L。
H₂ + I₂ ⇌ 2HI at 698 K (K = 54): from 1.00 M each, [HI] = 1.57 M at equilibrium.