14.2
平衡常数
考虑二氧化硫的氧化：
对于仅以反应物混合物开始的反应，生成物浓度最初等于零。 当反应朝着正向均衡方向前进时，反应物浓度会下降，生成物浓度会增加。 当达到平衡时，反应物和生成物的浓度保持不变。
如果反应开始时只有生成物存在，则反应将朝着相反方向逐渐达到平衡。 生成物浓度随着时间的推移而下降，反应物浓度…
质量作用定律引入平衡 常数，将反应物和产物的 平衡浓度关联起来。对于一个平衡的化学方程式，其中 A 和 B 是反应物，C 和 D 是产物，小写的 a、b、c 和 d 是它们各自的化学计量系数，平衡常数的表达式 由产物的摩尔浓度 除以反应物的 摩尔浓度得出，全部 加上它们的化学计量系数次方。平衡常数通常 用符号 K 或 Kc 表示，其中下标 c 表示相对于 摩尔浓度的平衡常数。作为摩尔浓度的比值，平衡常数是无单位的。一个远大于 1 的大平衡常数 意味着分子，即 产物的浓度，大于分母，即 反应物的浓度。这表明平衡朝向产物进行，有利于正向反应。相反，一个远小于 1 的小平衡常数 意味着分母大于分子。这意味着平衡 朝向反应物并且有利于逆向反应。对于一个量级等于或 接近 1 的平衡常数，反应物和 产物的相对浓度几乎相同。这表明，无论是正向反应还是逆向反应，都不被支持。对化学方程式的修改 也会改变平衡常数。如果一个平衡常数为 x 的化学反应 被逆转，则该逆转反应的 平衡常数表达式为 正向反应的倒数。所以，新的平衡常数就是 1 除以 x。如果这些系数乘以 另一个因子 n，则平衡常数 变为相同的因子次方。新的平衡常数现在 是 x 的 n 次方。当两个或更多单独的反应加在一起时，整个反应的平衡常数 是各个平衡常数的乘积。所以，新的平衡常数就是 y 乘以 z。对于非均相平衡，平衡常数 表达式不包括纯固体或纯液体 实体。例如，固体硝酸铵分解为 气态氧化亚氮和水蒸气的 平衡常数表达式，不包括硝酸铵—一种 纯固体。这是因为纯固体或液体的 相对摩尔浓度在 反应过程中保持不变。
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Q1: How is the equilibrium constant expression written for a chemical reaction?
The equilibrium constant expression is the ratio of molar concentrations of products divided by molar concentrations of reactants, each raised to their stoichiometric coefficients. For a reaction aA + bB ⇌ cC + dD, the expression is K = [C]^c[D]^d / [A]^a[B]^b. The subscript c in Kc indicates the constant is based on molar concentration. This unitless ratio quantifies the relative amounts of reactants and products at equilibrium.
Q2: What does a large equilibrium constant value tell you about a reaction?
A large equilibrium constant much greater than 1 indicates that the numerator (product concentrations) is greater than the denominator (reactant concentrations), meaning equilibrium lies toward the products and favors the forward reaction. Conversely, a small equilibrium constant much less than 1 means reactant concentrations exceed product concentrations, favoring the reverse reaction. An equilibrium constant near 1 indicates neither direction is favored.
Q3: How does reversing a chemical equation affect its equilibrium constant?
When a chemical reaction is reversed, the equilibrium constant for the reverse reaction becomes the reciprocal of the forward reaction's constant. If the forward reaction has equilibrium constant x, the reversed reaction has equilibrium constant 1/x. This reciprocal relationship reflects the swap of reactants and products in the reversed equation.
Q4: What happens to the equilibrium constant when stoichiometric coefficients are multiplied by a factor?
When stoichiometric coefficients in a chemical equation are multiplied by a factor n, the equilibrium constant is raised to that same factor. If the original equilibrium constant is x, the new equilibrium constant becomes x^n. This exponential relationship ensures the equilibrium expression remains consistent with the modified stoichiometry.
Q5: How do you find the equilibrium constant for coupled reactions?
When two or more equilibrium reactions are added together, the equilibrium constant for the overall reaction is the product of the individual equilibrium constants. If reaction 1 has constant y and reaction 2 has constant z, the combined reaction has equilibrium constant y × z. This multiplicative relationship allows prediction of complex equilibrium systems from simpler component reactions.
Q6: Why are pure solids and pure liquids excluded from heterogeneous equilibrium constant expressions?
Pure solids and pure liquids are excluded from heterogeneous equilibrium constant expressions because their relative molar concentrations remain constant during the reaction. Since the equilibrium constant depends only on concentrations that change, including constant values would be redundant. For example, in ammonium nitrate decomposition, the solid ammonium nitrate is excluded from the expression.
Q7: Does the equilibrium constant indicate how fast a reaction reaches equilibrium?
No, the magnitude of the equilibrium constant does not indicate the reaction rate or how quickly equilibrium is reached. Some equilibria establish nearly instantaneously, while others proceed so slowly that no perceptible change occurs over days, years, or longer. The equilibrium constant only reflects the composition of the reaction mixture at equilibrium, not the speed of reaching that state.