8.6
奥斯特瓦尔德稀释定律
考虑一种浓度为‘的二元电解质 ABc’ 可逆地解离为其组成离子。这种解离程度用 ⍺ 表示。这意味着每种离子物种的平衡浓度可表示为 ⍺c此外，平衡时未解离的电解质所占的分数由 (1−⍺) 给出。该未解离部分对应的平衡浓度则计算为 (1−⍺)c对于此类溶液，稀释定律可通过平衡常数表示 K，也称为电解质的…
根据阿伦尼乌斯的电解质解离理论，奥斯特瓦尔德稀释定律解释了水溶液中电解质的平衡状态。
考虑浓度为“c” mol/L 的电解质 AB，其可逆地解离为离子。
若 α 为解离度，则每种离子的平衡浓度为 cα，而未解离部分 (1 − α) 的浓度为 c(1 − α)。这些项定义了平衡常数 K，称为解离常数。
稀释定律仅适用于弱电解质，如乙酸和 NH₄OH。
像 HCl 和 NaF 这样的强电解质几乎完全解离，解离度 α 接近于 1。在这种情况下，(1 − α) 趋近于零，使得该定律在数学上不再适用。
强电解质不遵循该定律的原因在于，溶解时水的高介电常数削弱了静电作用力，导致几乎完全解离。因此，不存在离子-分子间的平衡，奥斯特瓦尔德稀释定律也因而不再适用。
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Q1: What is Ostwald's dilution law and how does it relate to electrolyte dissociation?
Ostwald's dilution law explains equilibrium in aqueous electrolyte solutions based on Arrhenius' theory of electrolytic dissociation. For an electrolyte AB at concentration c, the dissociation constant K equals cα²/(1−α), where α is the degree of dissociation. This law describes how weak electrolytes like acetic acid and NH₄OH reversibly dissociate into ions at equilibrium.
Q2: Why does Ostwald's dilution law fail for strong electrolytes?
Strong electrolytes like HCl and NaF are almost completely dissociated, with α approaching one, making (1−α) approach zero. This renders the law mathematically invalid. Water's high dielectric constant weakens electrostatic forces, causing near-complete dissociation and eliminating the ion-molecule equilibrium that the law requires.
Q3: How do you calculate equilibrium concentrations using the degree of dissociation?
For an electrolyte AB with concentration c and degree of dissociation α, each ion has equilibrium concentration cα, while the undissociated fraction has concentration c(1−α). These concentrations define the dissociation constant K, which governs the equilibrium state of weak electrolyte solutions.
Q4: What is the difference between weak and strong electrolytes in terms of dissociation?
Weak electrolytes like acetic acid partially dissociate, with α significantly less than one, allowing Ostwald's dilution law to apply. Strong electrolytes like NaCl and NaF are electrovalent compounds formed by electron transfer; they dissociate almost completely with α near one, making the law inapplicable to them.
Q5: How does the dielectric constant of a solvent affect electrolyte dissociation?
According to Coulomb's law, the dielectric constant of the medium affects electrostatic force strength and conductivity. Water's high dielectric constant weakens electrostatic forces between ions, promoting near-complete dissociation of strong electrolytes and enabling ion mobility for electrical conduction in solutions.
Q6: What is the simplified form of Ostwald's dilution law for very weak electrolytes?
For very weak electrolytes where α is much less than one, the term (1−α) approximates to one. The dissociation constant simplifies to K ≈ cα², making calculations more straightforward for solutions with minimal dissociation and allowing easier prediction of equilibrium behavior.
Q7: What structural difference exists between strong electrolytes and weak electrolytes?
Strong electrolytes like NaCl and NaF are electrovalent compounds composed solely of ions in their crystal structure, not molecules. When dissolved in water, these ions gain mobility and conduct electricity. Weak electrolytes exist as molecules that partially dissociate into ions upon dissolution in solution.