8.6
オストヴァルトの希釈法則
濃度「c」 を持ち、そのイオンに可逆的に解離する二元電解質ABを考えます。この解離の度合いは⍺で表されます。これは各イオン種の平衡濃度を⍺cで表せることを意味します。これに加えて、平衡状態で解離していない電解質の割合は(1−⍺)で与えられます。この解離されていない部分の対応する平衡濃度は(1−⍺)c…
アレニウスの電解解離理論に基づき、オストヴァルトの希釈則は水溶液中の平衡を説明します。
電解質ABを1リットルあたりの濃度で「c」molと考えると、可逆的にイオンに解離します。
αが解離度であれば、各イオンの平衡濃度は cαであり、解離されていない分画 (1−α) は c(1−α)の濃度を持ちます。これらの用語は平衡定数 K、すなわち解離定数を定義します。
オストヴァルトの希釈則は酢酸やNH₄OHのような弱い電解質にのみ適用されます。
HClやNaFのような強い電解質はほぼ完全に解離しており、そのαほぼ1です。これらの条件下では、 (1− α) はゼロに近づき、法則は数学的に無効となります。
強い電解質の法則が失敗するのは、溶解時に水の高い誘電率が静電気力を弱め、ほぼ完全な解離を引き起こすためです。その結果、イオン・分子平衡は存在せず、オストヴァルトの希釈則は適用されません。
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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.