7.2
イオニア協会
イオン結合とは、電解質溶液中の反対に帯電したイオンがイオン対を形成する結合体です。ビジェラムはイオン対を、系の熱エネルギー(通常2kT)を超える静電気引力を持つ2つの反対に帯電したイオンと定義しました。静電気引力はイオン電荷、分離距離、媒質の誘電率に依存します。熱エネルギーは kTで表され、分子の運…
強い電解質はしばしば水溶液中の自由イオンとして完全に存在すると仮定されます。実際には、反対に帯電したイオンは部分的に結合してイオン対を形成できますが、NaClやNaOHのような1:1の電解質では例外です。
ビェルムはイオン対を、電的引力が熱エネルギーを上回るほど近い2つの反対に帯電したイオン(kはボルツマン定数、Tは絶対温度)と定義しました。この条件を満たすと、イオン結合は有利になります。
イオンペアリングはイオン電荷が高まるほど増加し、例えば2:1や2:2の電解質では、低濃度でもイオン対の割合がかなり高まります。この予測は実験的に支持されています。イオン対中のカチオンの割合をモラリティと比較すると、結果はビジェラムの理論と非常に一致します。
また、溶媒の影響も強く受けます。水の誘電率が高いため静電気引力が弱まりイオン対が制限される一方で、誘電率が低い溶媒はイオン間引力を強めるため、1:1の電解質でもイオン対の形成が重要になります。
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Q1: What is an ion pair according to Bjerrum's definition?
An ion pair consists of two oppositely charged ions positioned close enough that their electrostatic attraction exceeds the thermal energy, quantified as 2kT, where k is Boltzmann's constant and T is absolute temperature. When this condition is met, the ions remain associated rather than freely dissolved in solution.
Q2: How does ionic charge affect ion pair formation?
Ion pairing increases significantly with higher ionic charges, such as in 2:1 or 2:2 electrolytes, leading to substantial ion-pair fractions even at low concentrations. In contrast, 1:1 electrolytes like NaCl show minimal ion pairing in aqueous solutions due to weaker electrostatic interactions between singly charged ions.
Q3: Why does water limit ion pair formation compared to other solvents?
Water's high dielectric constant weakens electrostatic attraction between oppositely charged ions, reducing ion-pair formation. Solvents with lower dielectric constants are less effective at stabilizing separated charges, resulting in stronger electrostatic attraction and increased ion pairing even for 1:1 electrolytes.
Q4: How does ionic association affect electrical conductivity?
Ionic association reduces electrical conductivity because ions form associated species like CaSO₄ and MgF₂, decreasing the number of free charge carriers in solution. The extent of association can be estimated from conductivity measurements, with association becoming significant in concentrated solutions and negligible at infinite dilution.
Q5: What is the difference between ion pairs and complex ions?
Ion pairs are held together by electrostatic forces and often retain part of their solvent shells, whereas complex ions involve bonds with significant covalent character, commonly forming in transition-metal salt solutions. Absorption spectroscopy can distinguish between these species, and solutions may contain both types simultaneously.
Q6: How do temperature and molality influence ionic association?
Both temperature and molality affect the extent of ionic association in solution. At infinite dilution, the degree of association approaches zero, while association becomes significant in concentrated solutions. Higher temperatures increase thermal energy, reducing favorable ion-pair formation conditions.
Q7: How does Bjerrum's theory compare to experimental observations of ion pairing?
Bjerrum's theory predictions closely match experimental data when the percentage of cations in ion pairs is plotted against molality. This agreement validates the theoretical framework that electrostatic attraction must exceed thermal energy for ion-pair formation, supporting the Debye–Hückel Theory of Electrolyte Solutions as a foundation for understanding nonideal behavior.