7.2
이오닉 협회
이온 결합은 전해질 용액 내에서 반대 전하를 띤 이온들이 이온 쌍을 형성하는 결합입니다. 비에럼은 이온 쌍을 계의 열에너지(일반적으로2kT)보다 정전기 인력이 초과하는 두 개의 반대 전하를 띤 이온으로 정의했다. 정전기 인력은 이온 전하, 분리 거리, 매질의 유전체 상…
강한 전해질은 종종 수용 내에서 완전히 자유 이온으로 존재한다고 가정합니다. 실제로는 반대 전하를 띤 이온이 부분적으로 결합해 이온 쌍을 형성할 수 있지만, NaCl이나 NaOH 같은 많은 1:1 전해질에서는 예외입니다.
비에럼은 이온 쌍을 서로 반대 전하를 띤 두 이온으로, 이들의 정전기적 인력이 열에너지(2kT)를 초과하는 것으로 정의했으며, 여기서 k는 볼츠만 상수, T는 절대 온도이다. 이 조건이 충족되면 이온 결합이 유리해집니다.
이온 쌍은 2:1 또는 2:2 전해질 비율처럼 이온 전하가 높아질수록 증가하여 낮은 농도에서도 상당한 비율의 이온 쌍이 형성됩니다. 이 예측은 실험적으로 뒷받침됩니다. 이온 쌍 내 양이온 비율을 몰리티와 비교했을 때, 결과는 비에럼의 이론과 매우 일치합니다.
또한 용매의 영향도 강하는데, 물의 유전체 상수가 높으면 정전기적 인력을 약화시키고 이온 쌍을 제한하는 반면, 유전체 상수가 낮은 용매는 이온-이온 인력을 증가시켜 1:1 전해질에서도 이온쌍 형성이 중요하게 만듭니다.
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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.