2.6
化学平衡論と溶解度平衡
1 リットルの溶媒に溶質を溶解する際の自由エネルギー変化は、溶液の自由エネルギー（ΔG_solution）と呼ばれます。 全体的な ΔG_solution は、常に有利な混合自由エネルギー ΔG_mixing に対する ΔG_interaction のバランスとして表されます。 ΔG_solutio…
溶液の形成は、混合のエントロピー、ΔS混合によって調節されることを思い出してください。撹拌の関連自由エネルギーであるΔG撹拌は、ギブス自由エネルギー方程式にΔS撹拌を代入することによって表されます。
ΔSの混合は分子間相互作用に依存しないため、これらの相互作用に関連するエネルギー変化は無視され、方程式のΔHの混合の値は0です。したがって、ΔGの混合は、負のT×ΔSの混合に等しくなります。
溶質を1リットルの溶媒に溶解することに伴う自由エネルギーの変化は、溶液ΔG溶液の自由エネルギーと呼ばれます。数学的には、成分粒子間の混合の自由エネルギーであるΔGの混合と相互作用の自由エネルギーであるΔGの相互作用の合計として表されます。
溶媒-溶質相互作用が溶質-溶質および溶媒-溶媒相互作用に打ち勝つことができない場合、ΔG相互作用は0より大きくなります。
ΔGの交互作用が十分に大きい場合、ΔGの解はゼロより大きくなります。ΔG溶液が増加すると、溶媒に溶解する溶質の量が減少します。
対照的に、ΔGの相互作用が無視できるほど小さい場合、全体のΔG溶液はΔGの混合に等しくなり、溶質は溶媒に完全に溶解します。
ペンタンをヘキサンに溶かすことを検討してください。溶液中のヘキサン-ペンタン引力は、純粋な液体中のペンタン-ペンタンおよびヘキサン-ヘキサン引力の一部を置き換えます。両方の分子が同じタイプであるため、ΔG相互作用は0に近く、ΔG混合が支配的であり、溶液が形成されます。
溶液の形成は、ΔG相互作用とΔG混合のバランスに依存します。ΔG相互作用が正の場合でも、高すぎない限り、解が形成されます。
要するに、ΔGの解が0より小さい場合は溶液の形成が有利であり、ΔGの解が0より大きい場合は好ましくありません。
View the full transcript and gain access to JoVE Core videos
Q1: What is the free energy of a solution and how does it determine whether a solute dissolves?
The free energy of a solution, ΔGsolution, represents the total free energy change when a solute dissolves in a solvent. It combines the free energy of mixing, ΔGmixing, and the free energy of interaction, ΔGinteraction, between component particles. Solution formation is favorable when ΔGsolution is less than zero, allowing the solute to dissolve. When ΔGsolution is greater than zero, dissolution is unfavorable and the solute remains insoluble.
Q2: How do intermolecular interactions affect whether two substances will mix to form a solution?
Solution formation depends on the balance between ΔGinteraction and ΔGmixing. If solvent-solute interactions cannot overcome solute-solute and solvent-solvent interactions, ΔGinteraction becomes positive and large, making ΔGsolution positive and preventing dissolution. However, if ΔGinteraction is negligible, the overall ΔGsolution equals ΔGmixing, and the solute dissolves completely in the solvent.
Q3: Why do nonpolar liquids mix easily with each other?
Nonpolar liquids are miscible because there is no appreciable difference in the strengths of solute-solute, solvent-solvent, and solute-solvent intermolecular attractions. Since the intermolecular forces are similar across all interactions, ΔGinteraction is negligible. The free energy of mixing dominates, making ΔGsolution negative and allowing complete dissolution and mixing.
Q4: What role does entropy of mixing play in solution formation?
Solution formation is regulated by the entropy of mixing, ΔSmixing, which is independent of intermolecular interactions. The free energy of mixing, ΔGmixing, equals negative T times ΔSmixing. Since ΔSmixing is always favorable, ΔGmixing is always negative and drives solution formation. This entropy effect can overcome positive ΔGinteraction values, provided they are not too large.
Q5: How do polar substances with hydrogen bonding behave when mixed together?
Polar substances that form hydrogen bonds mix easily because solute particles' dipole-dipole attractions with solvent particles are as strong as attractions between molecules in pure solute or solvent. Since the intermolecular forces are comparable, ΔGinteraction is negligible, and the two kinds of molecules readily combine. The free energy of mixing dominates, making solution formation favorable.
Q6: Can two significantly different substances form a solution even if their interaction free energy is positive?
Yes, two significantly different substances can form a solution even if ΔGinteraction is positive, provided it is not too high. The always-favorable free energy of mixing, ΔGmixing, can offset a moderate positive ΔGinteraction. Solution formation depends on the overall balance between these two terms; if the combined ΔGsolution remains negative, dissolution occurs.
Q7: What happens when pentane dissolves in hexane, and why is this a favorable process?
When pentane dissolves in hexane, hexane-pentane attractions in the solution replace some pentane-pentane and hexane-hexane attractions in pure liquids. Since both molecules are similar in type, ΔGinteraction is close to zero, and ΔGmixing dominates. The overall ΔGsolution becomes negative, making the solution formation favorable and resulting in complete miscibility.