2.6
Équilibres chimiques et de solubilité
Le changement d'énergie libre associé à la dissolution d'un soluté dans un litre de solvant est appelé énergie libre d'une solution, ΔGsolution. La va…
Rappelons que la formation de la solution est régulée par l’entropie du mélange, ΔSmélange. L’énergie libre associée au mélange, le mélange ΔG, est exprimée en substituant le mélange ΔS dans l’équation de l’énergie libre de Gibbs.
Étant donné que le mélange ΔS est indépendant des interactions intermoléculaires, les changements d’énergie associés à ces interactions sont négligés, et la valeur du mélange ΔH dans l’équation est 0. Ainsi, le mélange ΔG est égal à moins T fois le mélange ΔS.
La variation d’énergie libre associée à la dissolution d’un soluté dans un litre de solvant est appelée énergie libre d’une solution, solution ΔG. Mathématiquement, elle s’exprime comme la somme de l’énergie libre de mélange, ΔG de mélange, et de l’énergie libre d’interaction, ΔG interaction, entre les particules composantes.
Si les interactions solvant-soluté ne peuvent pas surmonter les interactions soluté-soluté et solvant-solvant, l’interaction ΔG est supérieure à 0.
Si l’interaction ΔG est suffisamment grande, la solution ΔG est supérieure à zéro. À mesure que la solution de ΔG augmente, la quantité de soluté qui se dissout dans un solvant diminue.
En revanche, si l’interaction ΔG est négligeable, la solution globale de ΔG est égale au mélange de ΔG, et le soluté se dissout complètement dans le solvant.
Envisagez de dissoudre le pentane dans l’hexane. Les attractions hexane-pentane dans la solution remplacent certaines des attractions pentane-pentane et hexane-hexane dans le liquide pur. Comme les deux molécules sont du même type, l’interaction ΔG est proche de 0, le mélange ΔG domine et une solution se forme.
La formation de la solution dépend de l’équilibre entre l’interaction ΔG et le mélange ΔG. Une solution peut se former même si l’interaction ΔG est positive, à condition qu’elle ne soit pas trop élevée.
En bref, la formation d’une solution est favorable si la solution de ΔG est inférieure à 0, alors qu’elle est défavorable si la solution de ΔG est supérieure à 0.
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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.