2.6
化学平衡和溶解平衡
将溶质溶解在一升溶剂中所伴随的自由能变化称为溶液的自由能，ΔGsolution。 总体 ΔGsolution 表示为 ΔGinteraction 与始终有利的混合自由能 ΔGmixing 之间的平衡。 如果 ΔGsolution 小于零，则溶液形成是有利的，而如果 ΔGsolution 大于零，则不…
请记住，溶液的形成受混合熵 ΔSmixing 的调控。相应的混合自由能 ΔGmixing 可通过将 ΔSmixing 代入吉布斯自由能方程得到。
由于ΔSmixing与分子间相互作用无关，因此与这些相互作用相关的能量变化被忽略，方程中ΔHmixing的值为0。因此，ΔGmixing等于负的T乘以ΔSmixing。
将溶质溶解于一升溶剂中所伴随的自由能变化称为溶液的自由能，ΔGsolution。从数学上讲，它表示为混合自由能ΔGmixing与组分粒子间相互作用自由能ΔGinteraction之和。
如果溶剂-溶质相互作用无法克服溶质-溶质和溶剂-溶剂相互作用，则ΔGinteraction大于0。
如果 ΔGinteraction 足够大，则 ΔGsolution 大于零。随着 ΔGsolution 增大，溶质在溶剂中溶解的量减少。
相反，如果 ΔGinteraction 可忽略不计，则总 ΔGsolution 等于 ΔGmixing，溶质将在溶剂中完全溶解。
考虑将戊烷溶解于己烷中。溶液中的己烷-戊烷相互作用取代了部分纯液体中的戊烷-戊烷和己烷-己烷相互作用。由于两种分子属于同类型，ΔGinteraction 接近于 0，因此 ΔGmixing 占主导地位，从而形成溶液。
溶液的形成取决于Δ之间的平衡G相互作用 和 ΔG混合即使 ΔG相互作用 呈阳性，前提是数值不过高。
简言之，若溶液形成的ΔGsolution小于0，则该过程有利；若ΔGsolution大于0，则该过程不利。
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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.