2.6
Equilibrio químico y de solubilidad
El cambio de energía libre asociado con la disolución de un soluto en un litro de disolvente es llamada energía libre de una solución, ΔG_solución . L…
Recordemos que la formación de la solución está regulada por la entropía de la mezcla, la mezcla ΔS. La energía libre asociada a la mezcla, la mezcla ΔG, se expresa sustituyendo la mezcla ΔS en la ecuación de energía libre de Gibbs.
Dado que la mezcla de ΔS es independiente de las interacciones intermoleculares, los cambios de energía asociados con estas interacciones se descuidan, y el valor de la mezcla de ΔH en la ecuación es 0. Por lo tanto, la mezcla ΔG es igual a T negativo por la mezcla ΔS.
El cambio de energía libre asociado con la disolución de un soluto en un litro de solvente se denomina energía libre de una solución, solución ΔG. Matemáticamente, se expresa como la suma de la energía libre de mezcla, mezcla ΔG, y la energía libre de interacción, interacción ΔG, entre las partículas componentes.
Si las interacciones solvente-soluto no pueden superar las interacciones soluto-soluto y solvente-solvente, la interacción ΔG es mayor que 0.
Si la interacción ΔG es lo suficientemente grande, la solución ΔG es mayor que cero. A medida que aumenta la solución de ΔG, la cantidad de soluto que se disuelve en un solvente disminuye.
Por el contrario, si la interacción ΔG es insignificante, la solución total de ΔG es igual a la mezcla de ΔG y el soluto se disuelve completamente en el disolvente.
Considera disolver el pentano en hexano. Las atracciones de hexano-pentano en la solución reemplazan algunas de las atracciones de pentano-pentano y hexano-hexano en el líquido puro. Dado que ambas moléculas son del mismo tipo, la interacción ΔG es cercana a 0, domina la mezcla ΔG y se forma una solución.
La formación de la solución depende del equilibrio entre la interacción ΔG y la mezcla ΔG. Se puede formar una solución incluso si la interacción ΔG es positiva, siempre que no sea demasiado alta.
En resumen, la formación de la solución es favorable si la solución ΔG es menor que 0, mientras que es desfavorable si la solución ΔG es mayor que 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.