2.6:
Chemical and Solubility Equilibria
Recall that solution formation is regulated by the entropy of mixing, ΔSmixing. The associated free energy of mixing, ΔGmixing, is expressed by substituting ΔSmixing in the Gibbs free energy equation.
Since ΔSmixing is independent of intermolecular interactions, the energy changes associated with these interactions are neglected, and the value of ΔHmixing in the equation is 0. Thus, ΔGmixing is equal to negative T times ΔSmixing.
The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, ΔGsolution. Mathematically, it is expressed as the sum of the free energy of mixing, ΔGmixing, and the free energy of interaction, ΔGinteraction, between the component particles.
If the solvent–solute interactions cannot overcome the solute–solute and solvent–solvent interactions, ΔGinteraction is greater than 0.
If ΔGinteraction is large enough, the ΔGsolution is greater than zero. As ΔGsolution increases, the amount of solute that dissolves in a solvent decreases.
In contrast, if ΔGinteraction is negligible, the overall ΔGsolution equals ΔGmixing, and the solute dissolves completely in the solvent.
Consider dissolving pentane in hexane. Hexane–pentane attractions in the solution replace some of the pentane–pentane and hexane–hexane attractions in the pure liquid. Since both the molecules are of the same type, ΔGinteraction is close to 0, ΔGmixing dominates, and a solution is formed.
Solution formation depends on the balance between ΔGinteraction and ΔGmixing. A solution can form even if ΔGinteraction is positive, provided it’s not too high.
In short, solution formation is favorable if the ΔGsolution is less than 0, whereas it is unfavorable if the ΔGsolution is greater than 0.
2.6:
Chemical and Solubility Equilibria
The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, ΔGsolution. The overall ΔGsolution is expressed as the balance of ΔGinteraction against the always-favorable free-energy of mixing, ΔGmixing. Solution formation is favorable if ΔGsolution is less than zero, whereas it is unfavorable if ΔGsolution is greater than zero. In short, for a solution to form and complete dissolution to take place, the Gibbs energy change must be negative. The amount to which a solute dissolves in a solvent is governed by the strength of intermolecular forces between the components. This property of dissolution is termed as solubility.
In addition to the free energy of mixing, non-covalent interactions also contribute to solution formation. Substances that form hydrogen bonds are termed polar substances. In such cases, solute particles' dipole–dipole attractions with solvent particles are as strong as that between molecules in a pure solute or solvent. Hence, the two kinds of molecules mix easily. Likewise, nonpolar liquids are miscible with each other because there is no appreciable difference in the strengths of solute–solute, solvent–solvent, and solute–solvent intermolecular attractions. If a solute and solvent pair have intermolecular attractions similar to that in a pure liquid, ΔGinteraction is negligible. The overall ΔGsolution equals ΔGmixing, and the solute dissolves completely in the solvent. However, if two significantly different substances are mixed, the probability of solution formation depends on the balance between ΔGinteraction and ΔGmixing. They can form a solution even if ΔGinteraction is positive, provided it is not too high.