![]() What this means is that a greater volume of solvent will be required to completely dissolve a given mass of solute. In such a case, we may describe the solute as being slightly soluble in a certain solvent. Even if the energetics are slightly endothermic, the entropy effect can still allow the solution to form, although perhaps limiting the maximum concentration that can be achieved. If the energetics of dissolution are favorable, this increase in entropy means that the conditions for solubility will always be met. This is the same as saying that the entropy of the solute increases. Thus in the very common case in which a small quantity of solid or liquid dissolves in a much larger volume of solvent, the solute becomes more spread out in space, and the number of equivalent ways in which the solute can be distributed within this volume is greatly increased. If you are not there yet, do not despair you are hereby granted temporary permission to think of molecular "disorder" and entropy simply in terms of "spread-outedness". A proper understanding of these considerations requires some familiarity with thermodynamics, which most students do not encounter until well into their second semester of Chemistry. Readers of this section who have had some exposure to thermodynamics will know that solubility, like all equilibria, is governed by the Gibbs free energy change for the process, which incorporates the entropy change at a fundamental level. Chemists use the term "entropy" to denote this aspect of molecular randomness. However, in doing so, the thermal energy they carry with them is also spread and dispersed, so the availability of this energy, as measured by the temperature, is also of importance. The numbers of objects (molecules) we deal with in Chemistry is so huge that their tendency to become as spread out as possible becomes overwhelming. And as the number of objects increases, the more does statistics govern their most likely arrangements. Even if the dissolution process is slightly endothermic, there is a third important factor, the entropy increase, that will very often favor the dissolved state.Īs anyone who has shuffled a deck of cards knows, disordered arrangements of objects are statistically more favored simply because there are more ways in which they can be realized. If step 2 releases more energy than is consumed in step 1, this will favor solution formation, and we can generally expect the solute to be soluble in the solvent. If the solute is A and the solvent is B, then what is important is the strength of the attractive forces between A-A and B-B molecules, compared to those between A-B pairs if the latter are greater, then the potential energy will be lower when the substances are mixed and solution formation will be favored. Whether this is energetically favorable or unfavorable depends on the nature of the solute and solvent. The solute must then be introduced into the solvent.This requires energy, and so this step always works against solution formation. If the solute is a solid or liquid, it must first be dispersed - that is, its molecular units must be pulled apart.To see how these considerations are applied to solutions, think about the individual steps that must be carried out when a solute is dissolved in a solvent: This is a general principle that applies throughout the world of matter the stable form at any given temperature will always be that which leads to the best balance between low potential energy and high molecular disorder. ![]() You may recall that in the earlier unit on phase equilibria, we pointed out that aggregations of molecules that are more disordered tend to be the ones that are favored at higher temperature, whereas those that possess the lowest potential energy are favored at lower temperatures. What do we mean when we describe a liquid such as water as "associated"? Explain how this relates to the the solubility of solutes in such liquids. ![]() What is the principal physical property of a molecule that defines this "likeness"?
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