The dissolution of a substance solute can be separated into three steps: The solute particles must separate from one another. The solvent particles must separate enough to make space for the solute molecules to come between them. The solute and solvent particles must interact to form the solution. The free energy G describes both the energetics i.
The enthalpy and entropy changes that occur in the dissolution process are shown in Figure 2, below. In the dissolution process, steps 1 and 2 listed above require energy because interactions between the particles solute or solvent are being broken. Step 3 usually releases energy because solute-solvent interactions are being formed.
In fact, for a large number of dissolution reactions, the entropic effect the change in randomness is more important than the enthalpic effect the change in energy in determining the spontaneity of the process. The figure on the left schematically shows the enthalpy changes accompanying the three processes that must occur in order for a solution to form: 1 separation of solute molecules, 2 separation of solvent molecules, and 3 interaction of solute and solvent molecules.
In general, if the solute and solvent interactions are of similar strength i. Therefore, the increase in entropy determines spontaneity in the process. However, if the solute and solvent interactions are of differing strength i. Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Aqueous Reactions. Search for:. Key Points Solubility is the relative ability of a solute to dissolve into a solvent.
Several factors affect the solubility of a given solute in a given solvent. If less than 1 gram of the substance will dissolve in milliliters or g of solvent, the substance is considered insoluble. If more than 10 grams of substance will dissolve in milliliters or g of solvent, the substance is considered soluble.
If between 1 and 10 grams of a substance will dissolve in milliliters or g of solvent, the substance is considered moderately soluble. Although it is difficult to determine specific solubilities without either finding them by experiment or referring to a table of solubilities, we do have guidelines that allow us to predict relative solubilities. Principal among these is. For example, this guideline could be used to predict that ethanol, which is composed of polar molecules, would be soluble in water, which is also composed of polar molecules.
Likewise, pentane C5H12 , which has nonpolar molecules, is miscible with hexane, which also has nonpolar molecules. We will use the Like Dissolve Like guideline to predict whether a substance is likely to be more soluble in water or in hexane. It can also be used to predict which of two substances is likely to be more soluble in water and which of two substances is likely to be more soluble in a nonpolar solvent, such as hexane: Polar substances are likely to dissolve in polar solvents.
For example, ionic compounds, which are very polar, are often soluble in the polar solvent water. Nonpolar substances are likely to dissolve in nonpolar solvents. For example, nonpolar molecular substances are likely to dissolve in hexane, a common nonpolar solvent. Nonpolar substances are not likely to dissolve to a significant degree in polar solvents.
For example, nonpolar molecular substances, like hydrocarbons, are likely to be insoluble in water. Polar substances are not likely to dissolve to a significant degree in nonpolar solvents.
For example, ionic compounds are insoluble in hexane. It is more difficult to predict the solubility of polar molecular substances than to predict the solubility of ionic compounds and nonpolar molecular substances.
Many polar molecular substances are soluble in both water and hexane. For example, ethanol is miscible with both water and hexane. The following generalization is helpful:. Substances composed of small polar molecules, such as acetone and ethanol, are usually soluble in water. They are also often soluble in hexane. The process of dissolving is a process which involves the breaking and making of bonds , and that involves energy.
From Hess's law we know that we can add the energies of each step in the cycle to determine the energy of the overall process. Mercury, a metal that is liquid at room temperature, can dissolve copper and gold, other types of metals.
Mercury adopted from Wikipedia. Of course things are more complicated for molecules that form different types of bonds. For example, alcohols contain a polar region that forms hydrogen bonds -OH and a non-polar carbon backbone. Hence, the solubility of alcohols in water is determined by the length of the non-polar carbon backbone — the longer it is, the less soluble the alcohol.
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