Kaplan MCAT General Chemistry Review

Nature of Solutions

Many important chemical reactions, both in the laboratory and in nature, take place in solution, including almost all reactions in living organisms. Solutions are homogenous (the same throughout) mixtures of two or more substances that combine to form a single phase, usually the liquid phase. The MCAT will focus generally on solids dissolved into liquid aqueous solutions, but it’s important to remember that solutions can be formed from different combinations of the three phases of matter. For example, gases can be dissolved in liquids (e.g., the carbonation of soda); liquids can be dissolved in other liquids (e.g., ethanol in water); solids can even be dissolved in other solids (e.g., metal alloys). Incidentally, gases “dissolved” into other gases can be thought of as solutions but are more properly defined as mixtures, because gas molecules don’t really interact all that much (one of the postulates of the kinetic molecular theory of gases). Just a point of minor clarification: All solutions are considered mixtures, but not all mixtures are considered solutions.

A solution consists of a solute (e.g., NaCl, NH₃, C₆H₁₂O₆, CO₂, etc.) dissolved (dispersed) in a solvent (e.g., H₂O, benzene, ethanol, etc.). The solvent is the component of the solution whose phase remains the same after mixing. If the two substances are already in the same phase (for example, a solution of two liquids), the solvent is the component present in greater quantity. And if the two same-phase components are in equal proportions in the solution, the component considered the solvent is the one that is more commonly identified as a solvent. Solute molecules move about freely in the solvent and interact with the solvent by way of interparticle forces such as ion–dipole, dipole–dipole, or hydrogen bonding. Dissolved solute molecules are also relatively free to interact with other dissolved molecules of different chemical identity; consequently, chemical reactions occur easily in solution.

SOLVATION

Not to be confused with the path to eternal life, solvation is the electrostatic interaction between solute and solvent molecules. This is also known as dissolution, and when water is the solvent, it is called hydration, and the resulting solution is an aqueous solution. Solvation involves breaking intermolecular interactions between solute molecules and between solvent molecules and forming new intermolecular interactions between solute and solvent molecules.

When the new interactions (attractions) are stronger than the original ones, solvation is exothermic, and the process is favored at low temperatures. The dissolution of gases into liquids, such as CO₂ into water, is an exothermic process because the only significant interactions that must be broken are those between water molecules. CO₂, as a gas, demonstrates minimal intermolecular interaction, and thus the dissolution of CO₂ gas into water is overall exothermic (and Le Châtelier’s principle tells us this is the reason that lowering the temperature of a liquid favors solubility of a gas in the liquid.) This is also why opening a two-liter container of warm soda is risky: The warm soda has very low solubility for the CO₂ gas, and when the pressure is released upon twisting the cap open, a lot of the CO₂ gas escapes (precipitates, comes out of the solution) with some amount of violence, carrying with it a lot of the liquid itself. We’ve all seen—or suffered—the aftermath of a warm soda too hastily opened.

When the new interactions (attractions) are weaker than the original ones, solvation is endothermic, and the process is favored at high temperatures. Most dissolutions are of this type. Two such examples have already been given: dissolving sugar or ammonium nitrate into water. Since the new interactions between the solute and solvent are weaker than the original interactions between the solute molecules and between the solvent molecules, energy (heat) must be supplied to facilitate the formation of these weaker, less stable interactions. Sometimes the overall strength of the new interactions is approximately equal to the overall strength of the original interactions. In this case, the overall enthalpy change for the dissolution is close to zero. These types of solutions approximate the formation of an ideal solution, for which the enthalpy of dissolution is equal to zero.