Kaplan MCAT General Chemistry Review

One of the most important properties of liquids is their ability to mix—both with each other and with other phases—to form solutions (see Chapter 9, Solutions). The degree to which two liquids can mix is called their miscibility. While ethanol and water are completely miscible, oil and water are almost completely immiscible; that is, their molecules tend to repel each other due to their polarity differences. You’re certainly familiar with the expression “Like dissolves like.” Oil and water normally form separate layers when mixed, with the oil layer above the water because it is less dense. Organic chemistry takes advantage of the solubility differences of immiscible liquids to separate compounds through the method of liquid-liquid extraction. Agitation of two immiscible liquids can result in the formation of a fairly homogenous mixture called an emulsion. Although they look like solutions, emulsions are actually mixtures of discrete particles too small to be seen distinctly. Shaking a cruet of extra virgin olive oil and balsamic vinegar, seasoned with sea salt and fresh ground pepper, makes for a simple but delicious emulsion for your mixed baby greens salad.

Phase Equilibria

In an isolated system, phase changes (solid liquid gas) are reversible, and equilibrium of phases will eventually be reached. For example, at 1 atm and 0°C in an isolated system, an ice cube and the water in which it is floats are in equilibrium. In other words, some of the ice may absorb heat (from the liquid water) and melt, but since that heat is being removed from the liquid water, an equal amount of the liquid water will freeze and form ice. Thus, the relative amounts of ice and water remain constant. Equilibrium between the liquid and gas states of water will be established in a closed container, such as a plastic water bottle with the cap screwed on tightly. At room temperature and atmospheric pressure, most of the water in the bottle will be in the liquid phase, but a small number of molecules at the surface will gain enough kinetic energy to escape into the gas phase; likewise, a small number of gas molecules will lose sufficient kinetic energy to re-enter the liquid phase. After a while, equilibrium is established, and the relative amounts of water in the liquid and gas phases become constant—at room temperature and atmospheric pressure, equilibrium occurs when the air above the water has about 3 percent humidity. Phase equilibria are analogous to the dynamic equilibria of reversible chemical reactions for which the concentrations of reactants and products are constant because the rates of the forward and reverse reactions are equal.

Key Concept

As with all equilibria, the rates of the forward and reverse processes will be the same.

GAS-LIQUID EQUILIBRIUM

The temperature of any substance in any phase is related to the average kinetic energy of the molecules that make up the substance. However, as we saw in Chapter 7, not all the molecules have exactly the same instantaneous speeds. Therefore, the molecules possess a range of instantaneous kinetic energy values. In the liquid phase, the molecules have relatively large degrees of freedom of movement. Some of the molecules near the surface of the liquid may have enough kinetic energy to leave the liquid phase and escape into the gaseous phase. This process is known as evaporation (or vaporization). Each time the liquid loses a high-energy particle, the temperature of the remaining liquid decreases. Evaporation is an endothermic process for which the heat source is the liquid water. Of course, the liquid water itself may be receiving thermal energy from some other source, as in the case of a puddle of water drying up under the hot summer sun or a pot of water on the stove-top. Given enough energy, the liquid will completely evaporate.