Kaplan MCAT General Chemistry Review

Curves such as this show the different compositions of the liquid phase and the vapor phase above a solution for different temperatures. The upper curve is the composition of the vapor, while the lower curve is that of the liquid. It is this difference in composition that forms the basis of distillation, an important separation technique in organic chemistry. For example, if we were to start with a mixture of A and B at a proportion of 40 percent A and 60 percent B and heat it to that solution’s boiling point (85°C), the resulting vapor would not have the same composition as the liquid solution because the two compounds have different volatilities. Compound B is more volatile because it has the lower boiling point. Therefore, vapor B should be in larger proportion to vapor A, compared to the proportion of B to A in the liquid phase. Because the boiling point temperature for this 60–40 mixture is 85°C, the vapor will also be at 85°C. At this temperature, we can tell from the graph that the vapor composition will be 30 percent A and 70 percent B. Indeed, the proportion of the more volatile compound, in this case compound B, has been enhanced. Repeated rounds of boiling (vaporization) and condensation will ultimately yield a pure sample of compound B.

MCAT Expertise

On the MCAT, you should be able to identify and understand each area and every line of a phase diagram.

Colligative Properties

All the way back at the start of this book, we suggested that adding salt to water will yield a solution whose boiling point is higher than that of the pure water. While this is a true statement, we also suggested that the quantity of salt that is normally added to a pot of cooking water is not sufficient to cause a significant rise in the boiling point or a significant decrease in the cooking time. In culinary practice, adding salt to your cooking water merely (but importantly) contributes to the flavor of the food. The measurable change in boiling point of a solution compared to that of the pure solvent is one of the colligative properties of solutions. The colligative properties are physical properties of solutions that are dependent upon the concentration of dissolved particles but not upon the chemical identity of the dissolved particles. These properties—vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure—are usually associated with dilute solutions (see Chapter 9, Solutions).

VAPOR PRESSURE DEPRESSION

When you add solute to a solvent and the solute dissolves, the solvent in solution has a vapor pressure that is lower than the vapor pressure of the pure solvent for all temperatures. For example, consider compound A in Figure 8.7. Compound A in its pure form (mole fraction = 1.0) has a boiling point of 100°C. Based on this information alone, we can assume that compound A is water. Compound B, which is more volatile than water and boils in pure form (mole fraction = 1.0) at around 80°C could be ethanol, which has a boiling point of 78.3°C. When a small amount of ethanol is added to water to create a dilute solution, say 90 percent water and 10 percent alcohol, the boiling point is around 95°C, and the vapor composition above the solution will be about 80 percent water and 20 percent ethanol. The relative decrease in the proportion of water in the vapor above the dilute water-alcohol solution is related to the decrease in the vapor pressure of water above the solution.

Key Concept

This goes hand in hand with boiling point elevation. The lowering of a solution’s vapor pressure would mean that a higher temperature is required to overcome atmospheric pressure, thereby raising the boiling point.

If the vapor pressure of A above pure solvent A is designated by and the vapor pressure of A above the solution containing B is P_A, the vapor pressure decreases as follows:

In the late 1800s, the French chemist François Marie Raoult determined that this vapor pressure decrease is also equivalent to

where X_B is the mole fraction of the solute B in solution with solvent A. Because X_B = 1 - X_A and substitution into the previous equation leads to the common form of Raoult’s law:

where X_A is the mole fraction of the solvent A in the solution. Similarly, the expression for the vapor pressure of the solute in solution (assuming it is volatile) is given by:

Raoult’s law holds only when the attraction between the molecules of the different components of the mixture is equal to the attraction between the molecules of any one component in its pure state. When this condition does not hold, the relationship between mole fraction and vapor pressure will deviate from Raoult’s law. Solutions that obey Raoult’s law are called ideal solutions.

BOILING POINT ELEVATION