Kaplan MCAT General Chemistry Review

In covered or closed container, the escaping molecules are trapped above the solution. These molecules exert a countering pressure, which forces some of the gas back into the liquid phase; this process is called condensation. Condensation is facilitated by lower temperature or higher pressure. Atmospheric pressure acts on a liquid in a manner similar to that of an actual physical lid. As evaporation and condensation proceed, the respective rates of the two processes become equal, and equilibrium is reached. The pressure that the gas exerts over the liquid at equilibrium is the vapor pressure of the liquid. Vapor pressure increases as temperature increases, since more molecules have sufficient kinetic energy to escape into the gas phase. The temperature at which the vapor pressure of the liquid equals the ambient (also known as external, applied, or atmospheric) pressure is called the boiling point: At boiling, vaporization happens throughout the entire volume of the liquid, not just near its surface. The bubbles of gas that rise from your pot of boiling pasta water are gaseous water molecules, plus the small amounts of atmospheric gases (mostly nitrogen and oxygen gas) that had been dissolved in the water.

LIQUID-SOLID EQUILIBRIUM

We’ve already illustrated the equilibrium that can exist between the liquid and the solid phases of water at 0°C. Even though the atoms or molecules of a solid are confined to definite locations, each atom or molecule can undergo motions about some equilibrium position. These vibrational motions increase when heat is applied. From our understanding of entropy, we can say that the availability of energy microstates increases as the temperature of the solid increases. In basic terms, this means that the molecules have greater freedom of movement, and energy disperses. If atoms or molecules in the solid phase absorb enough energy, the three-dimensional structure of the solid will break down, and the atoms or molecules will escape into the liquid phase. The transition from solid to liquid is called fusion or melting. The reverse process, from liquid to solid, is called solidification, crystallization, or freezing. The temperature at which these processes occur is called the melting point or freezing point, depending on the direction of the transition. Whereas pure crystalline solids have distinct, very sharp melting points, amorphous solids, such as glass, plastic, and candle wax, tend to melt (or solidify) over a larger range of temperatures due to their less-ordered molecular distribution. One of the surest signs that a dinner party has come to its natural and necessary conclusion is when the dinner guests start playing with the liquid wax dripping down the side of the candlesticks, allowing it to cool and solidify on their fingertips.

GAS-SOLID EQUILIBRIUM

The final phase equilibrium is that which exists between the gas and solid phase. When a solid goes directly into the gas phase, the process is called sublimation. Dry ice (solid CO₂) sublimes at room temperature and atmospheric pressure; the absence of the liquid phase makes it a convenient refrigerant and a fun addition to punch bowls at parties. The reverse transition, from the gaseous to the solid phase, is called deposition. In organic chemistry, a device known as the cold finger (the name sounds more malicious than it actually is) is used to purify a product that is heated under reduced pressure to cause it to sublimate. The desired product is usually more volatile than the impurities, so the gas is purer than the original product and the impurities are left in the solid state. The gas then deposits onto the cold finger, which has cold water flowing through it, yielding a purified solid product that can be collected. Another common instance of sublimation is “freezer burn” on meats and vegetables that are stored for long periods in the freezer. The frozen water in the meat or vegetables will slowly sublimate over time, and what is left is fit only for the garbage. On the other hand, sublimation is used to produce freeze-dried foods, such as coffee, “emergency survival meals,” and perhaps most (in)famously, astronaut ice cream. The crackly frost of late autumn mornings in colder climates is formed by deposition of water vapor in the air.

THE GIBBS FUNCTION

As with all equilibria, the thermodynamic criterion for each of the phase equilibria is that the change in Gibbs free energy must be equal to zero (G = 0). For an equilibrium between a gas and a solid,

G = G(g) - G(s) = 0

Therefore,

G(g) = G(s)

The same is true of the Gibbs functions for any other phase equilibria.

HEATING CURVES