Kaplan MCAT General Chemistry Review

In this chapter, we have reviewed the important concepts and calculations related to the condensed phases of the solid and liquid states of matter. For solids, we learned that the organization of the particles is either in a three-dimensional lattice formation, producing a crystalline structure, or in a less-ordered arrangement described as amorphous. Ionic and metallic solids have crystalline structure, while glass, plastic, and candle wax have amorphous structure. The particles that make up a crystalline structure can be organized in many ways; the basic repeating unit of that organization is called the unit cell. We reviewed the three cubic unit cells. Liquids, like gases, are defined by their ability to flow in response to shearing forces. Liquids that can mix together are called miscible, while those that repel each other and separate into different layers, like oil and water, are called immiscible. We examined the equilibria that exist between the different phases and noted that the change in Gibbs function for each phase change in equilibrium is zero, as is the case for all equilibria. Finally, we examined the colligative properties of solutions and the mathematics that govern them. The colligative properties—vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure—are physical properties of solutions that depend upon the concentration of dissolved particles but not upon their chemical identity.

We concluded this chapter with an overview of the colligative properties of solutions; the next chapter will continue the review of the behaviors and characteristics of solutions and the mathematics of solution chemistry that will earn you points on Test Day.

CONCEPTS TO REMEMBER

In the solid and liquid phases, the atoms, ions, or molecules are sufficiently condensed to allow the intermolecular forces, such as van der Waals, dipole–dipole, and hydrogen bonds, to hold the particles together and restrict their degrees of freedom of movement.

Solids are defined by their rigidity (ability to maintain a shape independent of a container) and resistance to flow.

The molecular arrangement in solids can be either crystalline or amorphous. Crystalline structure is a three-dimensional lattice arrangement of repeating units called the unit cell. Amorphous solids lack this lattice arrangement.

Liquids are defined as fluids because they flow in response to shearing forces and assume the shape of their container.

Liquids like water and alcohol will mix together and are called miscible; liquids like water and oil will not mix together and are called immiscible. Agitation of immiscible liquids will result in an emulsion.

Phase equilibria will exist at certain temperatures and pressures for each of the different phase changes: fusion (crystallization), vaporization (condensation), and sublimation (deposition).

The change in Gibbs free energy for phase equilibria is zero.

The phase diagram of a given system graphs the phases and phase equilibria as a function of temperature and pressure.

The phase diagram for a solution consisting of multiple components indicates the composition of the liquid and the vapor at different temperatures and pressures.

The colligative properties—vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure—are physical properties of solutions that depend upon the concentration of dissolved particles but not upon their chemical identity.

EQUATIONS TO REMEMBER

P_A = X_A°P_A

T_b = iK_bm

T_f = iK_fm

= iMRT

Practice Questions

1. Which of the following substances is illustrated by the phase diagram in the figure below?

A. CO₂

B. NaCl

C. Ne

D. H₂O

2. Which of the following proportionalities best describes the relationship between the number of intermolecular forces and heat of vaporization for a given substance?

A. They are proportional.

B. They are inversely proportional.

C. Their relationship cannot be generalized.

D. There is no relationship between them.

3. Which of the following molecules is likely to have the highest melting point?

4. Which of the following physical conditions favors a gaseous state for most substances?

A. High pressure and high temperature

B. Low pressure and low temperature

C. High pressure and low temperature

D. Low pressure and high temperature

5. Which of the following explanations best describes the mechanism by which solute particles affect the melting point of ice?

A. Melting point elevates because the kinetic energy of the substance increases.

B. Melting point elevates because the kinetic energy of the substance decreases.

C. Melting point depresses because solute particles interfere with lattice formation.

D. Melting point depresses because solute particles enhance lattice formation.

6. In the figure below, which phase change is represented by the arrow?

A. Condensation

B. Deposition

C. Sublimation

D. Vaporization