CHM 2046C Sample Exam Answers

Module Eight Part II: Solutions Chapter 14

Part X: Discussion Questions Chapter 14 10 points

Write the answer to four of the following that are circled:

1. What is an ideal solution?

An ideal solution is a solution that obeys Raoult’s law! It is a solution in which the forces of the intermolecular attraction in the two components (solute and solvent) are very similar to one another and to the forces of attraction in the solution. Ideal solutions are therefore ones for which the enthalpy of solution is zero.

2. Why does the freezing point decrease, while the boiling point increases of a solvent upon addition of a solute to a solvent?

Freezing is a process in the direction of increased order; vaporization (boiling) is a process in the direction of increased disorder. The directions are opposite. A lower temperature is required to create order from a more disordered solution (compared to the pure solvent). A higher temperature is required to create the disordered vapor from a more disordered solution. From a molecular point of view, the solute particles in the solution interfere with the freezing and boiling process. At equilibrium, at the phase interface, the number of solvent molecules traveling in both directions must be equal. Because of the presence of the solute particles in liquid phase, the temperature must be lower at the freezing point, and must be higher at the boiling point, to achieve the equal rates.

3. Describe the terms isotonic, hypotonic, and hypertonic.

Isotonic: equal osmotic pressures (equal solute concentrations).

Hypotonic: The osmotic pressure is less (than a standard).

Hypertonic: The osmotic pressure is greater (than a standard).

4. What is a colloidal dispersion?

A colloidal dispersion is not a solution, nor is a suspension; it is somewhere between. If the solute particle is large in size (about 1 μm or larger) or large in mass (molar mass in the thousands), but not so large that the particles settle out, a colloidal dispersion is produced.

5. Summarize the different types of colloids and give an example of each.

Sol: a colloidal dispersion of a solid in a liquid.

E.g. milk of magnesia

Gel: a colloidal dispersion of a liquid in a solid.

E.g. jello

Aerosol: a colloidal dispersion of a liquid or solid in a gas:

E.g. fog or smoke

Foam: a colloidal dispersion of a gas in a liquid or solid.

E.g. whipped cream

Emulsion: a colloidal dispersion of one liquid in another liquid.

E.G. mayonnaise

6. Why is ammonia much more soluble than oxygen in water?

Ammonia interacts with water to much a greater extent than oxygen does. Hydrogen bonding between ammonia molecules and the water molecules lead to an exceptionally high solubility. A solution of ammonia in water is often referred to as ammonium hydroxide although the ammonia is present predominantly as ammonia molecules. The interaction between oxygen and water is only a dipole-induced attraction and its solubility is much less.

7. Define the terms miscible and immiscible.

Miscible means that the two liquids making up the solution can mix (usually in all proportions).

Immiscible means two liquids do not dissolve in one another (for example, oil and water are immiscible.)

8. What two factors are largely responsible for determining the magnitude and sign of the enthalpy of solution?

As in any reaction, the energy released or absorbed is a measure of the strength of the bonds that have to be broken, the strength of the bonds that are made. In a solution process, there are bonds in the solute and bonds in the solvent that need to be broken. Bonds are formed between the two components. For example, when sodium chloride dissolves in water, the lattice energy of the NaCl crystal must be supplied. In addition, some hydrogen bonds in the water are broken. The ions are solvated by the water; strong ion-molecule bonds are formed. Therefore the two factors largely responsible are: lattice energy of the solute and the salvation energy. The energy required or released is the difference between these two.