Ch 14 Test Review Questions #9-13

9. Why does the process of vaporization require an input of energy?

The intermolecular forces that hold the molecules together in the liquid state must be overcome.

10. Explain how the process of vaporization and condensation represent equilibrium in

    a closed container. Use a sketch to emphasize your point.

First the liquid phase begins to evaporate into the empty space. When the air space above the liquid becomes saturated with gas molecules, the gas molecules re-enter the liquid phase. There is constant movement between the liquid/gas phase.

11. How is the boiling point of a liquid related to the atmospheric pressure.

A liquid boils when its vapor pressure is equal to atmospheric pressure. As the temperature increases, the particles of the liquid are more likely to have enough energy to escape the liquid. However, this amount of energy is dependent on the pressure around the liquid. Thus, as the atmospheric pressure decreases, less energy is required and the boiling point of the liquid decreases.

12. Describe in detail 3 important types of crystalline solids and give an example of each.

Ionic - alternating positive Na⁺ and negative Cl^- ions. Each positive ion is surrounded by several negative ions, and each negative ion is surrounded by several positive ions. The electrostatic forces that develop in such an arrangement are very large, the resulting substance is very stable, and has high melting and boiling points.

Ice is a molecular solid. The crystals consist of polar water molecules arranged in three dimensions so as to maximize dipole-dipole interactions (hydrogen bonding). Figure 14.18c shows a representation of an ice crystal, showing how the negative end of one water molecule is oriented towards the positive end of another water molecule, and how this arrangement repeats. Since dipole-dipole forces are weaker than ionic bonding forces, substances that exist as molecular solids typically have much lower melting and boiling points.

Atomic solids vary as to how the atoms are held together in the crystal. Substances such as the noble gases are held together in the solid only by very weak London dispersion forces. Such substances have extremely low melting and boiling points because these forces are so weak. In other atomic solids, such as the diamond form of carbon, adjacent atoms may actually form covalent bonds with each other, causing the entire crystal to be one giant molecule. Such atomic solids have much higher boiling and melting points than those substances held together by only London forces. Finally, the metallic substances are also atomic solids, in which there are strong, but nondirectional, bonding leading to the properties associated with metals. Metals are envisioned in terms of the "electron" sea model in which a regular array of metal atoms exist in a "sea" of freely moving valence electrons.

13. List 3 alloys and the metals from which they are made.

sterling silver - silver and copper

brass - copper and zinc

steel - iron and carbon