Chemistry (Chang), 9th Edition

Chapter 5: Gases

Chapter Summary

1. At 25^°C and 1 atm, a number of elements and molecular compounds exist as gases. Ionic compounds are solids rather than gases under atmospheric conditions.

2. Gases exert pressure because their molecules move freely and collide with any surface with which they make contact. Units of gas pressure include millimeters of mercury (mmHg), torr, pascals, and atmospheres. One atmosphere equals 760 mmHg, or 760 torr.

3. The pressure-volume relationships of ideal gases are governed by Boyle’s law: Volume is inversely proportional to pressure (at constant T and n).

4. The temperature-volume relationships of ideal gases are described by Charles’s and Gay-Lussac’s law: Volume is directly proportional to temperature (at constant P and n).

5. Absolute zero (273.15^°C) is the lowest theoretically attainable temperature. The Kelvin temperature scale takes 0 K as absolute zero. In all gas law calculations, temperature must be expressed in kelvins.

6. The amount-volume relationships of ideal gases are described by Avogadro’s law: Equal volumes of gases contain equal numbers of molecules (at the same T and P).

7. The ideal gas equation, PV = nRT, combines the laws of Boyle, Charles, and Avogadro. This equation describes the behavior of an ideal gas.

8. Dalton’s law of partial pressures states that each gas in a mixture of gases exerts the same pressure that it would if it were alone and occupied the same volume.

9. The kinetic molecular theory, a mathematical way of describing the behavior of gas molecules, is based on the following assumptions: Gas molecules are separated by distances far greater than their own dimensions, they possess mass but have negligible volume, they are in constant motion, and they frequently collide with one another. The molecules neither attract nor repel one another.

10. A Maxwell speed distribution curve shows how many gas molecules are moving at various speeds at a given temperature. As temperature increases, more molecules move at greater speeds.

11. Gas diffusion demonstrates random molecular motion.

12. The van der Waals equation is a modification of the ideal gas equation that takes into account the nonideal behavior of real gases. It corrects for the fact that real gas molecules do exert forces on each other and that they do have volume. The van der Waals constants are determined experimentally for each gas.