NOTES 25 - Topic 3 - Thermal Physics *

1. NOTES 25 - Topic 3 - Thermal Physics* ---------------------------------------------------------------------------------- 3.2.9 Define Pressure STP = Standard Temperature, Pressure = 273 K and 101.3 kPa* (that is, 0oC and one atmosphere of pressure ) Pressure (P) = Force per unit of Area; P = N / m2 *Unit of Pressure = Pascal (Pa) 1 Pa = 1 N m-2 1.000 atmosphere of pressure (atm) = 1.013 x 105 Pa = 101.3 kPa 1.00 atm = 14.6 lb in-2 = 29.9 in. Hg** = 760. mm Hg = 76.0 cm Hg (**Bob Ryan’s atmospheric pressure reading on Channel 4 Weather)

2. 3.2.10 The Kinetic Model of an Ideal Gas The Kinetic Model (aka Kinetic Molecular Theory) - an ideal gas is made up of atoms/molecules which are in continual random motion; Postulate 1: There are large numbers of atoms/molecules moving in random directions with a variety of speeds. Postulate 2: The average separation of the atoms/molecules is much greater than the diameter of the molecules. Postulate 3: Atoms/molecules obey Newton’s Laws and interact only when they collide. Postulate 4: All collisions with container walls and other atoms/molecules are perfectly elastic. KMT 1 http://lectureonline.cl.msu.edu/~mmp/kap10/cd283.htm KMT 2 http://mc2.cchem.berkeley.edu/Java/molecules/index.html - The pressure exerted by a gas on the walls of its container is due to the constant bombardment of molecules. If the volume is reduced, the molecules are closer together, the density of the molecules is greater, and the number striking the walls of the container increases.

3. 3.2.11 Temperature - a measure of the average KE of the molecules/atoms of a material; for gases, Kelvin is always used; 3.2.12 Explain the macroscopic model of an ideal gas in terms of the molecular model (PV = nRT); Qualitatively, gas properties are determined by KE of molecules. 1. PRESSURE (P = nRT/V) a. If V, n, and R are constant, an increase in T causes increased KE which results in more and harder collisions of molecules with the walls of the container...P increases; b. If T, n, and R are constant, an increase in V will spread molecules farther apart and decrease the number of collisions with the walls...P decreases; c. If V, T, and R are constant, and the number of gas molecules is increased, the number of collisions with the walls increases...P increases; 2. VOLUME (V = nRT/P): a. If P, n, and R are constant, an increase in T causes increased KE which results in more and harder collisions of molecules with the walls of the container...V increases; b. If T, n, and R are constant, an increase in P will push molecules closer together and increase the number of collisions with the walls...V decreases; c. If P, T, and R are constant, and the number of gas molecules is increased, the number of collisions with the walls increases...V increases;