Chapter: Kinetic Theory

Kinetic Theory, Summary

The molecules of gases, at temperatures above the absolute zero, are in motion. They exert practically no forces on each other unless the gas is strongly compressed, and the pressure exerted by a gas is due to the bombardment of the walls of the containing vessel by the molecules.

If p is the pressure, D the density, of the gas, the mean square speed, G, of the molecules at any given temperature is given by: p = 1/3 DG². The mean speed, Ω, is 0.921 √(G²). At 0° C. the speed of the hydrogen molecule is 1700 m. per sec.; those of other molecules are inversely proportional to the square roots of the molecular weights. The speed is proportional to the square root of the absolute temperature.

The kinetic energy of translation of the molecules in 1 mol of gas is ½MG², where M = mol. wt.; this depends only on the temperature and is the same for all gases. At 0°C. it is 3.41 x 10¹⁰ ergs.

The velocities of effusion of two gases are inversely proportional to the square roots of the molecular weights.

The molecular diameter is of the order of 10^-8 cm.; the mean free path, i.e., the distance traversed by a molecule before collision, is about 10^-5 cm. at S.T.P.

Avogadro's constant, N₀, is the number of molecules in a mol; with a probable accuracy of 1 per cent, it is 6.06 x 10²³.

The molecules of liquids and solids are much closer together than those of gases, and exert forces on one another.

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