Page 23 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:35 9in x 6in b985-ch01 Elementary Physical Chemistry

8 Elementary Physical Chemistry

Variation of the number of molecules with speed.
Fig. 1.2
collides with other molecules (i.e. number of collisions/second). It is obvious
that the root-mean-square speed is c = λz.
The kinetic theory developed sofar cannot be usedtoderive λ.Wemust
take into account the finite size of the particles. The result (not derived
here) is
√
λ = RT/( 2N A σP) (1.15)
√
z = 2N AσcP/(RT ) (1.16)
2
where σ is the area, σ = πd ,and d the diameter.
Note:
1) λz = c.
2) When P increases, λ increases and z increases.
3) Gases with larger σ have smaller λ and greater z.

1.9. Diffusion of Gases. Graham’s Law

Diffusion is the tendency of a substance to spread uniformly through space
available to it. Effusion is escape of a gas through a small hole.
The rate at which gases diffuse depends on the density. Graham’s Law
states that the rate of diffusion is inversely proportional to the square root
of the density. If D 1 and D 2 represent the rate of diffusion of Gas 1 and
of Gas 2, Graham’s Law suggests that the rate of diffusion is inversely