Hydrogen : Diffusion Of Gases

Hydrogen contained in an open inverted jar rapidly diffuses out, and air enters; this movement takes place in opposition to gravity. Dobereiner in 1823 found that hydrogen confined over water in a cracked flask escaped into the surrounding air, the water rising in the neck of the flask. Graham showed that as the hydrogen escaped, air entered the flask, and since the pressure inside is reduced, it follows that the hydrogen diffuses out more rapidly than air diffuses in. If the flask was covered with a bell-jar of hydrogen, no change in the level of water occurred.

Graham devised a more convenient apparatus for measuring the rates of diffusion of gases, consisting of a glass tube closed at one end with a thin dry plug of plaster of Paris. This tube is filled with water, the air being removed by a siphon so as to avoid wetting the plug, and the water displaced by hydrogen.

Fig: Graham’s diffusion apparatus

The water rises in the tube, and the latter may be sunk in a jar so as to keep the level constant.

After a time all the hydrogen diffuses out, and the tube contains only air which has diffused inwards. No further change of volume then occurs. If the volume of residual air is measured it gives the volume diffusing in the same time as the whole of the hydrogen originally contained in the tube. The inverse ratio of these volumes gives the ratio of the times required for the diffusion of equal volumes. In this way Graham found the following table, the velocity of diffusion being the ratio of the volumes diffusing in equal times:

Gas	Density (Air=1)		Velocity of diffusion (Air=1)
H₂	0.069	3.78	3.83
CH₄	0.559	1.34	1.34
N₂	0.971	1.015	1.014
O₂	1.1056	0.951	0.950
CO₂	1.529	0.809	0.812

Thus, the velocity of diffusion of a gas is inversely proportional to the square root of its density. This is known as Graham's law (1833).

Example. - One hundred c.c. of hydrogen are confined in a diffusion tube exposed to air. When change of volume ceases, what volume of air will be left in the tube ?

The volumes diffusing are in the inverse ratio of the square-roots of the densities;

Expt. 9. - A porous clay pot, such as is used in batteries, is fitted by a rubber bung to a tube passing into a Woulfe's bottle containing coloured water, as shown in

Fig: Experiment on diffusion

Dipping into the coloured water is a glass tube drawn out to a jet above. A large beaker of hydrogen is inverted over the pot; hydrogen diffuses into the latter more rapidly than air passes out, and the increase of pressure causes the water to issue from the jet in the form of a fountain. If the beaker is removed, hydrogen inside the pot diffuses out more rapidly than air enters, so that the pressure is reduced and the water rises in the vertical tube.

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