Page:Outlines of Physical Chemistry - 1899.djvu/261

 dissociated into equal volumes of hydrogen sulphide and ammonia. At 251°C, the dissociation pressure of this substance is equal to 50*1 centimetres of mercury : this corresponds to a partial pressure of 25*05 cm. for each of the constituents.

If the dissociation takes place in an atmosphere of hydrogen sulphide at a pressure of 8-6 cm., the dissociation- pressure attains the value 50*4 cm. — there is an increase of 41*8 cm. ; the partial pressure due to the ammonia is equal to half the increase, viz. 20*9 cm., and that due to the hydrogen sulphide is the other half of the increase plus the original pressure, 20-9 + 8*6 = 29*5 cm. In agree- ment with theory, the products of the partial pressures (25*05 x 25-05 in the first case, and 29-5 x 20'9 in the second) have the same value. Various experiments per-

Ammonium carbamate, on heating, is decomposed as

follows :

N 2 H 6 C0 2 = 2NH 3 + C0 2.

The equation of equilibrium in this case 2 is

kir = k l u l 2 u 2 ,

and it tells us that at a given temperature the product of the variable concentrations (partial pressures) has a con- stant value.

All the preceding examples relate to the dissociation of solid substances. Even ammonium hydrosulphide and ammonium carbamate may be considered as passing directly from the solid to the dissociated state, because their vapour contains only a negligible proportion of tmdecomposed molecules.

��1 From this we see that the initial presence of one of the products of decomposition offers a resistance to the dissociation.

2 As two molecules of ammonia take part in the reaction, it figures in the equation of equilibrium as the square of the active mass.

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