molecule decomposes independently of the others. The number of molecules decomposing in unit time is therefore given by —
N = kCs
where A; is a constant depending on the temperature.
For the formation of a molecule of ammonium chloride from a molecule of ammonia and one of hydrochloric acid it is necessary for these to meet. The number of molecules formed must consequently be proportional to the number of such collisions. The possibility of a single molecule of ammonia coming into collision with a molecule of hydro- chloric acid in unit volume is evidently proportional to the number of hydrochloric acid molecules present, i,e, to C2. For Ci molecules of ammonia the number of collisions with Cg molecules of hydrochloric acid will be Ci times as great. The number of collisions iVi between ammonia and hydrochloric acid molecules is therefore proportional to CiCa, or
Now, it is required that —
therefore kCs must be equal to kiCiC^, or
To take another example, if water vapour at high tempe- rature decomposes into hydrogen and oxygen according to —
2H2O ^ 2H2 + O2,
then, in order that a decomposition may take place, one water molecu}e must collide with another one ; and on the other hand, in order that a molecule of water may be formed, two molecules of hydrogen and one molecule of oxygen must collide. If the concentrations of water, hydrogen and oxygen, are Ca* (^h and C2, and JT is a constant, then in the condition of equilibrium —
Ca ^ ATC'i 62.
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