EQUILIBRIUM, Chemical. In mechanics a system is said to be in equilibrium when the forces that act upon it are precisely balanced, so that their resultant is everywhere zero. A system or mass is similarly said to be in chemi cal equilibrium when its state is such that there is no tendency toward a sensible chemical change in any of its parts. As set forth by Berthollet, chemical equilibrium is not a condi tion of rest, but one in which the velocity of reartion in one direction is equal to the velocity in the opposite direction. The absence of chemi cal action may be absolute, or merely apparent.
In other words, there may be no chemical changes going on at all, or there may be op posite changes going on simultaneously in such a way that no resultant modification can be ob served in any part of the system, however small the part selected for observation may be. In the latter case the existence of the simultaneous and opposite reactions can only be indirectly inferred from a study of the system when it is not in equilibrium.
According to the theory of chemical affinity that was held before the importance of mass action was understood, two substances either would combine or would not, according as their ((affinities') were more or less completely satis fied in the combined state, or in the uncombined state. That this view is entirely inadequate to explain the facts of chemistry is made evident by the following simple example: When steam is passed over red-hot iron filings it is decom posed into oxygen and hydrogen, the iron ab sorbing the oxygen with the fonnation of oxide of iron, while the hydrogen escapes in the free state; but if hydrogen is passed over red-hot oxide of iron the oxide is reduced to the metal lic state, its oxygen combining with the hydro gen to produce steam, which passes on in the hydrogen current. This apparent contradiction may be best explained by assuming that when a mixture of steam and hydrogen is in contact with a red-hot mixture of iron oxide and me tallic iron, both of the foregoing reactions take place simultaneously. If the metallic iron and the steam are present in excessive amounts, the resultant action will be, on the whole, the oxida tion of the iron and the decomposition of the steam; while if the iron oxide and the hydrogen are present in excess, the resultant action will be the reduction of the oxide to the metallic state and the simultaneous formation of steam.
It is therefore apparent that in some reactions, at any rate, the relative masses in which the various constituents are present must be con sidered with much care before any prediction of the chemical deportment of the mixture can be made. Attention was first directed to this fact by Claude Louis Berthollet ((Essai de statique chimique) 1803). In the example cited above, if the iron, iron oxide, hydrogen and steam were left in contact in a closed vessel, a state of apparent equilibrium would be finally at tained in which the forrnation and decomposi tion of the steam would occur with equal rapidity, so that no visible change would take place thereafter. When this state of ((chemical equilibrium)) is attained the abstraction of hy drogen or of iron oxide, or the addition of steam or of metallic iron, will destroy. the equilibrium, and more iron will be oxidized, until a new state of equilibrium is established. Similarly, the abstraction of steam or of me tallic iron or the addition of hydrogen or of iron oxide when the system is in equilibrium will be followed by the reduction of a portion of the iron oxide and the establishment of a new state of equilibrium. According to Ber thollet, all reactions are fundamentally of this kind. When sodium chloride is added to a solu tion of silver nitrate we know that the silver is all thrown down in the form of an insoluble chloride. In this case, Berthollet would con sider that the sodium chloride, soditim nitrate, silver chloride and silver nitrate tend toward a state of chemical equilibrium ; but that since the silver chloride is continually removed from the solution by rea,son of its insolubility, it is im possible for the state of equilibrium ever to be attained, just as it would be impossible for such a state to be attained in our previous illustra tion, if one of the constituents (say the iron oxide) were removed, or rendered inactive in any way, as fast as it were formed.