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Mass action in science is the idea that a large number of small units (especially atoms or molecules) acting randomly by themselves can in fact have a larger pattern. For example, consider a cloud of gas is moving in a given direction. Individual molecules will move in a semi-random walk, but if taken as a whole, they have direction.
The Law of Mass Action, first expressed by Waage and Guldberg in 1864 (Waage, P.; Guldberg, C. M. Forhandlinger: Videnskabs-Selskabet i Christiana 1864, 35) states that the speed of a chemical reaction is proportional to the quantity of the reacting substances. More formally the change of a product quantity is proportional to the product of reactant activities. In the case of a reaction occurring in a gas phase, the activities are equal to the partial pressures. In the case of a well-stirred aqueous medium, the activities are equal to the concentrations. Therefore, if we have a reaction:
The speed of A2B formation is where k is a constant.
A general case is given by a reversible reaction such as:
Which means C and D are produced by the onward reaction, and consumed by the backward reaction. A consequence of the Mass Action Law is that after a sufficient amount of time, there will be an equilibrium between the two reactions and:
resulting in the following equation, often called Mass Action Law itself
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