The Law of Mass Action

For a reversible chemical reaction , the equilibrium constant is given by , where [] represents the concentration of the reactant or product in moles/liter (M). The law of mass action states that adding reactant or , or removing product , will shift the equilibrium to the right (in the forward direction, as written). Conversely, removing or , or adding , will shift the equilibrium to the left. For purposes of visualization, imagine that , , and are immiscible red, green, and blue liquids, respectively. You can vary the equilibrium constant on a logarithmic scale between =10-4 and =104 using the last slider. You can vary the initial concentrations , , and to observe the effect on the equilibrium concentrations.
The law of mass action is one example of Le Chatelier's principle, that the equilibrium in a chemical system responds to a change in concentration, temperature, or pressure by shifting in the direction which partially counteracts the imposed perturbation. Thus if the reaction considered were exothermic, such that heat could formally be regarded as one of the products, an increase in temperature would shift the equilibrium to the left. If A, B and C were gases, an increase in total pressure would shift the equilibrium to the right.



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Snapshot 1: when is large, the reactants and are almost completely converted into product
Snapshot 2: conversely, for small , pure will almost entirely convert to and
Snapshot 3: adding more to the reaction mixture will shift the equilibrium toward higher concentrations of and
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