Law of Mass Action and Chemical Equilibrium
This Demonstration illustrates the law of mass action, which is an example of Le Chatelier's principle, that if a system in chemical equilibrium is disturbed it tends to change in such a way as to counteract the disturbance.[more]
The final concentrations in a reaction are related to initial concentrations by the equation for the equilibrium constant :
This allows us to obtain the final concentrations by using: , and .
The initial concentrations and are assumed equal and fixed, so the value of is affected only by .
A simple mechanical analogy of a ball in a concave surface helps to explain how the reaction is shifted: when it is on the left, it tends to roll to the right, like the reaction is shifted to the right (the value is assumed positive because reactant concentrations decrease, while product concentrations increase). The opposite happens on the other side. At the bottom, the equilibrium has already been reached, so there is no change in the concentrations. This shifting can be obtained by changing the initial concentration of .
For "mechanical analogy", on the left the reaction is mostly forward and on the right it is mostly backward; these are fixed and are useful to represent the extreme cases when one reaction is predominant. In the middle, the opacity of the arrows is proportional to the forward and backward reaction rate and changes every time the initial concentration is modified. Iodine in its gas phase is colored purple, while the other components are colorless. Thus the reaction rate can be monitored by the intensity of the color.
For both the "product favored" and "reactant favored" windows, the mass effect is shown.
For "product favored", adding reactant to or subtracting products from a steady-state reaction makes it possible to increase the amount of the reaction product . The magnitude of the green dashed arrows is proportional to the quantity added or subtracted, while the green arrow represents the overall effect on the reaction.
For "reactant favored", the situation is the exact opposite. The magnitude of the blue dashed arrows is proportional to the quantity added or subtracted, while the blue arrow represents the overall effect on the reaction.[less]
Snapshot 1: mechanical analogy for the chemical equilibrium; the initial concentration fulfills the equilibrium constant equation, so there is no variation
Snapshot 2: law of mass action: adding reactants or subtracting products in the equilibrium state causes a shift of the equilibrium toward the side of products
Snapshot 3: adding products or subtracting reactants in the equilibrium state causes a shift of the equilibrium toward the side of reactants
 C. H. P. Lupis, Chemical Thermodynamics of Materials, New York: North-Holland, 1983.
 S. V. Lavagnino. Chemical Equilibrium [Video]. (Jun 25, 2020) www.youtube.com/watch?v=TDBQOF7M-W8&list=PLswwssc6Q2yac7AM3x5UjmesLQaye-xMP&index=3.