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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.
The final concentrations in a reaction are related to initial concentrations by the equation for the equilibrium constant for a generic reaction with all stoichiometric coefficients set equal to 1 and fixed temperature [1]:
[+x][+x]
C
0
D
0
[-x][-x]
A
0
B
0
K
c
This allows us to obtain the final concentrations by using:
[C]
eq
[C]
0
[D]
eq
[D]
0
[A]
eq
[A]
0
[B]
eq
[B]
0
The concentrations at the equilibrium are set as:
[A]
0
[B]
0
[C]
0
[D]
0
so is equal to 1. The domain for a physically valid solution (all the final concentrations must assume positive values) is determined by and as a consequence of the solution of the system of inequalities: +x>0,+x>0,-x>0,-x>0.
K
c
[A]
0
[D]
0
C
0
D
0
A
0
B
0
As the concentrations change, the plot is shifted (brown plot) following Le Chatelier's principle. A red line shows the magnitude of the shifting and the value.
x
Selecting "reaction behavior" shows the numerical values and which side is favored after altering the concentrations.