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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.