Quantitative Approach to Law of Mass Action

​
quantitative interpretation
reaction behavior
[C]
0
0.5
[D]
0
0.2
[A]
0
0.2
[B]
0
0.5
zoom y
25
zoom x
0.7
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]:
[
C
0
+x][
D
0
+x]
[
A
0
-x][
B
0
-x]
=
K
c
.
This allows us to obtain the final concentrations by using:
[C]
eq
=
[C]
0
+x
,
[D]
eq
=
[D]
0
+x
,
[A]
eq
=
[A]
0
-x
and
[B]
eq
=
[B]
0
-x
.
The concentrations at the equilibrium are set as:
[A]
0
=0.2
,
[B]
0
=0.5
,
[C]
0
=0.5
and
[D]
0
=0.2
so
K
c
is equal to 1. The domain for a physically valid solution (all the final concentrations must assume positive values) is determined by
[A]
0
and
[D]
0
as a consequence of the solution of the system of inequalities:
C
0
+x>0,
D
0
+x>0,
A
0
-x>0,
B
0
-x>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
x
value.
Selecting "reaction behavior" shows the numerical values and which side is favored after altering the concentrations.

Details

Snapshot 1: system at the equilibrium; the initial concentration fulfils the equilibrium constant equation so there is no variation
Snapshot 2: Le Chatelier's principle: raising one reactant concentration and lowering one product concentration in the equilibrium state causes a shift toward the side of products (
x>0
)
Snapshot 3: raising reactant concentrations and lowering product concentrations in the equilibrium state causes a shift toward the side of products (
x<0
)

References

[1] C. H. P. Lupis, Chemical Thermodynamics of Materials, New York: North-Holland, 1983.
[2] S. Z. Lavagnino. Chemical Equilibrium[Video]. (Jun 25, 2020) www.youtube.com/watch?v=TDBQOF7M-W8&list=PLswwssc6Q2yac7AM3x5UjmesLQaye-xMP&index=3.

External Links

Chemical Equilibrium and Kinetics for HI Reaction
Single-Step Reaction Kinetics Using Collision Theory

Permanent Citation

D. Meliga, L. Lavagnino, S. Z. Lavagnino
​
​"Quantitative Approach to Law of Mass Action"​
​http://demonstrations.wolfram.com/QuantitativeApproachToLawOfMassAction/​
​Wolfram Demonstrations Project​
​Published: July 17, 2020