On-Line Resources:
Dynamic Equilibrium
Jodye Selco, jiselco@csupomona.edu
Center for Excellence in Mathematics
and Science Teaching, Cal Poly
Pomona
8 November 2014
NGSS
• HS PS-1 (5, 6); Practices (use models, math
thinking, construct explanation); Crosscutting
(stability and change)
Website
• http://www.csupomona.edu/
~jiselco/dynamicequilibrium/
Student Assignment
• 1) Kinetics: 3 different rates
– What rates of change for A converting into B did
you choose?
– Describe what is shown in each of the three
windows in the simulation upon completion of
each reaction.
– Describe how the reaction changes as the
conversion rate changes.
Student Assignment
• 2) Equilibrium:
• 4 different pairs of conversion rates:
–A converts into B at a larger rate than B
converts back into A
–A converts into B at a smaller rate than
B converts back into A
–A converts into B at the same rate as B
converts into A
–Any conversion rates you wish to
examine.
Student Assignment
• Describe how the reaction is different
from the one-way reaction observed in
kinetics.
• How are the “final” amounts of A and B
determined by the chosen rates of
conversion?
• What do you think the term “dynamic
equilibrium” means? (What changes and
what remains the same?)
Student Assignment
• Make some predictions:
– If your system contained 200 molecules
instead of 100, in what ways would the
“results” of the simulation change?
– If you started with 100 molecules of B instead
of A, would the results of your simulations be
any different? Explain.
– Once dynamic equilibrium has been attained,
20 additional molecules of A were added to
the system. What do you predict the final
results would be if the reaction were to
continue with the same rates of conversion?
Kinetics Take Home Messages
• The reaction does not go to “completion”
• First-order decay kinetics (exponential decay)
• Can see the amount of A disappearing at the
same time the amount of B increases
• Step number represents time
• Number of objects represents concentration
Equilibrium Take Home Messages
• Regardless of percentages exchanged, there is
still some large amount of both A and B
remaining.
• When B/A = 40/60 = 20%/30% = Keq
• Dynamic equilibrium occurs when the total
numbers of A and B remain constant, but
there is still an exchange of AB and BA;
the numbers exchanged are the same
What happens if…
• From equilibrium position, remove 12 squares
and hand them to the group next door.
• Reestablish equilibrium…
• The ratio of B/A is still 2/3!!!!
• This allows you to “organically” demonstrate
what happens during Le Chatelier’s Principle!
(which should state that equilibrium is
reestablished – not establish a “new”
equilibrium)
Take home messages
• As long as the rate constants are the same,
the equilibrium constant remains the same
regardless of the starting amounts, and total
amounts of material.
• Equilibrium is reestablished regardless of how
the amounts (concentrations) are changed.
Students UNDERSTAND that
• Dynamic equilibrium is when the exchange
continues, but the total numbers of A or B
(and molecular concentrations) remain
constant.
• After assignment, students discussed answers
to questions and came up with valid working
definitions of dynamic equilibrium.
• 1 week later (on quiz) 95% of students that did
the assignment were able to describe what
dynamic equilibrium was (in writing).
Students UNDERSTAND that
• Le Châtelier’s Principle is not where a new
equilibrium is established, but a case where
equilibrium is reestablished – with the same K
as before.
• Equilibrium is NOT when there are equal
amounts of products and reactants, nor when
the rate constants are equal.
Beyond General Chemistry
• Even upper division and graduate students
finally realize, with the help of this
visualization tool, what equilibrium actually is.
• Their comments usually include something
about how they wished they “knew” this
before and wonder why it is not universally
used.
For More Information
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Feel free to contact me at
jiselco@csupomona.edu
Jodye Selco
Center for Excellence in Mathematics and
Science Teaching
• Cal Poly Pomona