On-Line Resources: Dynamic Equilibrium Jodye Selco, 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 ~jiselco/dynamicequilibrium/ ~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% = K eq 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 Ch â telier’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 Feel free to contact me at Jodye Selco Center for Excellence in Mathematics and Science Teaching Cal Poly Pomona