Lesson Scenario - Reverse Consecutive First Order (Vensim)
Basic Model:
Description
This model represents a two-stage reversible reaction. Many organic processes operate in which the
reaction can both transform reactants to products, and products back to reactants. In such a
scenario, rather than going to completion the reaction will reach equilibrium. There are three
stages in the reaction, represented by groups A, B, and C. As time goes by, A transforms to B, and
B to C, and upon equilibrium the reverse can also occur
Background Information
A reversible reaction is a different type of chemical reaction where reactants form products, and
when reacted together give reactants back. Reversible reactions can be distinguished from normal
reactions by double arrows, which symbolize the idea that equations can flow in two directions.
Reversible reactions will never complete if they are performed within a closed vessel. In a
reversible chemical process, a state of equilibrium is obtained when the rate of forward chemical
reaction is equal to the rate of reverse chemical reaction.
Science/Math
Le Chatelier's principle is regularly paired with reversible reactions to help explore the
connection of equilibrium concentrations and reverse reactions in chemistry. Le Chatelier states,
"The position of equilibrium shifts to try to cancel out any changes you make." Ex.
A + B <--> C + D Increasing the concentration of A means more C and D are produced to
counteract the change
A + B <--> C + D + Heat Heating the mixture means the equilibrium moves to the left to
counteract the change.
Teaching Strategies
Before teaching this semi-complex topic it is best to reference the generic chemical dynamics
learning scenario and resources. Next proceed to explain chemical equations and reversible
reactions with the following talking points:
The concept of reactants turning into products. Point out that in many cases chemical equations
are oversimplified
Equilibrium. Explain that many reactions reach an equilibrium and the reaction mixture contains
both reactants and products particles. The percent of reactants converted to products varies
considerably.
Le Chatelier's principle. Point out that the principle connects reverse reactions to
equilibrium.
Implementation:
How to use the Model
There are three stages in the 2-staged reaction model, represented by groups A, B, and C. As time
goes by, A transforms to B, and B to C, but the reverse also occurs. The quantity of each group is
shown in the graph on the right. Each stage has circular variables labeled K1, K2, K3, K4
connected with blue arrows showing their dependency, and serve as constant factors to the rate
outcome. In addition to circular variables there are rate flow arrows which control the
organizational flow of the model and 2 sets of box variable reactions labeled frxn1 (First order
reactions 1), frxn2 (First order reactions 2), rrxn1 (Reverse order reactions 1), and rrxn2
(Reverse order reactions 2). Each box variable has its own level that affects the results of the
model and serves as the quantities to the model. The parameters of interest are k1, k2, k3, and
k4, which determine the rate at which each stage of the reaction occurs. Once you have changed the
parameters to your liking, simply run the reaction to view the results. For more information on
Vensim, reference the Vensim tutorial at:
http://shodor.org/tutorials/VensimIntroduction/Preliminaries.
Learning Objectives:
Understand the concept of reversible reactions, equilibrium, and Le Chatelier's principle
Understand the main elements on the chemical reaction model
Objective 1
To accomplish this objective have students observe the current model without any added changes and
be sure they understand the flow, both first order and reversible, as well as the graph results.
Have students complete the reversible reactions review worksheet to keep the concept fresh in
their minds.
Write reversible reactions and ask students to use Le Chatelier's principle to predict the
outcome and how they could increase the yield of a reaction product.
Have students refer to CSERD for more examples and further detailed explanations.
Objective 2
Have students experiment with each of the manipulable parameters (k1, k2, k3, k4) to see the
effect on the simulation. Ask the following questions to guide their exploration:
What happens to the graph if you increase or decrease the amount of reactant/product A?
Reactant/product B? or Reactant/product C?
Do the k1, k2, k3, or k4 values have any effect on the product values on the graph? If so, what
is the effect?
What happens to the graph if you change the dependency of any of the K parameters? (Try
connecting the blue arrows to different boxes and see how the simulation model results change)
Extensions:
Look for other biological pathways that act as reverse reactions
Look at models of reverse reactions in agent modeling applets
Extension 1
Have students research real life examples of reversible chemical reactions. Once they find one
they understand have them sketch a picture of the process and draw a graph predicting what the end
results look like. Students can also try to make their own Vensim model to create a graph of the
processes end result.
Ex. A real life example of a reverse reaction includes: Glycolysis, a metabolic pathway that has 3
irreversible reactions coincided with 7 reversible reactions to carry out the processes in the
cytosol...
This model represents the progress of a chemical reaction from start to finish. It documents the
speed of a reaction as it relates to the concentration.
This is a simple dynamics model of generic chemical reaction relationships. As the model
progresses, reactants change into intermediaries, which then change into products. Users can
change the parameter of reactions to view the rates of transformation and concentration's of
compounds
The Michaelis-Menton model of chemical reactions states that a substrate combines with an enzyme
to form an activated complex. The reaction specified by this equation is the most common
representation of a chemical reaction, used in fields from biochemistry to neuroscience.